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Virginia Tech
1.
Akhtar, Kareem.
Numerical Investigation using RANS Equations of Two-dimensional Turbulent Jets and Bubbly Mixing layers.
Degree: MS, Engineering Science and Mechanics, 2010, Virginia Tech
URL: http://hdl.handle.net/10919/34512 
► This thesis presents numerical investigations of two-dimensional single-phase turbulent jets and bubbly mixing layers using Reynolds-Averaged Navier-Stokes (RANS) equations. The behavior of a turbulent jet…
(more)
▼ This thesis presents numerical investigations of two-dimensional
single-phase turbulent jets and bubbly mixing layers using
Reynolds-Averaged Navier-Stokes (RANS) equations.
The behavior of a turbulent jet confined in a channel depends
on the Reynolds number and geometry of the channel which
is given by the expansion ratio (channel width to jet thickness)
and offset ratio (eccentricity of the jet entrance). Steady
solutions to the RANS equations for a two-dimensional turbulent
jet injected in the middle of a channel have been obtained. When
no entrainment from the channel base is allowed, the flow is
asymmetric for a wide range of expansion ratio at high Reynolds
number. The jet attaches to one of the channel side walls. The
attachment length increases linearly with the channel
width for fixed value of Reynolds number. The attachment length is
also found to be independent of the (turbulent) jet Reynolds
number for fixed expansion ratio. By simulating half of the
channel and imposing symmetry, we can construct a steady symmetric
solution to the RANS equations. This implies that there are
possibly two solutions to the steady RANS equations, one is
symmetric but unstable, and the other solution is asymmetric (the
jet attaches to one of the side walls) but stable. A symmetric
solution is also obtained if entrainment from jet exit plane is
permitted. Fearn et al. (Journal of Fluid Mechanics, vol. 121,
1990) studied the laminar problem, and showed that the flow
asymmetry of a symmetric expansion arises at a symmetry-breaking
bifurcation as the jet Reynolds number is increased from zero. In
the present study the Reynolds number is high and the jet is
turbulent. Therefore, a symmetry-breaking bifurcation parameter
might be the level of entrainment or expansion ratio.
The two-dimensional turbulent bubbly mixing layer, which is a
multiphase problem, is investigated using RANS based models.
Available experimental data show that the spreading rate of
turbulent bubbly mixing layers is greater than that of the
corresponding single phase flow. The presence of bubbles also
increases the turbulence level. The global structure of the flow
proved to be sensitive to the void fraction. The present
RANS simulations predict this behavior, but different turbulence
models give different spreading rates. There is a significant
difference in turbulence kinetic energy between numerical
predictions and experimental data. The models tested include
k-ε, shear-stress transport (SST), and Reynolds stress
transport (SSG) models. All tested turbulence models under predict
the spreading rate of the bubbly mixing layer, even though they
accurately predict the spreading rate for single phase flow. The
best predictions are obtained by using SST model.
Advisors/Committee Members: Ragab, Saad A. (committeechair), Cramer, Mark S. (committee member), Hajj, Muhammad R. (committee member).
Subjects/Keywords: confined jets; bubbly mixing layers
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APA (6th Edition):
Akhtar, K. (2010). Numerical Investigation using RANS Equations of Two-dimensional Turbulent Jets and Bubbly Mixing layers. (Masters Thesis). Virginia Tech. Retrieved from http://hdl.handle.net/10919/34512
Chicago Manual of Style (16th Edition):
Akhtar, Kareem. “Numerical Investigation using RANS Equations of Two-dimensional Turbulent Jets and Bubbly Mixing layers.” 2010. Masters Thesis, Virginia Tech. Accessed October 19, 2019.
http://hdl.handle.net/10919/34512.
MLA Handbook (7th Edition):
Akhtar, Kareem. “Numerical Investigation using RANS Equations of Two-dimensional Turbulent Jets and Bubbly Mixing layers.” 2010. Web. 19 Oct 2019.
Vancouver:
Akhtar K. Numerical Investigation using RANS Equations of Two-dimensional Turbulent Jets and Bubbly Mixing layers. [Internet] [Masters thesis]. Virginia Tech; 2010. [cited 2019 Oct 19].
Available from: http://hdl.handle.net/10919/34512.
Council of Science Editors:
Akhtar K. Numerical Investigation using RANS Equations of Two-dimensional Turbulent Jets and Bubbly Mixing layers. [Masters Thesis]. Virginia Tech; 2010. Available from: http://hdl.handle.net/10919/34512
2.
Zhang, Hongbin.
Approaches to Simulation of an Underground Longwall Mine and Implications for Ventilation System Analysis.
Degree: MS, Mining and Minerals Engineering, 2015, Virginia Tech
URL: http://hdl.handle.net/10919/52977
► Carefully engineered mine ventilation is critical to the safe operation of underground longwall mines. Currently, there are several options for simulation of mine ventilation. This…
(more)
▼ Carefully engineered mine ventilation is critical to the safe operation of underground longwall mines. Currently, there are several options for simulation of mine ventilation. This research was conducted to rapidly simulate an underground longwall mine, especially for the use of tracer gas in an emergency situation. In an emergency situation, limited information about the state of mine ventilation system is known, and it is difficult to make informed decisions about safety of the mine for rescue personnel. With careful planning, tracer gases can be used to remotely ascertain changes in the ventilation system. In the meantime, simulation of the tracer gas can be conducted to understand the airflow behavior for improvements during normal operation.
Better informed decisions can be made with the help of both tracer gas technique and different modeling approaches. This research was made up of two main parts. One was a field study conducted in an underground longwall mine in the western U.S. The other one was a simulation of the underground longwall mine with different approaches, such as network modeling and Computational Fluid Dynamics (CFD) models. Networking modeling is the most prevalent modeling technique in the mining industry. However, a gob area, which is a void zone filled with broken rocks after the longwall mining, cannot be simulated in an accurate way with networking modeling. CFD is a powerful tool for modeling different kinds of flows under various situations. However, it requires a significant time investment for the expert user as well as considerable computing power. To take advantage of both network modeling and CFD, the hybrid approach, which is a combination of network modeling and CFD was established. Since tracer gas was released and collected in the field study, the tracer gas concentration profile was separately simulated in network modeling, CFD model, and hybrid model in this study. The simulated results of airflow and tracer gas flow were analyzed and compared with the experimental results from the field study.
Two commercial network modeling software packages were analyzed in this study. One of the network modeling software also has the capability to couple with CFD. A two-dimensional (2D) CFD model without gob was built to first analyze the accuracy of CFD. More 2D CFD models with gob were generated to determine how much detail was necessary for the gob model. Several three-dimensional (3D) CFD models with gob were then created. A mesh independence study and a sensitivity study for the porosity and permeability values were created to determine the optimal mesh size, porosity and permeability values for the 3D CFD model, and steady-state simulation and transient simulations were conducted in the 3D CFD models. In the steady-state simulation, a comparison was made between the 3D CFD models with and without taking the diffusivity of SF6 in air into account.
Finally, the different simulation techniques were compared to measured field data, and assessed to determine if the hybrid approach was…
Advisors/Committee Members: Luxbacher, Kramer Davis (committeechair), Ragab, Saad A. (committee member), Luttrell, Gerald H. (committee member).
Subjects/Keywords: CFD modeling; network modeling; hybrid model; underground mine ventilation
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APA (6th Edition):
Zhang, H. (2015). Approaches to Simulation of an Underground Longwall Mine and Implications for Ventilation System Analysis. (Masters Thesis). Virginia Tech. Retrieved from http://hdl.handle.net/10919/52977
Chicago Manual of Style (16th Edition):
Zhang, Hongbin. “Approaches to Simulation of an Underground Longwall Mine and Implications for Ventilation System Analysis.” 2015. Masters Thesis, Virginia Tech. Accessed October 19, 2019.
http://hdl.handle.net/10919/52977.
MLA Handbook (7th Edition):
Zhang, Hongbin. “Approaches to Simulation of an Underground Longwall Mine and Implications for Ventilation System Analysis.” 2015. Web. 19 Oct 2019.
Vancouver:
Zhang H. Approaches to Simulation of an Underground Longwall Mine and Implications for Ventilation System Analysis. [Internet] [Masters thesis]. Virginia Tech; 2015. [cited 2019 Oct 19].
Available from: http://hdl.handle.net/10919/52977.
Council of Science Editors:
Zhang H. Approaches to Simulation of an Underground Longwall Mine and Implications for Ventilation System Analysis. [Masters Thesis]. Virginia Tech; 2015. Available from: http://hdl.handle.net/10919/52977
Virginia Tech
3.
Barsallo Pacheco, Nilma Rosa.
Material and Damping Characterization of Discretized Adhesive Tapes in Cantilever Beams undergoing Free and Forced Vibration.
Degree: MS, Engineering Science and Mechanics, 2014, Virginia Tech
URL: http://hdl.handle.net/10919/64357
► The work is focused in investigating the effectiveness of discretized damping tapes applied to a cantilever beam subjected to free and forced vibrations. The work…
(more)
▼ The work is focused in investigating the effectiveness of discretized damping tapes applied to a cantilever beam subjected to free and forced vibrations. The work is divided into three main sections. First, we performed material characterization of the viscoelastic (VE) pressure sensitive adhesive layer of the damping tapes. To do so, we designed a novel quad shear specimen to measure shear storage and loss moduli, and tan delta from dynamic mechanical analyzer measurements. Second, the optimal discretization length for different damping tapes was experimentally determined and analytically verified using linear viscoelasticity and basic strength of materials and vibrations principles. These results showed a mean to improve the damping of a structure without increasing the weight of the added damping layer. Third, a nonlinear analysis was performed for cantilever beams with damping layers subjected to parametric excitation. Comparison of the response amplitude of the parametrically excited beam was performed for different discretization lengths, and system identification of the nonlinear parameters was carried out. The effects of large deflections of a beam under parametric excitation were analyzed; large deflections were found to induce localized buckling of the stiff constraining layer of the damping tape that would invalidate some of the assumptions and analytical solutions that do not take such phenomena into account.
Advisors/Committee Members: Dillard, David A. (committeechair), Hajj, Muhammad R. (committeechair), Ragab, Saad A. (committee member).
Subjects/Keywords: Damping tapes; Vibrations; Viscoelastic material; Damping ratio; Nonlinear damping
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APA (6th Edition):
Barsallo Pacheco, N. R. (2014). Material and Damping Characterization of Discretized Adhesive Tapes in Cantilever Beams undergoing Free and Forced Vibration. (Masters Thesis). Virginia Tech. Retrieved from http://hdl.handle.net/10919/64357
Chicago Manual of Style (16th Edition):
Barsallo Pacheco, Nilma Rosa. “Material and Damping Characterization of Discretized Adhesive Tapes in Cantilever Beams undergoing Free and Forced Vibration.” 2014. Masters Thesis, Virginia Tech. Accessed October 19, 2019.
http://hdl.handle.net/10919/64357.
MLA Handbook (7th Edition):
Barsallo Pacheco, Nilma Rosa. “Material and Damping Characterization of Discretized Adhesive Tapes in Cantilever Beams undergoing Free and Forced Vibration.” 2014. Web. 19 Oct 2019.
Vancouver:
Barsallo Pacheco NR. Material and Damping Characterization of Discretized Adhesive Tapes in Cantilever Beams undergoing Free and Forced Vibration. [Internet] [Masters thesis]. Virginia Tech; 2014. [cited 2019 Oct 19].
Available from: http://hdl.handle.net/10919/64357.
Council of Science Editors:
Barsallo Pacheco NR. Material and Damping Characterization of Discretized Adhesive Tapes in Cantilever Beams undergoing Free and Forced Vibration. [Masters Thesis]. Virginia Tech; 2014. Available from: http://hdl.handle.net/10919/64357
Virginia Tech
4.
Goodnight, Randall James Truett.
Derivation of an Evolution Equation for Two-Dimensional Waves on Thin Films.
Degree: MS, Engineering Science and Mechanics, 2013, Virginia Tech
URL: http://hdl.handle.net/10919/23125
► We examine wave propagation on thin liquid films subjected to gravity, fluid friction, surface tension, and Marangoni effects. The physical configuration is a thin liquid…
(more)
▼ We examine wave propagation on thin liquid films subjected to gravity, fluid friction, surface tension, and Marangoni effects. The physical configuration is a thin liquid layer on a planar incline. Following previous studies, the Marangoni effect is incorporated by a constant surface tension gradient and yields a non-convex flux function in our thin film equation. We extend previous studies by deriving the thin film equation governing two-dimensional waves on the liquid layer. We then derive a simplified evolution equation governing weakly nonlinear, quasi-planar, and weakly dissipative waves on the layer. When the undisturbed state is in the vicinity of an inflection point in the streamwise component of the flux function, the mixed nonlinearity, fourth order dissipation and the transverse modulations interact over time scales on the order of the scaled amplitude to the negative second power. The effect the transverse modulations is found to be intrinsically nonlinear.
Advisors/Committee Members: Cramer, Mark S. (committeechair), Ragab, Saad A. (committee member), Jung, Sunghwan (committee member).
Subjects/Keywords: Thin Films; Undercompressive Shock; Non-Convex Flux; Marangoni Stress; Surface Tensio
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APA (6th Edition):
Goodnight, R. J. T. (2013). Derivation of an Evolution Equation for Two-Dimensional Waves on Thin Films. (Masters Thesis). Virginia Tech. Retrieved from http://hdl.handle.net/10919/23125
Chicago Manual of Style (16th Edition):
Goodnight, Randall James Truett. “Derivation of an Evolution Equation for Two-Dimensional Waves on Thin Films.” 2013. Masters Thesis, Virginia Tech. Accessed October 19, 2019.
http://hdl.handle.net/10919/23125.
MLA Handbook (7th Edition):
Goodnight, Randall James Truett. “Derivation of an Evolution Equation for Two-Dimensional Waves on Thin Films.” 2013. Web. 19 Oct 2019.
Vancouver:
Goodnight RJT. Derivation of an Evolution Equation for Two-Dimensional Waves on Thin Films. [Internet] [Masters thesis]. Virginia Tech; 2013. [cited 2019 Oct 19].
Available from: http://hdl.handle.net/10919/23125.
Council of Science Editors:
Goodnight RJT. Derivation of an Evolution Equation for Two-Dimensional Waves on Thin Films. [Masters Thesis]. Virginia Tech; 2013. Available from: http://hdl.handle.net/10919/23125
Virginia Tech
5.
Epstein, Stephen David.
The Stochastic Dynamics of Optomechanical Sensors for Atomic Force Microscopy.
Degree: MS, Engineering Science and Mechanics, 2013, Virginia Tech
URL: http://hdl.handle.net/10919/23730
► This work explores the stochastic dynamics and important diagnostics of a mechanical resonator (nanobeam) used in cavity optomechanical sensors for atomic force microscopy. Atomic force…
(more)
▼ This work explores the stochastic dynamics and important diagnostics of a mechanical resonator (nanobeam) used in cavity optomechanical sensors for atomic force microscopy. Atomic force microscopy (AFM) is a tool to image surface topology down to the level of individual atoms. Conventional AFM has been an essential tool for micro and nanoscale studies in physics, chemistry, and biology. Cavity optomechanical sensors for AFM extend the utility of conventional AFM into a new regime of high sensitivity k is approximately 1 N/m and high frequency f0 is approximately 10 MHz. Cavity optomechanical sensors for AFM are unique because they use near field optics to transduce the position of a nanobeam. The nanobeam is not able to be transduced by more conventional AFM techniques, such as laser interferometry, because the nanobeam is smaller than the spot size of the laser.
This work determines the noise spectrum G of a nanobeam in water and in air. Also important diagnostics of the nanobeam are determined in air and in water. These important diagnostics include the quality factor Q and natural frequency in fluid omega_f. It is found that the nanobeam is overdamped in water. However, the nanobeam is underdamped in air and has quality factor of Q is approximately 4. The noise spectrum is determined from deterministic numerical calculations and the Fluctuation-Dissipation Theorem. This is possible because the same molecular processes, Brownian motion, cause both the fluctuations of the nanobeam and the dissipation of the nanobeam.
Advisors/Committee Members: Paul, Mark R. (committeechair), Ragab, Saad A. (committee member), Ross, Shane David (committee member).
Subjects/Keywords: optomechanics; atomic force microscopy; noise spectrum
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APA (6th Edition):
Epstein, S. D. (2013). The Stochastic Dynamics of Optomechanical Sensors for Atomic Force Microscopy. (Masters Thesis). Virginia Tech. Retrieved from http://hdl.handle.net/10919/23730
Chicago Manual of Style (16th Edition):
Epstein, Stephen David. “The Stochastic Dynamics of Optomechanical Sensors for Atomic Force Microscopy.” 2013. Masters Thesis, Virginia Tech. Accessed October 19, 2019.
http://hdl.handle.net/10919/23730.
MLA Handbook (7th Edition):
Epstein, Stephen David. “The Stochastic Dynamics of Optomechanical Sensors for Atomic Force Microscopy.” 2013. Web. 19 Oct 2019.
Vancouver:
Epstein SD. The Stochastic Dynamics of Optomechanical Sensors for Atomic Force Microscopy. [Internet] [Masters thesis]. Virginia Tech; 2013. [cited 2019 Oct 19].
Available from: http://hdl.handle.net/10919/23730.
Council of Science Editors:
Epstein SD. The Stochastic Dynamics of Optomechanical Sensors for Atomic Force Microscopy. [Masters Thesis]. Virginia Tech; 2013. Available from: http://hdl.handle.net/10919/23730
Virginia Tech
6.
Andreyev, Aleksandr Vladimirovich.
Theoretical And Computational Study of Steady Transonic Flows of Bethe-Zel\'dovich-Thompson Fluids.
Degree: MS, Engineering Science and Mechanics, 2013, Virginia Tech
URL: http://hdl.handle.net/10919/23734
► We examine steady transonic flows of Bethe-Zel\'dovich-Thompson (BZT) fluids over thin turbine blades or airfoils. BZT fluids are ordinary fluids having a region of negative…
(more)
▼ We examine steady transonic flows of Bethe-Zel\'dovich-Thompson (BZT) fluids over thin turbine blades or airfoils. BZT fluids are ordinary fluids having a region of negative fundamental derivative over a finite range of pressures and temperatures in the single phase regime. We derive the transonic small disturbance equation (TSDE) capable of capturing the qualitative behavior of BZT fluids. The shock jump conditions, and shock existence conditions consistent with the derived TSDE are presented. The flux function is seen to be quartic in the pressure or density perturbation rather than the quadratic (convex) flux function of the perfect gas theory. We show how this nonconvex flux function can be used to predict and explain the complex flows possible in transonic BZT fluids. Numerical solutions using a successive line relaxation (SLR) scheme are presented. New results of interest include shock-splitting, collisions between expansion and compression shocks, the prediction and observation of two compressive bow shocks in supersonic flows, and the observation of as many as three normal stern shocks following an oblique trailing edge shock.
Advisors/Committee Members: Cramer, Mark S. (committeechair), Jung, Sunghwan (committee member), Ragab, Saad A. (committee member).
Subjects/Keywords: Transonic Flow; Bethe-Zel'dovich-Thompson; Small Disturbance; Successive Line Relaxation; Shock Splitting
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APA (6th Edition):
Andreyev, A. V. (2013). Theoretical And Computational Study of Steady Transonic Flows of Bethe-Zel\'dovich-Thompson Fluids. (Masters Thesis). Virginia Tech. Retrieved from http://hdl.handle.net/10919/23734
Chicago Manual of Style (16th Edition):
Andreyev, Aleksandr Vladimirovich. “Theoretical And Computational Study of Steady Transonic Flows of Bethe-Zel\'dovich-Thompson Fluids.” 2013. Masters Thesis, Virginia Tech. Accessed October 19, 2019.
http://hdl.handle.net/10919/23734.
MLA Handbook (7th Edition):
Andreyev, Aleksandr Vladimirovich. “Theoretical And Computational Study of Steady Transonic Flows of Bethe-Zel\'dovich-Thompson Fluids.” 2013. Web. 19 Oct 2019.
Vancouver:
Andreyev AV. Theoretical And Computational Study of Steady Transonic Flows of Bethe-Zel\'dovich-Thompson Fluids. [Internet] [Masters thesis]. Virginia Tech; 2013. [cited 2019 Oct 19].
Available from: http://hdl.handle.net/10919/23734.
Council of Science Editors:
Andreyev AV. Theoretical And Computational Study of Steady Transonic Flows of Bethe-Zel\'dovich-Thompson Fluids. [Masters Thesis]. Virginia Tech; 2013. Available from: http://hdl.handle.net/10919/23734
Virginia Tech
7.
Bowling, John Robert Reid.
Design of an Experimental Mine Simulator for the Development of a Procedure for Utilization Multiple Tracer Gases in Underground Mines.
Degree: MS, Mining and Minerals Engineering, 2011, Virginia Tech
URL: http://hdl.handle.net/10919/32538
► An experimental mine simulator was constructed which will be used to conduct tracer gas experiments in the laboratory. The test apparatus simulates a mine in…
(more)
▼ An experimental mine simulator was constructed which will be used to conduct
tracer gas experiments in the laboratory. The test apparatus simulates a mine in a tabular
deposit and is modular and simple and can be easily rearranged to represent a variety of
mine geometries. The apparatus is appropriate for the use of tracer gases by being both
airtight and open-circuit (exhausting to the atmosphere) and by maintaining turbulent
flow throughout the model, ensuring the tracer gas is fully dispersed.
The model features ports for injection and sampling of tracer gases, which
represent boreholes present in an actual mine. The model is designed, in part, for the
practice of tracer gas release and sampling methods in the laboratory. Valves on the
apparatus represent ventilation controls, such as stoppings or regulators, or changing
resistances in a mine, such an increase in resistance due to a roof fall or a decrease in
resistance due to stoppings being destroyed. The relative resistances of airways can be
changed by changing the status of the valves to represent different states of the
ventilation controls.
The mine simulator should serve as a tool for identifying and investigating novel
tracer gases, developing a procedure for performing ventilation surveys using multiple
tracer gases, and eventually developing a method for remotely inferring ventilation
changes using tracer gases.
Advisors/Committee Members: Luxbacher, Kramer Davis (committeechair), Westman, Erik Christian (committee member), Ragab, Saad A. (committee member).
Subjects/Keywords: mine scale model; tracer gas; mine ventilation
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APA (6th Edition):
Bowling, J. R. R. (2011). Design of an Experimental Mine Simulator for the Development of a Procedure for Utilization Multiple Tracer Gases in Underground Mines. (Masters Thesis). Virginia Tech. Retrieved from http://hdl.handle.net/10919/32538
Chicago Manual of Style (16th Edition):
Bowling, John Robert Reid. “Design of an Experimental Mine Simulator for the Development of a Procedure for Utilization Multiple Tracer Gases in Underground Mines.” 2011. Masters Thesis, Virginia Tech. Accessed October 19, 2019.
http://hdl.handle.net/10919/32538.
MLA Handbook (7th Edition):
Bowling, John Robert Reid. “Design of an Experimental Mine Simulator for the Development of a Procedure for Utilization Multiple Tracer Gases in Underground Mines.” 2011. Web. 19 Oct 2019.
Vancouver:
Bowling JRR. Design of an Experimental Mine Simulator for the Development of a Procedure for Utilization Multiple Tracer Gases in Underground Mines. [Internet] [Masters thesis]. Virginia Tech; 2011. [cited 2019 Oct 19].
Available from: http://hdl.handle.net/10919/32538.
Council of Science Editors:
Bowling JRR. Design of an Experimental Mine Simulator for the Development of a Procedure for Utilization Multiple Tracer Gases in Underground Mines. [Masters Thesis]. Virginia Tech; 2011. Available from: http://hdl.handle.net/10919/32538
Virginia Tech
8.
Meesala, Vamsi Chandra.
Modeling and Analysis of a Cantilever Beam Tip Mass System.
Degree: MS, Engineering Science and Mechanics, 2018, Virginia Tech
URL: http://hdl.handle.net/10919/83378
► We model the nonlinear dynamics of a cantilever beam with tip mass system subjected to different excitation and exploit the nonlinear behavior to perform sensitivity…
(more)
▼ We model the nonlinear dynamics of a cantilever beam with tip mass system subjected to different excitation and exploit the nonlinear behavior to perform sensitivity analysis and propose a parameter identification scheme for nonlinear piezoelectric coefficients.
First, the distributed parameter governing equations taking into consideration the nonlinear boundary conditions of a cantilever beam with a tip mass subjected to principal parametric excitation are developed using generalized Hamilton's principle. Using a Galerkin's discretization scheme, the discretized equation for the first mode is developed for simpler representation assuming linear and nonlinear boundary conditions. We solve the distributed parameter and discretized equations separately using the method of multiple scales. We determine that the cantilever beam tip mass system subjected to parametric excitation is highly sensitive to the detuning. Finally, we show that assuming linearized boundary conditions yields the wrong type of bifurcation.
Noting the highly sensitive nature of a cantilever beam with tip mass system subjected to parametric excitation to detuning, we perform sensitivity of the response to small variations in elasticity (stiffness), and the tip mass. The governing equation of the first mode is derived, and the method of multiple scales is used to determine the approximate solution based on the order of the expected variations. We demonstrate that the system can be designed so that small variations in either stiffness or tip mass can alter the type of bifurcation. Notably, we show that the response of a system designed for a supercritical bifurcation can change to yield a subcritical bifurcation with small variations in the parameters. Although such a trend is usually undesired, we argue that it can be used to detect small variations induced by fatigue or small mass depositions in sensing applications.
Finally, we consider a cantilever beam with tip mass and piezoelectric layer and propose a parameter identification scheme that exploits the vibration response to estimate the nonlinear piezoelectric coefficients. We develop the governing equations of a cantilever beam with tip mass and piezoelectric layer by considering an enthalpy that accounts for quadratic and cubic material nonlinearities. We then use the method of multiple scales to determine the approximate solution of the response to direct excitation. We show that approximate solution and amplitude and phase modulation equations obtained from the method of multiple scales analysis can be matched with numerical simulation of the response to estimate the nonlinear piezoelectric coefficients.
Advisors/Committee Members: Hajj, Muhammad R. (committeechair), Ragab, Saad A. (committee member), Shahab, Shima (committee member).
Subjects/Keywords: Parametric excitation; Cantilever beam mass systems; Boundary conditions; Perturbation methods; Method of multiple scales; Sensitivity analysis; Uncertainty quantification; Mass/gas sensing; Damage detection; Energy harvesting; Piezoelectric material
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APA (6th Edition):
Meesala, V. C. (2018). Modeling and Analysis of a Cantilever Beam Tip Mass System. (Masters Thesis). Virginia Tech. Retrieved from http://hdl.handle.net/10919/83378
Chicago Manual of Style (16th Edition):
Meesala, Vamsi Chandra. “Modeling and Analysis of a Cantilever Beam Tip Mass System.” 2018. Masters Thesis, Virginia Tech. Accessed October 19, 2019.
http://hdl.handle.net/10919/83378.
MLA Handbook (7th Edition):
Meesala, Vamsi Chandra. “Modeling and Analysis of a Cantilever Beam Tip Mass System.” 2018. Web. 19 Oct 2019.
Vancouver:
Meesala VC. Modeling and Analysis of a Cantilever Beam Tip Mass System. [Internet] [Masters thesis]. Virginia Tech; 2018. [cited 2019 Oct 19].
Available from: http://hdl.handle.net/10919/83378.
Council of Science Editors:
Meesala VC. Modeling and Analysis of a Cantilever Beam Tip Mass System. [Masters Thesis]. Virginia Tech; 2018. Available from: http://hdl.handle.net/10919/83378
Virginia Tech
9.
Deshpande, Revati Rajeev.
Effect of Centrifugal Stiffening on the Natural Frequencies of Aircraft Wings During Rapid Roll Maneuvers.
Degree: MS, Aerospace and Ocean Engineering, 2018, Virginia Tech
URL: http://hdl.handle.net/10919/92698
► The rolling of an aircraft about its fuselage produces centrifugal forces which affect the stiffness of the wings. A number of previous studies explain the…
(more)
▼ The rolling of an aircraft about its fuselage produces centrifugal forces which affect the stiffness of the wings. A number of previous studies explain the effect of centrifugal stiffening in rotating beams and consequently on the frequencies of the beam. Multiple cases of the rotating beam are explored in this thesis to investigate effects of mass distribution and boundary conditions on the frequencies of centrifugally stiffened beams. It is found that for a uniform beam with all degrees of freedom free on both ends, the rigid modes of the beam are affected and are no longer zero when it is stiffened from centrifugal forces. This thesis aims to set up a model to investigate the stiffening effects using the mAEWing2 aircraft. A preliminary analysis is done for the mAEWing2 aircraft and the roll rate, control surface deflection and angle of attack are identified as the parameters to be studied. For a given angle of attack and control surface deflection, the centrifugal forces in the aircraft in steady roll are determined using trim analysis. These are used to pre-stress the model for modal analysis. It is found that in mAEWing2 aircraft in steady roll maneuvers, the centrifugal stiffening effect on the natural frequencies is not significant. It emphasizes the need to conduct a sensitivity analysis to include centrifugal stiffening in the dynamic analysis while designing an aircraft. This, along with some de-stiffening due to gravity loads might be important for the future N+3 aircraft with their high aspect ratio large wingspans.
Advisors/Committee Members: Kapania, Rakesh K (committeechair), Ragab, Saad A (committee member), Patil, Mayuresh J (committee member).
Subjects/Keywords: Centrifugal Stiffening; Rotating beam; Natural Frequencies
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APA (6th Edition):
Deshpande, R. R. (2018). Effect of Centrifugal Stiffening on the Natural Frequencies of Aircraft Wings During Rapid Roll Maneuvers. (Masters Thesis). Virginia Tech. Retrieved from http://hdl.handle.net/10919/92698
Chicago Manual of Style (16th Edition):
Deshpande, Revati Rajeev. “Effect of Centrifugal Stiffening on the Natural Frequencies of Aircraft Wings During Rapid Roll Maneuvers.” 2018. Masters Thesis, Virginia Tech. Accessed October 19, 2019.
http://hdl.handle.net/10919/92698.
MLA Handbook (7th Edition):
Deshpande, Revati Rajeev. “Effect of Centrifugal Stiffening on the Natural Frequencies of Aircraft Wings During Rapid Roll Maneuvers.” 2018. Web. 19 Oct 2019.
Vancouver:
Deshpande RR. Effect of Centrifugal Stiffening on the Natural Frequencies of Aircraft Wings During Rapid Roll Maneuvers. [Internet] [Masters thesis]. Virginia Tech; 2018. [cited 2019 Oct 19].
Available from: http://hdl.handle.net/10919/92698.
Council of Science Editors:
Deshpande RR. Effect of Centrifugal Stiffening on the Natural Frequencies of Aircraft Wings During Rapid Roll Maneuvers. [Masters Thesis]. Virginia Tech; 2018. Available from: http://hdl.handle.net/10919/92698
Virginia Tech
10.
Salem Said, Abdel-Halim Saber.
Large Eddy Simulation of Shear-Free Interaction of Homogeneous Turbulence with a Flat-Plate Cascade.
Degree: PhD, Engineering Science and Mechanics, 2007, Virginia Tech
URL: http://hdl.handle.net/10919/28520
► Studying the effects of free stream turbulence on noise, vibration, and heat transfer on structures is very important in engineering applications. The problem of the…
(more)
▼ Studying the effects of free stream turbulence on noise, vibration, and heat transfer on structures is very important in engineering applications. The problem of the interaction of large scale turbulence with a flat-plate cascade is a model of important problems in propulsion systems. Addressing the problem of large scale turbulence interacting with a flat plate cascade requires flow simulation over a large number of plates (6-12 plates) in order to be able to represent numerically integral length scales on the order of blade-to-blade spacing. Having such a large number of solid surfaces in the simulation requires very large computational grid points to resolve the boundary layers on the plates, and that is not possible with the current computing resources.
In this thesis we develop a computational technique to predict the distortion of homogeneous isotropic turbulence as it passes through a cascade of thin flat plates. We use Large-Eddy Simulation (LES) to capture the spatial development of the incident turbulence and its interaction with the plates which are assumed to be inviscid walls.
The LES is conducted for a linear cascade composed of six plates. Because suppression of the normal component of velocity is the main mechanism of distortion, we neglect the presence of mean shear in the boundary layers and wakes, and allow slip velocity on the plate surfaces. We enforce the zero normal velocity condition on the plates. This boundary condition treatment is motivated by rapid distortion theory (RDT) in which viscous effects are neglected, however, the present LES approach accounts for nonlinear and turbulence diffusion effects by a sub-grid scale model. We refer to this type of turbulence-blade interaction as shear-free interaction.
To validate our calculations, we computed the unsteady loading and radiated acoustic pressure field from flat plates interacting with vortical structures. We consider two fundamental problems: (1) A linear cascade of flat plates excited by a vortical wave (gust) given by a 2D Fourier mode, and (2) The parallel interaction of a finite-core vortex with a single plate. We solve the nonlinear Euler equations by a high-order finite-differece method. We use nonreflecting boundary conditions at the inflow and outflow boundaries. For the gust problem, we found that the cascade response depends sensitively on the frequency of the convicted gust. The unsteady surface pressure distribution and radiated pressure field agree very well with predictions of the linear theory for the tested range of reduced frequency. We have also investigated the effects of the incident gust frequency on the undesirable wave reflection at the inflow and outflow boundaries. For the vortex-plate interaction problem, we investigate the effects of the internal structure of the vortex on the strength and directivity of radiated sound.
Then we solved the turbulence cascade interaction problem. The normal Reynolds stresses and velocity spectra are analyzed ahead, within, and downstream of the cascade. Good agreement with…
Advisors/Committee Members: Ragab, Saad A. (committeechair).
Subjects/Keywords: large eddy simulation; acoustic radiation; cascade interaction; homogeneous turbulence; high order finite difference
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APA (6th Edition):
Salem Said, A. S. (2007). Large Eddy Simulation of Shear-Free Interaction of Homogeneous Turbulence with a Flat-Plate Cascade. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/28520
Chicago Manual of Style (16th Edition):
Salem Said, Abdel-Halim Saber. “Large Eddy Simulation of Shear-Free Interaction of Homogeneous Turbulence with a Flat-Plate Cascade.” 2007. Doctoral Dissertation, Virginia Tech. Accessed October 19, 2019.
http://hdl.handle.net/10919/28520.
MLA Handbook (7th Edition):
Salem Said, Abdel-Halim Saber. “Large Eddy Simulation of Shear-Free Interaction of Homogeneous Turbulence with a Flat-Plate Cascade.” 2007. Web. 19 Oct 2019.
Vancouver:
Salem Said AS. Large Eddy Simulation of Shear-Free Interaction of Homogeneous Turbulence with a Flat-Plate Cascade. [Internet] [Doctoral dissertation]. Virginia Tech; 2007. [cited 2019 Oct 19].
Available from: http://hdl.handle.net/10919/28520.
Council of Science Editors:
Salem Said AS. Large Eddy Simulation of Shear-Free Interaction of Homogeneous Turbulence with a Flat-Plate Cascade. [Doctoral Dissertation]. Virginia Tech; 2007. Available from: http://hdl.handle.net/10919/28520
Virginia Tech
11.
Chang, Brian Lida.
Crossing the Air-Water Interface: Inspiration from Nature.
Degree: PhD, Engineering Science and Mechanics, 2018, Virginia Tech
URL: http://hdl.handle.net/10919/83445
► This dissertation aims to contribute toward the understanding of water-entry and -exit behaviors in nature. Since water is nearly a thousand times denser than air,…
(more)
▼ This dissertation aims to contribute toward the understanding of water-entry and -exit behaviors in nature. Since water is nearly a thousand times denser than air, transitioning between the two mediums is often associated with significant changes in force. Three topics with implications in water-entry are discussed, along with a fourth topic on water-exit. For a plunge-diving seabird, the first two stages of water-entry (initial impact and air-cavity formation) create large stresses on the bird's neck. Linear stability analysis of a cone-beam system impacting water shows buckling and non-buckling behaviors on the beam, which is extended to the diving birds. The next topic is related to the third stage of water-entry (air-cavity pinch-off), in which the chest feathers come in contact with the water. Here, the elasticity of Northern Gannet contour feathers is calculated using the nonlinear bending equation. The third topic will explore the formation of ripples along air cavity walls and their resulting force after pinch-off. An acoustic model predicts the observed wavelengths of the ripples. The final topic will delve into the mechanics of how animals leap out of water. A scaling law that balances the power of thrust and drag will predict the height of the jump. Finally, a bio-inspired robot was built to help identify physical conditions required to jump out of water.
Advisors/Committee Members: Jung, Sunghwan (committeechair), Socha, John (committee member), Abaid, Nicole Teresa (committee member), Ragab, Saad A. (committee member), Barone, Justin Robert (committee member).
Subjects/Keywords: Water-entry; water-exit; feather; seabird; plunge-dive; air-cavity; jumping; impact
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APA (6th Edition):
Chang, B. L. (2018). Crossing the Air-Water Interface: Inspiration from Nature. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/83445
Chicago Manual of Style (16th Edition):
Chang, Brian Lida. “Crossing the Air-Water Interface: Inspiration from Nature.” 2018. Doctoral Dissertation, Virginia Tech. Accessed October 19, 2019.
http://hdl.handle.net/10919/83445.
MLA Handbook (7th Edition):
Chang, Brian Lida. “Crossing the Air-Water Interface: Inspiration from Nature.” 2018. Web. 19 Oct 2019.
Vancouver:
Chang BL. Crossing the Air-Water Interface: Inspiration from Nature. [Internet] [Doctoral dissertation]. Virginia Tech; 2018. [cited 2019 Oct 19].
Available from: http://hdl.handle.net/10919/83445.
Council of Science Editors:
Chang BL. Crossing the Air-Water Interface: Inspiration from Nature. [Doctoral Dissertation]. Virginia Tech; 2018. Available from: http://hdl.handle.net/10919/83445
Virginia Tech
12.
Ahmed, Sara Mohamed.
Modeling of Power Electronics Distribution Systems with Low-frequency, Large-signal (LFLS) Models.
Degree: PhD, Electrical and Computer Engineering, 2011, Virginia Tech
URL: http://hdl.handle.net/10919/27787
► This work presents a modeling methodology that uses new types of models called low-frequency, large-signal models in a circuit simulator (Saber) to model a complex…
(more)
▼ This work presents a modeling methodology that uses new types of models called low-frequency, large-signal models in a circuit simulator (Saber) to model a complex hybrid ac/dc power electronics system. The new achievement in this work is being able to model the different components as circuit-based models and to capture some of the large-signal phenomena, for example, real transient behavior of the system such as startup, inrush current and power flow directionality. In addition, models are capable of predicting most low frequency harmonics only seen in real switching detailed models. Therefore the new models system can be used to predict steady state performance, harmonics, stability and transients. This work discusses the modeling issues faced based on the author recent experiences both on component level and system level. In addition, it recommends proper solutions to these issues verified with simulations.
This work also presents one of the new models in detail, a voltage source inverter (VSI), and explains how the model can be modified to capture low frequency harmonics that are usually phenomena modeled only with switching models. The process of implementing these different phenomena is discussed and the model is then validated by comparing the results of the proposed low frequency large signal (LFLS) model to a complete detailed switching model. In addition, experimental results are also obtained with a 2 kW voltage source inverter prototype to validate the proposed improved average model (LFLS model).
In addition, a complete Verification, Validation, and Uncertainty Quantification (VV&UQ) procedures is applied to a two-level boost rectifier. The goal of this validation process is the improvement of the modeling procedure for power electronics systems, and the full assessment of the boost rectifier model predictive capabilities.
Finally, the performance of the new models system is compared with the detailed switching models system. The LFLS models result in huge cut in simulation time (about 10 times difference) and also the ability to use large time step with the LFLS system and still capture all the information needed. Even though this low frequency large signal (LFLS) models system has wider capabilities than ideal average models system, it still canâ t predict all switching phenomena. Therefore, another benefit of this modeling approach is the ability to mix different types of models (low frequency large signal (LFLS) and detailed switching) based on the application study they are used for.
Advisors/Committee Members: Boroyevich, Dushan (committeechair), Lindner, Douglas K. (committee member), Mattavelli, Paolo (committee member), Ragab, Saad A. (committee member), Wang, Fei Fred (committee member).
Subjects/Keywords: Low frequency large signal models; impedance; small-signal model; Modeling; average models
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APA (6th Edition):
Ahmed, S. M. (2011). Modeling of Power Electronics Distribution Systems with Low-frequency, Large-signal (LFLS) Models. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/27787
Chicago Manual of Style (16th Edition):
Ahmed, Sara Mohamed. “Modeling of Power Electronics Distribution Systems with Low-frequency, Large-signal (LFLS) Models.” 2011. Doctoral Dissertation, Virginia Tech. Accessed October 19, 2019.
http://hdl.handle.net/10919/27787.
MLA Handbook (7th Edition):
Ahmed, Sara Mohamed. “Modeling of Power Electronics Distribution Systems with Low-frequency, Large-signal (LFLS) Models.” 2011. Web. 19 Oct 2019.
Vancouver:
Ahmed SM. Modeling of Power Electronics Distribution Systems with Low-frequency, Large-signal (LFLS) Models. [Internet] [Doctoral dissertation]. Virginia Tech; 2011. [cited 2019 Oct 19].
Available from: http://hdl.handle.net/10919/27787.
Council of Science Editors:
Ahmed SM. Modeling of Power Electronics Distribution Systems with Low-frequency, Large-signal (LFLS) Models. [Doctoral Dissertation]. Virginia Tech; 2011. Available from: http://hdl.handle.net/10919/27787
Virginia Tech
13.
San, Omer.
Multiscale Modeling and Simulation of Turbulent Geophysical Flows.
Degree: PhD, Engineering Science and Mechanics, 2012, Virginia Tech
URL: http://hdl.handle.net/10919/28031
► The accurate and efficient numerical simulation of geophysical flows is of great interest in numerical weather prediction and climate modeling as well as in numerous…
(more)
▼ The accurate and efficient numerical simulation of geophysical flows is of great interest in numerical weather prediction and climate modeling as well as in numerous critical areas and industries, such as agriculture, construction, tourism, transportation, weather-related disaster management, and sustainable energy technologies. Oceanic and atmospheric flows display an enormous range of temporal and spatial scales, from seconds to decades and from centimeters to thousands of kilometers, respectively. Scale interactions, both spatial and temporal, are the dominant feature of all aspects of general circulation models in geophysical fluid dynamics. In this thesis, to decrease the cost for these geophysical flow computations, several types of multiscale methods were systematically developed and tested for a variety of physical settings including barotropic and stratified wind-driven large scale ocean circulation models, decaying and forced two-dimensional turbulence simulations, as well as several benchmark incompressible flow problems in two and three dimensions. The new models proposed here are based on two classes of modern multiscale methods: (i) interpolation based approaches in the context of the multigrid/multiresolution methodologies, and (ii) deconvolution based spatial filtering approaches in the context of large eddy simulation techniques. In the first case, we developed a coarse-grid projection method that uses simple interpolation schemes to go between the two components of the problem, in which the solution algorithms have different levels of complexity. In the second case, the use of approximate deconvolution closure modeling strategies was implemented for large eddy simulations of large-scale turbulent geophysical flows. The numerical assessment of these approaches showed that both the coarse-grid projection and approximate deconvolution methods could represent viable tools for computing more realistic turbulent geophysical flows that provide significant increases in accuracy and computational efficiency over conventional methods.
Advisors/Committee Members: Staples, Anne E. (committeechair), Stremler, Mark A. (committee member), De Vita, Raffaella (committee member), Iliescu, Traian (committee member), Ragab, Saad A. (committee member).
Subjects/Keywords: Geophysical Flows; Physical Oceanography; Multiscale Modeling; Multigrid; Large Eddy Simulation; Computational Fluid Dynamics
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APA (6th Edition):
San, O. (2012). Multiscale Modeling and Simulation of Turbulent Geophysical Flows. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/28031
Chicago Manual of Style (16th Edition):
San, Omer. “Multiscale Modeling and Simulation of Turbulent Geophysical Flows.” 2012. Doctoral Dissertation, Virginia Tech. Accessed October 19, 2019.
http://hdl.handle.net/10919/28031.
MLA Handbook (7th Edition):
San, Omer. “Multiscale Modeling and Simulation of Turbulent Geophysical Flows.” 2012. Web. 19 Oct 2019.
Vancouver:
San O. Multiscale Modeling and Simulation of Turbulent Geophysical Flows. [Internet] [Doctoral dissertation]. Virginia Tech; 2012. [cited 2019 Oct 19].
Available from: http://hdl.handle.net/10919/28031.
Council of Science Editors:
San O. Multiscale Modeling and Simulation of Turbulent Geophysical Flows. [Doctoral Dissertation]. Virginia Tech; 2012. Available from: http://hdl.handle.net/10919/28031
Virginia Tech
14.
Abdelkefi, Abdessattar.
Global Nonlinear Analysis of Piezoelectric Energy Harvesting from Ambient and Aeroelastic Vibrations.
Degree: PhD, Engineering Science and Mechanics, 2012, Virginia Tech
URL: http://hdl.handle.net/10919/28761
► Converting vibrations to a usable form of energy has been the topic of many recent investigations. The ultimate goal is to convert ambient or aeroelastic…
(more)
▼ Converting vibrations to a usable form of energy has been the topic of many recent investigations.
The ultimate goal is to convert ambient or aeroelastic vibrations to operate low-power consumption devices, such as microelectromechanical systems, heath monitoring
sensors, wireless sensors or replacing small batteries that have a nite life span or would require hard and expensive maintenance. The transduction mechanisms used for transforming
vibrations to electric power include: electromagnetic, electrostatic, and piezoelectric mechanisms. Because it can be used to harvest energy over a wide range of frequencies and because of its ease of application, the piezoelectric option has attracted significant interest.
In this work, we investigate the performance of different types of piezoelectric energy harvesters. The objective is to design and enhance the performance of these harvesters. To this end, distributed-parameter and phenomenological models of these harvesters are developed. Global analysis of these models is then performed using modern methods of nonlinear dynamics. In the first part of this Dissertation, global nonlinear distributed-parameter models
for piezoelectric energy harvesters under direct and parametric excitations are developed.
The method of multiple scales is then used to derive nonlinear forms of the governing equations
and associated boundary conditions, which are used to evaluate their performance and
determine the effects of the nonlinear piezoelectric coefficients on their behavior in terms of softening or hardening.
In the second part, we assess the influence of the linear and nonlinear parameters on the dynamic behavior of a wing-based piezoaeroelastic energy harvester. The system is composed of a rigid airfoil that is constrained to pitch and plunge and supported by linear and nonlinear torsional and flexural springs with a piezoelectric coupling attached to the plunge
degree of freedom. Linear analysis is performed to determine the effects of the linear spring
coefficients and electrical load resistance on the flutter speed. Then, the normal form of the Hopf bifurcation (flutter) is derived to characterize the type of instability and determine the effects of the aerodynamic nonlinearities and the nonlinear coefficients of the springs on the system's stability near the bifurcation. This is useful to characterize the effects of different parameters on the system's output and ensure that subcritical or "catastrophic" bifurcation does not take place. Both linear and nonlinear analyses are then used to design and enhance the performance of these harvesters.
In the last part, the concept of energy harvesting from vortex-induced vibrations of a
circular cylinder is investigated. The power levels that can be generated from these vibrations
and the variations of these levels with the freestream velocity are determined. A mathematical
model that accounts for the coupled lift force, cylinder motion and generated voltage is
presented. Linear analysis of the electromechanical model is performed to…
Advisors/Committee Members: Paul, Mark R. (committee member), Ragab, Saad A. (committee member), Woolsey, Craig A. (committee member), Nayfeh, Ali H. (committeecochair), Hajj, Muhammad R. (committeecochair).
Subjects/Keywords: Energy harvesting; piezoelectricity; aeroelasticity; electromechanical modeling; nonlinear dynamics; vortex-induced vibrations
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APA (6th Edition):
Abdelkefi, A. (2012). Global Nonlinear Analysis of Piezoelectric Energy Harvesting from Ambient and Aeroelastic Vibrations. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/28761
Chicago Manual of Style (16th Edition):
Abdelkefi, Abdessattar. “Global Nonlinear Analysis of Piezoelectric Energy Harvesting from Ambient and Aeroelastic Vibrations.” 2012. Doctoral Dissertation, Virginia Tech. Accessed October 19, 2019.
http://hdl.handle.net/10919/28761.
MLA Handbook (7th Edition):
Abdelkefi, Abdessattar. “Global Nonlinear Analysis of Piezoelectric Energy Harvesting from Ambient and Aeroelastic Vibrations.” 2012. Web. 19 Oct 2019.
Vancouver:
Abdelkefi A. Global Nonlinear Analysis of Piezoelectric Energy Harvesting from Ambient and Aeroelastic Vibrations. [Internet] [Doctoral dissertation]. Virginia Tech; 2012. [cited 2019 Oct 19].
Available from: http://hdl.handle.net/10919/28761.
Council of Science Editors:
Abdelkefi A. Global Nonlinear Analysis of Piezoelectric Energy Harvesting from Ambient and Aeroelastic Vibrations. [Doctoral Dissertation]. Virginia Tech; 2012. Available from: http://hdl.handle.net/10919/28761
Virginia Tech
15.
Zakaria, Mohamed Yehia.
Unsteady Nonlinear Aerodynamic Modeling and Applications.
Degree: PhD, Engineering Science and Mechanics, 2016, Virginia Tech
URL: http://hdl.handle.net/10919/79909
► Unsteady aerodynamic modeling is indispensable in the design process of rotary air vehicles, flapping flight and agile unmanned aerial vehicles. Undesirable vibrations can cause high-frequency…
(more)
▼ Unsteady aerodynamic modeling is indispensable in the design process of rotary air vehicles, flapping flight and agile unmanned aerial vehicles. Undesirable vibrations can cause high-frequency variations in motion variables whose effects cannot be well predicted using quasi-steady aerodynamics. Furthermore, one may exploit the lift enhancement that can be generated through an unsteady motion for optimum design of flapping vehicles. Additionally, undesirable phenomena like the flutter of fixed wings and ensuing limit cycle oscillations can be exploited for harvesting energy. In this dissertation, we focus on modeling the unsteady nonlinear aerodynamic response and present various applications where unsteady aerodynamics are very relevant.
The dissertation starts with experiments for measuring unsteady loads on an NACA-0012 airfoil undergoing a plunging motion under various operating conditions. We supplement these measurements with flow visualization to obtain better insight into phenomena causing enhanced lift. For the model, we present the frequency response function for the airfoil at various angles of attack. Experiments were performed at reduced frequencies between 0.1 and 0.95 and angles of attack up to 65 degrees. Then, we formulate an optimization problem to unify the transfer function coefficients for each regime independently to obtain one model that represents the global dynamics. An optimization-based finite-dimensional (fourth-order) approximation for the frequency responses is developed. Converting these models to state-space form and writing the entries of the matrices as polynomials in the mean angle of attack, a unified unsteady model was developed. In the second set of experiments, we measured the unsteady plunging forces on the same airfoil at zero forward velocity. The aim is to investigate variations of the added forces associated with the oscillation frequency of the wing section for various angles of attack. Data of the measured forces are presented and compared with predicted forces from potential flow approximations. The results show a significant departure from those estimates, especially at high frequencies indicating that viscous effects play a major role in determining these forces.
In the second part of this dissertation, we consider different applications where unsteady loads and nonlinear effects play an important role. We perform a multi-objective aerodynamic optimization problem of the wing kinematics and planform shape of a Pterosaur replica ornithopter. The objective functions included minimization of the required cycle-averaged aerodynamic power and maximization of the propulsive efficiency. The results show that there is an optimum kinematic parameter as well as planform shape to fulfill the two objectives. Furthermore, the effects of preset angle of attack, wind speed and load resistance on the levels of harvested power from a composite beam bonded with the piezoelectric patch are determined experimentally. The results point to a complex relation between the aerodynamic…
Advisors/Committee Members: Hajj, Muhammad R. (committeechair), Patil, Mayuresh J. (committee member), Ragab, Saad A. (committee member), Canfield, Robert Arthur (committee member).
Subjects/Keywords: Unsteady nonlinear aerodynamics; Wind tunnel testing; Flow Visualization; High angles of attack; Energy Harvesting
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APA (6th Edition):
Zakaria, M. Y. (2016). Unsteady Nonlinear Aerodynamic Modeling and Applications. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/79909
Chicago Manual of Style (16th Edition):
Zakaria, Mohamed Yehia. “Unsteady Nonlinear Aerodynamic Modeling and Applications.” 2016. Doctoral Dissertation, Virginia Tech. Accessed October 19, 2019.
http://hdl.handle.net/10919/79909.
MLA Handbook (7th Edition):
Zakaria, Mohamed Yehia. “Unsteady Nonlinear Aerodynamic Modeling and Applications.” 2016. Web. 19 Oct 2019.
Vancouver:
Zakaria MY. Unsteady Nonlinear Aerodynamic Modeling and Applications. [Internet] [Doctoral dissertation]. Virginia Tech; 2016. [cited 2019 Oct 19].
Available from: http://hdl.handle.net/10919/79909.
Council of Science Editors:
Zakaria MY. Unsteady Nonlinear Aerodynamic Modeling and Applications. [Doctoral Dissertation]. Virginia Tech; 2016. Available from: http://hdl.handle.net/10919/79909
Virginia Tech
16.
Qin, Tong.
Numerical Simulations of Interactions of Solid Particles and Deformable Gas Bubbles in Viscous Liquids.
Degree: PhD, Engineering Science and Mechanics, 2013, Virginia Tech
URL: http://hdl.handle.net/10919/19225
► Studying the interactions of solid particles and deformable gas bubbles in viscous liquids is very important in many applications, especially in mining and chemical industries.…
(more)
▼ Studying the interactions of solid particles and deformable gas bubbles in viscous liquids is very important in many applications, especially in mining and chemical industries. These interactions involve liquid-solid-air multiphase flows and an arbitrary-Lagrangian-Eulerican (ALE) approach is used for the direct numerical simulations. In the system of rigid particles and deformable gas bubbles suspended in viscous liquids, the Navier-Stokes equations coupled with the equations of motion of the particles and deformable bubbles are solved in a finite-element framework. A moving, unstructured, triangular mesh tracks the deformation of the bubble and free surface with adaptive refinement. In this dissertation, we study four problems. In the first three problems the flow is assumed to be axisymmetric and two dimensional (2D) in the fourth problem. Firstly, we study the interaction between a rising deformable bubble and a solid wall in highly viscous liquids. The mechanism of the bubble deformation as it interacts with the wall is described in terms of two nondimensional groups, namely the Morton number (Mo) and Bond number (Bo). The film drainage process is also considered. It is found that three modes of bubble-rigid wall interaction exist as Bo changes at a moderate Mo. The first mode prevails at small Bo where the bubble deformation is small. For this mode, the bubble is hard to break up and will bounce back and eventually attach to the rigid wall. In the second mode, the bubble may break up after it collides with the rigid wall, which is determined by the film drainage. In the third mode, which prevails at high Bo, the bubble breaks up due to the bottom surface catches up the top surface during the interaction. Secondly, we simulate the interaction between a rigid particle and a free surface. In order to isolate the effects of viscous drag and particle inertia, the gravitational force is neglected and the particle gains its impact velocity by an external accelerating force. The process of a rigid particle impacting a free surface and then rebounding is simulated. Simplified theoretical models are provided to illustrate the relationship between the particle velocity and the time variation of film thickness between the particle and free surface. Two film thicknesses are defined. The first is the thickness achieved when the particle reaches its highest position. The second is the thickness when the particle falls to its lowest position. The smaller of these two thicknesses is termed the minimum film thickness and its variation with the impact velocity has been determined. We find that the interactions between the free surface and rigid particle can be divided into three regimes according to the trend of the first film thickness. The three regimes are viscous regime, inertial regime and…
Advisors/Committee Members: Ragab, Saad A. (committeechair), Yue, Pengtao (committee member), Hajj, Muhammad R. (committee member), De Vita, Raffaella (committee member), Stremler, Mark A. (committee member).
Subjects/Keywords: Multiphase flow; Bubble-wall interaction; Particle- free surface interaction; Particle-bubble interaction; Film drainage; Bubble
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APA (6th Edition):
Qin, T. (2013). Numerical Simulations of Interactions of Solid Particles and Deformable Gas Bubbles in Viscous Liquids. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/19225
Chicago Manual of Style (16th Edition):
Qin, Tong. “Numerical Simulations of Interactions of Solid Particles and Deformable Gas Bubbles in Viscous Liquids.” 2013. Doctoral Dissertation, Virginia Tech. Accessed October 19, 2019.
http://hdl.handle.net/10919/19225.
MLA Handbook (7th Edition):
Qin, Tong. “Numerical Simulations of Interactions of Solid Particles and Deformable Gas Bubbles in Viscous Liquids.” 2013. Web. 19 Oct 2019.
Vancouver:
Qin T. Numerical Simulations of Interactions of Solid Particles and Deformable Gas Bubbles in Viscous Liquids. [Internet] [Doctoral dissertation]. Virginia Tech; 2013. [cited 2019 Oct 19].
Available from: http://hdl.handle.net/10919/19225.
Council of Science Editors:
Qin T. Numerical Simulations of Interactions of Solid Particles and Deformable Gas Bubbles in Viscous Liquids. [Doctoral Dissertation]. Virginia Tech; 2013. Available from: http://hdl.handle.net/10919/19225
Virginia Tech
17.
Valyrakis, Manousos.
Initiation of Particle Movement in Turbulent Open Channel Flow.
Degree: PhD, Civil Engineering, 2011, Virginia Tech
URL: http://hdl.handle.net/10919/27289
► The objective of this thesis is to investigate the flow conditions that lead to coarse grain entrainment at near incipient motion conditions. Herein, a new…
(more)
▼ The objective of this thesis is to investigate the flow conditions that lead to coarse grain entrainment at near incipient motion conditions. Herein, a new conceptual approach is proposed, which in addition to the magnitude of hydrodynamic force or flow power, takes into account the duration of the flow event. Two criteria for inception of grain entrainment, namely the critical impulse and critical energy concepts, are proposed and compared. These frameworks adopt a force or energy perspective, considering the momentum or energy transfer from each flow event to the particle respectively, to describe the phenomenon.
A series of conducted mobile particle experiments, are analyzed to examine the validity of the proposed approaches. First a set of bench-top experiments incorporates an electromagnet which applies pulses of known magnitude and duration to a steel spherical particle in a controlled fashion, so as to identify the critical level for entrainment. The utility of the above criteria is also demonstrated for the case of entrainment by the action of turbulent flow, via analysis of a series of flume experiments, where both the history of hydrodynamic forces exerted on the particle as well as its response are recorded simultaneously.
Statistical modeling of the distribution of impulses, as well as conditional excess impulses, is performed using distributions from Extreme Value Theory to effectively model the episodic nature of the occurrence of these events. For the examined uniform and low mobility flow conditions, a power law relationship is proposed for describing the magnitude and frequency of occurrence of the impulse events. The Weibull and exponential distributions provide a good fit for the time between particle entrainments. In addition to these statistical tools, a number of Adaptive Neuro-Fuzzy Inference Systems employing different input representations are used to learn the nonlinear dynamics of the system and perform statistical prediction. The performance of these models is assessed in terms of their broad validity, efficiency and forecast accuracy.
Even though the impulse and energy criteria are deeply interrelated, the latter is shown to be advantageous with regard to its performance, applicability and extension ability. The effect of single or multiple highly energetic events carried by certain coherent flow structures (mainly strong sweep events) with regard to the particle response is also investigated.
Advisors/Committee Members: Diplas, Panayiotis (committeechair), Schetz, Joseph A. (committee member), Ragab, Saad A. (committee member), Little, John C. (committee member), Dancey, Clinton L. (committeecochair).
Subjects/Keywords: extreme value theory; impulse and energy event based criteria; wavelet analysis; onset of particle entrainment; neuro-fuzzy inference systems; coherent flow structures
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APA (6th Edition):
Valyrakis, M. (2011). Initiation of Particle Movement in Turbulent Open Channel Flow. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/27289
Chicago Manual of Style (16th Edition):
Valyrakis, Manousos. “Initiation of Particle Movement in Turbulent Open Channel Flow.” 2011. Doctoral Dissertation, Virginia Tech. Accessed October 19, 2019.
http://hdl.handle.net/10919/27289.
MLA Handbook (7th Edition):
Valyrakis, Manousos. “Initiation of Particle Movement in Turbulent Open Channel Flow.” 2011. Web. 19 Oct 2019.
Vancouver:
Valyrakis M. Initiation of Particle Movement in Turbulent Open Channel Flow. [Internet] [Doctoral dissertation]. Virginia Tech; 2011. [cited 2019 Oct 19].
Available from: http://hdl.handle.net/10919/27289.
Council of Science Editors:
Valyrakis M. Initiation of Particle Movement in Turbulent Open Channel Flow. [Doctoral Dissertation]. Virginia Tech; 2011. Available from: http://hdl.handle.net/10919/27289
Virginia Tech
18.
Park, Seungwoo.
Modeling Bubble Coarsening in Froth Phase from First Principles.
Degree: PhD, Mining and Minerals Engineering, 2015, Virginia Tech
URL: http://hdl.handle.net/10919/73351
► Between two neighboring air bubbles in a froth (or foam), a thin liquid film (TLF) is formed. As the bubbles rise upwards, the TLFs thin…
(more)
▼ Between two neighboring air bubbles in a froth (or foam), a thin liquid film (TLF) is formed. As the bubbles rise upwards, the TLFs thin initially due to the capillary pressure created by curvature changes. As the film thicknesses (H) reach approximately 200 nm, the disjoining pressure created by surface forces in the films also begins to control the film drainage rate and affect the waves motions at the air/water interfaces. If the disjoining pressure is negative, both the film drainage and the capillary wave motion accelerate. When the TLF thins to a critical film thickness (Hcr), the amplitude of the wave motion grows suddenly and the two air/water interfaces touch each other, causing the TLF to rupture and bubbles to coalesce.
In the present work, a new model that can predict Hcr has been developed by considering the film drainage due to both viscous film thinning and capillary wave motion. Based on the Hcr model, bubble-coarsening in a dynamic foam has been predicted by deriving the geometric relation between the thickness of the lamella film, which controls bubble-coalescence rate, and the Plateau border area, which controls liquid drainage rate.
Furthermore, a model for predicting bubble-coarsening in froth (3-phase foam) has been developed by developing a film drainage model quantifying the effect of particles on pc. The parameter pc is affected by the number of particles and the local capillary pressure around particles, which in turn vary with the hydrophobicity and size of the particles in the film. Assuming that films rupture at free films, the pc corrected for the particles in lamella films has been used to determine the critical rupture time (tcr), at which the film thickness reaches Hcr, using the Reynolds equation. Assuming that the number of bubbles decrease exponentially with froth height, and knowing that bubbles coalesce when film drains to a thickness Hcr, a bubble coarsening model has been developed. The model predictions are in agreement with the experimental data obtained using particle of varying hydrophobicity and size.
Advisors/Committee Members: Yoon, Roe-Hoan (committeechair), Luttrell, Gerald H. (committee member), Adel, Gregory T. (committee member), Jung, Sunghwan (committee member), Ragab, Saad A. (committee member).
Subjects/Keywords: Bubble-Coarsening; Froth Stability; Thin Liquid Film; Hydrophobic Force
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APA (6th Edition):
Park, S. (2015). Modeling Bubble Coarsening in Froth Phase from First Principles. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/73351
Chicago Manual of Style (16th Edition):
Park, Seungwoo. “Modeling Bubble Coarsening in Froth Phase from First Principles.” 2015. Doctoral Dissertation, Virginia Tech. Accessed October 19, 2019.
http://hdl.handle.net/10919/73351.
MLA Handbook (7th Edition):
Park, Seungwoo. “Modeling Bubble Coarsening in Froth Phase from First Principles.” 2015. Web. 19 Oct 2019.
Vancouver:
Park S. Modeling Bubble Coarsening in Froth Phase from First Principles. [Internet] [Doctoral dissertation]. Virginia Tech; 2015. [cited 2019 Oct 19].
Available from: http://hdl.handle.net/10919/73351.
Council of Science Editors:
Park S. Modeling Bubble Coarsening in Froth Phase from First Principles. [Doctoral Dissertation]. Virginia Tech; 2015. Available from: http://hdl.handle.net/10919/73351
19.
Tan, Ting.
Mechanical Characterization of Swine Uterosacral and Cardinal Ligaments.
Degree: PhD, Engineering Science and Mechanics, 2015, Virginia Tech
URL: http://hdl.handle.net/10919/77863
► The uterosacral ligament (USL) and cardinal ligament (CL) are the two major suspensory tissues of the uterus, cervix, and vagina. These supportive structures can be…
(more)
▼ The uterosacral ligament (USL) and cardinal ligament (CL) are the two major suspensory tissues of the uterus, cervix, and vagina. These supportive structures can be weakened or damaged, leading to the development of pelvic floor disorders (PFDs) such as urinary incontinence, fecal incontinence, and pelvic organ prolapse. In the surgical treatment for PFDs, the USL and CL are extensively used as anchor structures to restore the normal position of the prolapsed organs. Therefore, the mechanical properties of the USL and CL may be critical for the development of new surgical reconstruction strategies for PFDs.
In chapter 1, we present the first histo-mechanical characterization of the swine USL and CL using histological analysis, scanning electron microscopy and quasi-static uniaxial tensile tests. Our results suggest that the histological and uniaxial tensile properties of the swine CL and USL are very similar to those in humans. The swine is found to be a suitable animal model for studying the mechanical properties of these ligaments.
To capture both the active and passive mechanical responses of biological tissues containing SMCs such as the USL and CL, a new structural constitutive model is proposed in chapter 2. The deformation of the active component in such tissues during isometric and isotonic contractions is described using an evolution law. This model is tested with published active and passive, uniaxial and biaxial, experimental data on pig arteries due to lack of data on the active properties of the USL and CL.
Subjected to constant forces in-vivo, the structure and length of the USL and CL are sig- nificantly altered over time. In chapter 3, we present the first rigorous characterization of the fiber microstructure and creep properties of the USL/CL complex by using scanning electron microscopy and planar biaxial testing. Fibers are found to be oriented primarily along the main in-vivo loading direction. In such direction, the creep proceeds significantly faster under lower load.
Overall, our experimental findings advance our knowledge about the passive elastic and viscoelastic properties of the USL/CL complex. The novel structural constitutive model proposed enhances our understanding of the active mechanical behavior of biological tissues containing SMCs. Knowledge about the mechanical behavior of the USL and CL from experimental and theoretical studies such as those presented here will help to improve, in the long term, the medical treatment for PFDs.
Advisors/Committee Members: De Vita, Raffaella (committeechair), Ragab, Saad A. (committee member), Case, Scott W. (committee member), Leonessa, Alexander (committee member).
Subjects/Keywords: Uterosacral Ligament; Cardinal Ligament; Scanning Electron Microscopy; Histology; Uniaxial and Biaxial Tests; Nonlinear Elasticity; Viscoelasticity; Isometric Contraction; Isotonic Contraction; Constitutive Modeling
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APA (6th Edition):
Tan, T. (2015). Mechanical Characterization of Swine Uterosacral and Cardinal Ligaments. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/77863
Chicago Manual of Style (16th Edition):
Tan, Ting. “Mechanical Characterization of Swine Uterosacral and Cardinal Ligaments.” 2015. Doctoral Dissertation, Virginia Tech. Accessed October 19, 2019.
http://hdl.handle.net/10919/77863.
MLA Handbook (7th Edition):
Tan, Ting. “Mechanical Characterization of Swine Uterosacral and Cardinal Ligaments.” 2015. Web. 19 Oct 2019.
Vancouver:
Tan T. Mechanical Characterization of Swine Uterosacral and Cardinal Ligaments. [Internet] [Doctoral dissertation]. Virginia Tech; 2015. [cited 2019 Oct 19].
Available from: http://hdl.handle.net/10919/77863.
Council of Science Editors:
Tan T. Mechanical Characterization of Swine Uterosacral and Cardinal Ligaments. [Doctoral Dissertation]. Virginia Tech; 2015. Available from: http://hdl.handle.net/10919/77863
Virginia Tech
20.
Ahmad, Naveed.
Passive Damping in Stiffened Structures Using Viscoelastic Polymers.
Degree: PhD, Engineering Science and Mechanics, 2016, Virginia Tech
URL: http://hdl.handle.net/10919/79566
► Noise and vibration suppression is an important aspect in the design process of structures and machines. Undesirable vibrations can cause fatigue in a structure and…
(more)
▼ Noise and vibration suppression is an important aspect in the design process of structures and machines. Undesirable vibrations can cause fatigue in a structure and are, therefore, a risk to the safety of a structure. One of the most effective and widely used methods of mitigating these unwanted vibrations from a system is passive damping, by using a viscoelastic material. This dissertation will primarily focus on constrained layer passive damping treatments in structures and the investigation of associated complex modes. The key idea behind constrained damping treatment is to increase damping as affected by the presence of a highly damped core layer vibrating mainly in shear. Our main goal was to incorporate viscoelastic material in a thick stiffened panel with plate-strip stiffeners, to enhance the damping characteristics of the structure.
First, we investigated complex damped modes in beams in the presence of a viscoelastic layer sandwiched between two elastic layers. The problem was solved using two approaches, (1) Rayleigh beam theory and analyzed using the principle of virtual work, and (2) by using 2D plane stress elasticity based finite-element method. The damping in the viscoelastic material was modeled using the complex modulus approach. We used FEM without any kinematic assumptions for the transverse shear in both the core and elastic layers. Moreover, numerical examples were studied, by including complex modulus in the base and constraining layers. The loss factor was calculated by modal strain energy method, and by solving a complex eigenvalue problem. The efficiency of the modal strain energy method was tested for different loss factors in the core layer. Complex mode shapes of the beam were also examined in the study, and a comparison was made between viscoelastically damped and non-proportionally damped structures.
Secondly, we studied the free vibration response of an integrally stiffened and/or stepped plate. The stiffeners used here were plate-strip stiffeners, unlike the rib stiffeners often investigated by researchers. Both plate and stiffeners were analyzed using the first-order shear deformation theory. The deflections and rotations were assumed as a product of Timoshenko beam functions, chosen appropriately according to the given boundary conditions. Unlike Navier and Levy solution techniques, the approach used here can also be applied to fully clamped, free and cantilever supported stiffened plates. The governing differential equations were solved using the Rayleigh-Ritz method. The development of the stiffness and the mass matrices in the Ritz analysis was found to consume a huge amount of CPU time due to the recursive integration of Timoshenko beam functions. An approach is suggested to greatly decrease this amount of CPU time, by replacing the recursive integration in a loop structure in the computer program, with the analytical integration of the integrand in the loop. The numerical results were compared with the exact solutions available in the literature and the commercially available…
Advisors/Committee Members: Kapania, Rakesh K. (committeechair), Rojiani, Kamal (committee member), Ragab, Saad A. (committee member), Hajj, Muhammad R. (committee member), Thangjitham, Surot (committee member).
Subjects/Keywords: Passive Damping; Finite Element; Viscoelasticity; Modal Strain Energy; Free Vibration; Constrained Layer damping; Integrally Stiffened Plates; Stepped Plates; Adhesively Bonded Plates
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APA ·
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APA (6th Edition):
Ahmad, N. (2016). Passive Damping in Stiffened Structures Using Viscoelastic Polymers. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/79566
Chicago Manual of Style (16th Edition):
Ahmad, Naveed. “Passive Damping in Stiffened Structures Using Viscoelastic Polymers.” 2016. Doctoral Dissertation, Virginia Tech. Accessed October 19, 2019.
http://hdl.handle.net/10919/79566.
MLA Handbook (7th Edition):
Ahmad, Naveed. “Passive Damping in Stiffened Structures Using Viscoelastic Polymers.” 2016. Web. 19 Oct 2019.
Vancouver:
Ahmad N. Passive Damping in Stiffened Structures Using Viscoelastic Polymers. [Internet] [Doctoral dissertation]. Virginia Tech; 2016. [cited 2019 Oct 19].
Available from: http://hdl.handle.net/10919/79566.
Council of Science Editors:
Ahmad N. Passive Damping in Stiffened Structures Using Viscoelastic Polymers. [Doctoral Dissertation]. Virginia Tech; 2016. Available from: http://hdl.handle.net/10919/79566
Virginia Tech
21.
Al Haik, Mohammad Yousef.
Nanoparticle-based Organic Energy Storage with Harvesting Systems.
Degree: PhD, Engineering Science and Mechanics, 2016, Virginia Tech
URL: http://hdl.handle.net/10919/79815
► A new form of organic energy storage devices (organic capacitors) is presented in the first part of this dissertation. The storage devices are made out…
(more)
▼ A new form of organic energy storage devices (organic capacitors) is presented in the first part of this dissertation. The storage devices are made out of an organic semiconductor material and charge storage elements from synthesized nanoparticles. The semiconducting polymer is obtained by blending poly (vinyl alcohol) and poly (acrylic acid) in crystal state polymers with a known plasticizer; glycerol or sorbitol. Synthesized nanoparticles namely, zinc-oxide (ZnO), erbium (Er), cadmium sulfide (CdS), palladium (Pd) and silver-platinum (AgPt) were used as charge storage elements in fabrication of metal-insulator-semiconductor (MIS) structure. The organic semiconductor and synthesized nanoparticles are tested to evaluate and characterize their electrical performance and properties. Fabrication of the organic capacitors consisted of layer-by-layer deposition and thermal evaporation of the electrode terminals. Capacitance versus voltage (C-V) measurement tests were carried out to observe hysteresis loops with a window gate that would indicate the charging, discharging and storage characteristics. Experimental investigation of various integrated energy harvesting techniques combined with these organic based novel energy storage devices are performed in the second part of this dissertation. The source of the energy is the wind and is harvested by means of miniature wind turbines and vibrations, using piezoelectric transduction. In both cases, the generated electric charge is stored in these capacitors. The performance of the organic capacitors are evaluated through their comparison with commercial capacitors. The results show that the voltage produced from the two energy harvesters was high enough to store the harvested energy in the organic capacitors. The charge and energy levels of the organic capacitors are also reported.
The third part of this dissertation focuses on harvesting energy from a self-induced flutter of a thin composite beam. The composite beam consisted of an MFC patch bonded near the clamped end and placed vertically in the center of a wind tunnel test section. The self sustaining energy harvesting from the unimorph composite beam is exploited. The effects of different operational parameters including the optimum angle of attack, wind speed and load resistance are determined.
Advisors/Committee Members: Hajj, Muhammad R. (committeechair), Case, Scott W. (committee member), Philen, Michael Keith (committee member), Ragab, Saad A. (committee member).
Subjects/Keywords: Energy Storage Devices; Energy Harvesting; Organic Semiconducting Polymers; Nanoparticles; Composites; Fluttering
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APA (6th Edition):
Al Haik, M. Y. (2016). Nanoparticle-based Organic Energy Storage with Harvesting Systems. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/79815
Chicago Manual of Style (16th Edition):
Al Haik, Mohammad Yousef. “Nanoparticle-based Organic Energy Storage with Harvesting Systems.” 2016. Doctoral Dissertation, Virginia Tech. Accessed October 19, 2019.
http://hdl.handle.net/10919/79815.
MLA Handbook (7th Edition):
Al Haik, Mohammad Yousef. “Nanoparticle-based Organic Energy Storage with Harvesting Systems.” 2016. Web. 19 Oct 2019.
Vancouver:
Al Haik MY. Nanoparticle-based Organic Energy Storage with Harvesting Systems. [Internet] [Doctoral dissertation]. Virginia Tech; 2016. [cited 2019 Oct 19].
Available from: http://hdl.handle.net/10919/79815.
Council of Science Editors:
Al Haik MY. Nanoparticle-based Organic Energy Storage with Harvesting Systems. [Doctoral Dissertation]. Virginia Tech; 2016. Available from: http://hdl.handle.net/10919/79815
Virginia Tech
22.
Basu, Saikat.
Dynamics of vortices in complex wakes: modeling, analysis, and experiments.
Degree: PhD, Engineering Science and Mechanics, 2014, Virginia Tech
URL: http://hdl.handle.net/10919/51749
► The thesis develops singly-periodic mathematical models for complex laminar wakes which are formed behind vortex-shedding bluff bodies. These wake structures exhibit a variety of patterns…
(more)
▼ The thesis develops singly-periodic mathematical models for complex laminar wakes which are formed behind vortex-shedding bluff bodies. These wake structures exhibit a variety of patterns as the bodies oscillate or are in close proximity of one another. The most well-known formation comprises two counter-rotating vortices in each shedding cycle and is popularly known as the vk vortex street. Of the more complex configurations, as a specific example, this thesis investigates one of the most commonly occurring wake arrangements, which consists of two pairs of vortices in each shedding period. The paired vortices are, in general, counter-rotating and belong to a more general definition of the 2P mode, which involves periodic release of four vortices into the flow. The 2P arrangement can, primarily, be sub-classed into two types: one with a symmetric orientation of the two vortex pairs about the streamwise direction in a periodic domain and the other in which the two vortex pairs per period are placed in a staggered geometry about the wake centerline. The thesis explores the governing dynamics of such wakes and characterizes the corresponding relative vortex motion.
In general, for both the symmetric as well as the staggered four vortex periodic arrangements, the thesis develops two-dimensional potential flow models (consisting of an integrable Hamiltonian system of point vortices) that consider spatially periodic arrays of four vortices with their strengths being +/-1 and +/-2. Vortex formations observed in the experiments inspire the assumed spatial symmetry. The models demonstrate a number of dynamic modes that are classified using a bifurcation analysis of the phase space topology, consisting of level curves of the Hamiltonian. Despite the vortex strengths in each pair being unequal in magnitude, some initial conditions lead to relative equilibrium when the vortex configuration moves with invariant size and shape.
The scaled comparisons of the model results with experiments conducted in a flowing soap film with an airfoil, which was imparted with forced oscillations, are satisfactory and validate the reduced order modeling framework. The experiments have been performed by a collaborator group at the Department of Physics and Fluid Dynamics at the Technical University of Denmark (DTU), led by Dr. Anders Andersen. Similar experiments have also been run at
Virginia Tech as part of this dissertation and the preliminary results are included in this treatise.
The thesis also employs the same dynamical systems techniques, which have been applied to study the 2P regime dynamics, to develop a mathematical model for the P+S mode vortex wakes, with three vortices present in each shedding cycle. The model results have also been compared favorably with an experiment and the predictions regarding the vortex circulation data match well with the previous results from literature.
Finally, the thesis introduces a novel concept of clean and renewable energy extraction from vortex-induced vibrations of bluff bodies. The slow-moving…
Advisors/Committee Members: Stremler, Mark A. (committeechair), Jung, Sunghwan (committee member), Iliescu, Traian (committee member), Ross, Shane David (committee member), Ragab, Saad A. (committee member).
Subjects/Keywords: Vortex dynamics; Point vortices; Bluff body wake; Fluid-structure interactions; Vortex-induced vibrations
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APA ·
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Export
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APA (6th Edition):
Basu, S. (2014). Dynamics of vortices in complex wakes: modeling, analysis, and experiments. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/51749
Chicago Manual of Style (16th Edition):
Basu, Saikat. “Dynamics of vortices in complex wakes: modeling, analysis, and experiments.” 2014. Doctoral Dissertation, Virginia Tech. Accessed October 19, 2019.
http://hdl.handle.net/10919/51749.
MLA Handbook (7th Edition):
Basu, Saikat. “Dynamics of vortices in complex wakes: modeling, analysis, and experiments.” 2014. Web. 19 Oct 2019.
Vancouver:
Basu S. Dynamics of vortices in complex wakes: modeling, analysis, and experiments. [Internet] [Doctoral dissertation]. Virginia Tech; 2014. [cited 2019 Oct 19].
Available from: http://hdl.handle.net/10919/51749.
Council of Science Editors:
Basu S. Dynamics of vortices in complex wakes: modeling, analysis, and experiments. [Doctoral Dissertation]. Virginia Tech; 2014. Available from: http://hdl.handle.net/10919/51749
23.
Jrad, Mohamed.
Multidisciplinary Optimization and Damage Tolerance of Stiffened Structures.
Degree: PhD, Engineering Science and Mechanics, 2015, Virginia Tech
URL: http://hdl.handle.net/10919/52276
► The structural optimization of a cantilever aircraft wing with curvilinear spars and ribs and stiffeners is described. The design concept of reinforcing the wing structure…
(more)
▼ The structural optimization of a cantilever aircraft wing with curvilinear spars and ribs and stiffeners is described. The design concept of reinforcing the wing structure using curvilinear stiffening members has been explored due to the development of novel manufacturing technologies like electron-beam-free-form-fabrication (EBF3). For the optimization of a complex wing, a common strategy is to divide the optimization procedure into two subsystems: the global wing optimization which optimizes the geometry of spars, ribs and wing skins; and the local panel optimization which optimizes the design variables of local panels bordered by spars and ribs. The stiffeners are placed on the local panels to increase the stiffness and buckling resistance. The panel thickness, size and shape of stiffeners are optimized to minimize the structural weight. The geometry of spars and ribs greatly influences the design of stiffened panels. During the local panel optimization, the stress information is taken from the global model as a displacement boundary condition on the panel edges using the so-called "Global-Local Approach". The aircraft design is characterized by multiple disciplines: structures, aeroelasticity and buckling. Particle swarm optimization is used in the integration of global/local optimization to optimize the SpaRibs. The interaction between the global wing optimization and the local panel optimization is usually computationally expensive. A parallel computing technology has been developed in Python programming to reduce the CPU time. The license cycle-check method and memory self-adjustment method are two approaches that have been applied in the parallel framework in order to optimize the use of the resources by reducing the license and memory limitations and making the code robust. The integrated global-local optimization approach has been applied to subsonic NASA common research model (CRM) wing, which proves the methodology's application scaling with medium fidelity FEM analysis. Both the global wing design variables and local panel design variables are optimized to minimize the wing weight at an acceptable computational cost. The structural weight of the wing has been, therefore, reduced by 40% and the parallel implementation allowed a reduction in the CPU time by 89%. The aforementioned Global-Local Approach is investigated and applied to a composite panel with crack at its center. Because of composite laminates' heterogeneity, an accurate analysis of these requires very high time and storage space. In the presence of structural discontinuities like cracks, delaminations, cutouts etc., the computational complexity increases significantly. A possible alternative to reduce the computational complexity is the global-local analysis which involves an approximate analysis of the whole structure followed by a detailed analysis of a significantly smaller region of interest. We investigate here the performance of the global-local scheme based on the finite element method by comparing it to the traditional finite element…
Advisors/Committee Members: Kapania, Rakesh K. (committeechair), De Vita, Raffaella (committee member), Hajj, Muhammad R. (committee member), Ragab, Saad A. (committee member), Adjerid, Slimane (committee member).
Subjects/Keywords: Multidisciplinary optimization; stiffened panels; damage tolerance; stress intensity factor; composite panel; buckling; crack; global/local approach; finite element method; parallel computing
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APA ·
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CSE |
Export
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APA (6th Edition):
Jrad, M. (2015). Multidisciplinary Optimization and Damage Tolerance of Stiffened Structures. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/52276
Chicago Manual of Style (16th Edition):
Jrad, Mohamed. “Multidisciplinary Optimization and Damage Tolerance of Stiffened Structures.” 2015. Doctoral Dissertation, Virginia Tech. Accessed October 19, 2019.
http://hdl.handle.net/10919/52276.
MLA Handbook (7th Edition):
Jrad, Mohamed. “Multidisciplinary Optimization and Damage Tolerance of Stiffened Structures.” 2015. Web. 19 Oct 2019.
Vancouver:
Jrad M. Multidisciplinary Optimization and Damage Tolerance of Stiffened Structures. [Internet] [Doctoral dissertation]. Virginia Tech; 2015. [cited 2019 Oct 19].
Available from: http://hdl.handle.net/10919/52276.
Council of Science Editors:
Jrad M. Multidisciplinary Optimization and Damage Tolerance of Stiffened Structures. [Doctoral Dissertation]. Virginia Tech; 2015. Available from: http://hdl.handle.net/10919/52276
Virginia Tech
24.
Ben Ayed, Samah.
Analysis, Simulation and Control of Peak Pressure Loads on Low-Rise Structures.
Degree: PhD, Engineering Science and Mechanics, 2013, Virginia Tech
URL: http://hdl.handle.net/10919/23671
► Wind storms pose dangerous threats to human lives and are an enormous drain on the economy. Their damage to buildings usually starts with the failure…
(more)
▼ Wind storms pose dangerous threats to human lives and are an enormous drain on the economy. Their damage to buildings usually starts with the failure of structural components that are subjected to excessive wind loads. In this dissertation, we investigate the characteristics of extreme loads on low-rise structures through analysis of full-scale and numerical data. We also use numerical simulations to evaluate different approaches to control the separated flow over a surface-mounted prism with the objective of reducing extreme pressure coefficients or loads on its surface. In the first part, we use a probabilistic approach to characterize peak loads as measured on a subject house during Hurricane Ivan on 2004. Time series of pressure coefficients collected on the roof of that house are analyzed. Rather than using peak values, which could vary due to the stochastic nature of the data, a probabilistic analysis is used to determine the probability of non-exceedence of specific values of pressure coefficients and associated wind loads. The results show that the time series of the pressure coefficients follow a three-parameter Gamma distribution, while the peak pressure follows a two-parameter Gumbel distribution. The results of the analysis are contrasted with the design values. In the second part, we perform numerical simulations of the flow over a surface-mounted prism as a simplified example for the flow over a low-rise structure. A Direct Numerical Simulation (DNS) code is developed to solve the unsteady two-dimensional incompressible Navier-Stokes equations of the flow past the prism. The pressure coefficients are then computed on the prism surface in order to assess the wind loads. The code is written on a parallel platform using the Message Passing Interface (MPI) library. We use the simulations to study the effects of inflow disturbances on the extreme loads on structures. The sensitivities of peak loads on a surface mounted prism to variations in incident gust parameters are determined. Latin Hypercube Sampling (LHS) is applied to obtain different combinations of inflow parameters. A non-intrusive polynomial chaos expansion is then applied to determine the sensitivities. The results show that the gust enhances the destabilization of the separation shear layer, forces it to break down and moves it closer to the roof of the prism. As for the sensitivities, the results show that the extreme loads are most sensitive to the transverse amplitude of the disturbance. Because the separated flow over sharp edges is responsible for the extreme pressure peaks, we investigate the use of active and passive control strategies to reduce wind loads. The studied active flow control strategies include blowing, suction, and synthetic jets. We implement them by using different flux injections, different slot locations and different angles. Investigation of the possible peak pressure reduction for two Reynolds numbers is performed. For Re = 1000, a reduction by nearly 50% of the peak pressure is obtained.…
Advisors/Committee Members: Hajj, Muhammad R. (committeechair), Ragab, Saad A. (committeechair), Paul, Mark R. (committee member), Jung, Sunghwan (committee member), Puri, Ishwar K. (committee member).
Subjects/Keywords: wind loads; peak pressure distribution; sensitivity analysis; flow control; fluid-structure interaction
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APA (6th Edition):
Ben Ayed, S. (2013). Analysis, Simulation and Control of Peak Pressure Loads on Low-Rise Structures. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/23671
Chicago Manual of Style (16th Edition):
Ben Ayed, Samah. “Analysis, Simulation and Control of Peak Pressure Loads on Low-Rise Structures.” 2013. Doctoral Dissertation, Virginia Tech. Accessed October 19, 2019.
http://hdl.handle.net/10919/23671.
MLA Handbook (7th Edition):
Ben Ayed, Samah. “Analysis, Simulation and Control of Peak Pressure Loads on Low-Rise Structures.” 2013. Web. 19 Oct 2019.
Vancouver:
Ben Ayed S. Analysis, Simulation and Control of Peak Pressure Loads on Low-Rise Structures. [Internet] [Doctoral dissertation]. Virginia Tech; 2013. [cited 2019 Oct 19].
Available from: http://hdl.handle.net/10919/23671.
Council of Science Editors:
Ben Ayed S. Analysis, Simulation and Control of Peak Pressure Loads on Low-Rise Structures. [Doctoral Dissertation]. Virginia Tech; 2013. Available from: http://hdl.handle.net/10919/23671
Virginia Tech
25.
Mehmood, Arshad.
Active/Passive Controls and Energy Harvesting from Vortex-Induced Vibrations.
Degree: PhD, Engineering Science and Mechanics, 2013, Virginia Tech
URL: http://hdl.handle.net/10919/23902
► Fluid-structure interactions occur in many enginnering and industrial applications. Such interactions may result in undesirable forces acting on the structure that may cause fatigue and…
(more)
▼ Fluid-structure interactions occur in many enginnering and industrial applications. Such
interactions may result in undesirable forces acting on the structure that may cause fatigue
and degradation of the structural components. The purpose of this research is to develop
a solver that simulates the fluid-structure interaction, assess tools that can be used to control
the resulting motions and analyze a system that can be used to convert the structure's
motion to a useful form of energy. For this purpose, we develop a code which encompasses
three-dimensional numerical simulations of a flow interacting with a freely-oscillating cylinder.
The solver is based on the accelerated reference frame technique (ARF), in which the
momentum equations are directly coupled with the cylinder motion by adding a reference
frame acceleration term; the outer boundary conditions of the flow domain are updated using
the response of the cylinder.
We develop active linear and nonlinear velocity feedback controllers that suppress VIV
by directly controlling the cylinder's motion. We assess their effectiveness and compare their
performance and required power levels to suppress the motion of the cylinder. Particularly,
we determine the most effective control law that requires minimum power to achieve a desired
controlled amplitude. Furthermore, we investigate, in detail, the feasibility of using a
nonlinear energy sink to control the vortex-induced vibrations of a freely oscillating circular
cylinder. It has been postulated that such a system, which consists of a nonlinear spring,
can be used to control the motion over a wide range of frequencies. However, introducing
an essential nonlinearity of the cubic order to a coupled system could lead to multiple stable
solutions depending on the initial conditions, system's characteristics and parameters. Our
investigation aims at determining the effects of the sink parameters on the response of the
coupled system.
We also investigate the extent of drag reduction that can be attained through rotational oscillations of the circular cylinder. An optimization is performed by combining the CFD
solver with a global deterministic optimization algorithm. The use of this optimization tool
allows for a rapid determination of the rotational amplitude and frequency domains that
yield minimum drag. We also perform three-dimensional numerical simulations of an inlinevibrating
cylinder over a range of amplitudes and frequencies with the objective of suppressing
the lift force. We compare the amplitude-frequency response curves, levels of lift suppression,
and synchronization maps for two- and three-dimensional flows.
Finally, we evaluate the possibility of converting vortex-induced vibrations into a usable
form of electric power. Different transduction mechanisms can be employed for converting
these vibrations to electric power, including electrostatic, electromagnetic, and piezoelectric
transduction. We consider the piezoelectric option because it can be used to harvest energy
over a wide range of frequencies and…
Advisors/Committee Members: Hajj, Muhammad R. (committeechair), Borggaard, Jeffrey T. (committee member), Ribbens, Calvin J. (committee member), Ragab, Saad A. (committee member).
Subjects/Keywords: Vortex-induced vibrations; Actice/Passive controls; Nonlinear energy sink; Rotary oscillations; Inline oscillations; Energy harvesting
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APA (6th Edition):
Mehmood, A. (2013). Active/Passive Controls and Energy Harvesting from Vortex-Induced Vibrations. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/23902
Chicago Manual of Style (16th Edition):
Mehmood, Arshad. “Active/Passive Controls and Energy Harvesting from Vortex-Induced Vibrations.” 2013. Doctoral Dissertation, Virginia Tech. Accessed October 19, 2019.
http://hdl.handle.net/10919/23902.
MLA Handbook (7th Edition):
Mehmood, Arshad. “Active/Passive Controls and Energy Harvesting from Vortex-Induced Vibrations.” 2013. Web. 19 Oct 2019.
Vancouver:
Mehmood A. Active/Passive Controls and Energy Harvesting from Vortex-Induced Vibrations. [Internet] [Doctoral dissertation]. Virginia Tech; 2013. [cited 2019 Oct 19].
Available from: http://hdl.handle.net/10919/23902.
Council of Science Editors:
Mehmood A. Active/Passive Controls and Energy Harvesting from Vortex-Induced Vibrations. [Doctoral Dissertation]. Virginia Tech; 2013. Available from: http://hdl.handle.net/10919/23902
Virginia Tech
26.
Fayed, Hassan El-Hady Hassan.
Particles and Bubbles Collisions Frequency in Homogeneous Turbulence and Applications to Minerals Flotation Machines.
Degree: PhD, Engineering Science and Mechanics, 2014, Virginia Tech
URL: http://hdl.handle.net/10919/24910
► The collisions frequency of dispersed phases (particles, droplets, bubbles) in a turbulent carrier phase is a fundamental quantity that is needed for modeling multiphase flows…
(more)
▼ The collisions frequency of dispersed phases (particles, droplets, bubbles) in a turbulent carrier phase is a fundamental quantity that is needed for modeling multiphase flows with applications to chemical processes, minerals flotation, food science, and many other industries.
In this dissertation, numerical simulations are performed to determine collisions frequency of bi-dispersed particles (solid particles and bubbles) in homogeneous isotropic turbulence. Both direct numerical simulations (DNS) and Large Eddy simulations (LES) are conducted to determine velocity fluctuations of the carrier phase. The DNS results are used to validate existing theoretical models as well as the LES results. The dissertation also presents a CFD-based flotation model for predicting the pulp recovery rate in froth flotation machines.
In the direct numerical simulations work, particles and bubbles suspended in homogeneous isotropic turbulence are tracked and their collisions frequency is determined as a function of particle Stokes number. The effects of the dispersed phases on the carrier phase are neglected. Particles and bubbles of sizes on the order of Kolmogorov length scale are treated as point masses. Equations of motion of dispersed phases are integrated simultaneously with the equations of the carrier phase using the same time stepping scheme. In addition to Stokes drag, the pressure gradient in the carrier phase and added-mass forces are also included. The collision model used here allows overlap of particles and bubbles. Collisions kernel, radial relative velocity, and radial distribution function found by DNS are compared to theoretical models over a range of particle Stokes number. In general, good agreement between DNS and recent theoretical models is obtained for radial relative velocity for both particle-particle and particle-bubble collisions. The DNS results show that around Stokes number of unity particles of the same group undergo expected preferential concentration while particles and bubbles are segregated. The segregation behavior of particles and bubbles leads to a radial distribution function that is less than one. Existing theoretical models do not account for effects of this segregation behavior of particles and bubbles on the radial distribution function.
In the large-eddy simulations efforts, the dissertation addresses the importance of the subgrid fluctuations on the collisions frequency and investigates techniques for predicting those fluctuations. The cases studied are of particles-particles and particles-bubbles collisions at Reynolds number Re = 96. A study is conducted first by neglecting the effects of subgrid velocity fluctuations on particles and bubbles motions. It is found that around Stokes number of unity solid particles of the same group undergo the well known preferential concentration as observed in the DNS. Effects of pressure gradient on the particles are negligible due to their small sizes. Bubbles as a low inertia particles are very sensitive to subgrid velocity and acceleration fields…
Advisors/Committee Members: Ragab, Saad A. (committeechair), Luttrell, Gerald H. (committee member), Adjerid, Slimane (committee member), Thangjitham, Surot (committee member), Hajj, Muhammad R. (committee member).
Subjects/Keywords: Collisions Frequency; Multiphase flow; Homogeneous turbulence; Direct numerical simulation; Large-eddy simulation; Multifractal modeling; Bubble size distribution; Minerals flotation machines
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APA (6th Edition):
Fayed, H. E. H. (2014). Particles and Bubbles Collisions Frequency in Homogeneous Turbulence and Applications to Minerals Flotation Machines. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/24910
Chicago Manual of Style (16th Edition):
Fayed, Hassan El-Hady Hassan. “Particles and Bubbles Collisions Frequency in Homogeneous Turbulence and Applications to Minerals Flotation Machines.” 2014. Doctoral Dissertation, Virginia Tech. Accessed October 19, 2019.
http://hdl.handle.net/10919/24910.
MLA Handbook (7th Edition):
Fayed, Hassan El-Hady Hassan. “Particles and Bubbles Collisions Frequency in Homogeneous Turbulence and Applications to Minerals Flotation Machines.” 2014. Web. 19 Oct 2019.
Vancouver:
Fayed HEH. Particles and Bubbles Collisions Frequency in Homogeneous Turbulence and Applications to Minerals Flotation Machines. [Internet] [Doctoral dissertation]. Virginia Tech; 2014. [cited 2019 Oct 19].
Available from: http://hdl.handle.net/10919/24910.
Council of Science Editors:
Fayed HEH. Particles and Bubbles Collisions Frequency in Homogeneous Turbulence and Applications to Minerals Flotation Machines. [Doctoral Dissertation]. Virginia Tech; 2014. Available from: http://hdl.handle.net/10919/24910
Virginia Tech
27.
Bahmani, Fatemeh.
Three Problems Involving Compressible Flow with Large Bulk Viscosity and Non-Convex Equations of State.
Degree: PhD, Engineering Science and Mechanics, 2013, Virginia Tech
URL: http://hdl.handle.net/10919/25885
► We have examined three problems involving steady flows of Navier-Stokes fluids. In each problem non-classical effects are considered. In the first two problems, we consider…
(more)
▼ We have examined three problems involving steady flows of Navier-Stokes fluids. In each problem non-classical effects are considered. In the first two problems, we consider fluids which have bulk viscosities which are much larger than their shear viscosities. In the last problem, we examine steady supersonic flows of a Bethe-Zel'dovich-Thompson (BZT) fluid over a thin airfoil or turbine blade. BZT fluids are fluids in which the fundamental derivative of gasdynamics changes sign during an isentropic expansion or compression.
In the first problem we consider the effects of large bulk viscosity on the structure of the inviscid approximation using the method of matched asymptotic expansions. When the ratio of bulk to shear viscosity is of the order of the square root of the Reynolds number we find that the bulk viscosity effects are important in the first corrections to the conventional boundary layer and outer inviscid flow. At first order the outer flow is found to be frictional, rotational, and non-isentropic for large bulk viscosity fluids. The pressure is found to have first order variations across the boundary layer and the temperature equation is seen to have two additional source terms at first order when the bulk viscosity is large.
In the second problem, we consider the reflection of an oblique shock from a laminar flat plate boundary layer. The flow is taken to be two-dimensional, steady, and the gas model is taken to be a perfect gas with constant Prandtl number. The plate is taken to be adiabatic. The full Navier-Stokes equations are solved using a weighted essentially non-oscillatory (WENO) numerical scheme. We show that shock-induced separation can be suppressed once the bulk viscosity is large enough.
In the third problem, we solve a quartic Burgers equation to describe the steady, two-dimensional, inviscid supersonic flow field generated by thin airfoils. The Burgers equation is solved using the WENO technique. Phenomena of interest include the partial and complete disintegration of compression shocks, the formation of expansion shocks, and the collision of expansion and compression shocks.
Advisors/Committee Members: Cramer, Mark S. (committeechair), Ragab, Saad A. (committee member), Ross, Shane David (committee member), Paul, Mark R. (committee member), Jung, Sunghwan (committee member).
Subjects/Keywords: Compressible flow; shock; boundary layer; WENO; CFD; bulk viscosity; BZT fluids
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APA (6th Edition):
Bahmani, F. (2013). Three Problems Involving Compressible Flow with Large Bulk Viscosity and Non-Convex Equations of State. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/25885
Chicago Manual of Style (16th Edition):
Bahmani, Fatemeh. “Three Problems Involving Compressible Flow with Large Bulk Viscosity and Non-Convex Equations of State.” 2013. Doctoral Dissertation, Virginia Tech. Accessed October 19, 2019.
http://hdl.handle.net/10919/25885.
MLA Handbook (7th Edition):
Bahmani, Fatemeh. “Three Problems Involving Compressible Flow with Large Bulk Viscosity and Non-Convex Equations of State.” 2013. Web. 19 Oct 2019.
Vancouver:
Bahmani F. Three Problems Involving Compressible Flow with Large Bulk Viscosity and Non-Convex Equations of State. [Internet] [Doctoral dissertation]. Virginia Tech; 2013. [cited 2019 Oct 19].
Available from: http://hdl.handle.net/10919/25885.
Council of Science Editors:
Bahmani F. Three Problems Involving Compressible Flow with Large Bulk Viscosity and Non-Convex Equations of State. [Doctoral Dissertation]. Virginia Tech; 2013. Available from: http://hdl.handle.net/10919/25885
28.
Hussein, Ahmed Abd Elmonem Ahmed.
Dynamical System Representation and Analysis of Unsteady Flow and Fluid-Structure Interactions.
Degree: PhD, Engineering Science and Mechanics, 2018, Virginia Tech
URL: http://hdl.handle.net/10919/85626
► A dynamical system approach is utilized to reduce the representation order of unsteady fluid flows and fluid-structure interaction systems. This approach allows for significant reduction…
(more)
▼ A dynamical system approach is utilized to reduce the representation order of unsteady fluid flows and fluid-structure interaction systems. This approach allows for significant reduction in the computational cost of their numerical simulations, implementation of optimization and control methodologies and assessment of their dynamic stability. In the first chapter, I present a new Lagrangian function to derive the equations of motion of unsteady point vortices. This representation is a reconciliation between Newtonian and Lagrangian mechanics yielding a new approach to model the dynamics of these vortices. In the second chapter, I investigate the flutter of a helicopter rotor blade using finite-state time approximation of the unsteady aerodynamics. The analysis showed a new stability region that could not be determined under the assumption of a quasi-steady flow. In the third chapter, I implement the unsteady vortex lattice method to quantify the effects of tail flexibility on the propulsive efficiency of a fish. I determine that flexibility enhances the propulsion. In the fourth chapter, I consider the stability of a flapping micro air vehicle and use different approaches to design the transition from hovering to forward flight. I determine that first order averaging is not suitable and that time periodic dynamics are required for the controller to achieve this transition. In the fifth chapter, I derive a mathematical model for the free motion of a two-body planar system representing a fish under the action of coupled dynamics and hydrodynamics loads. I conclude that the psicform fish family are inherently stable under certain conditions that depend on the location of the center of mass.
Advisors/Committee Members: Hajj, Muhammad Ramiz (committeechair), Woolsey, Craig A. (committee member), Canfield, Robert Arthur (committee member), Ragab, Saad A. (committee member).
Subjects/Keywords: Finite-State Unsteady Aerodynamics; Lagrangian; Point Vortices; Flutter; Aeroelasticity; Finite Element Method; Flexibility; Hydroelasticity; Nonlinear Time-Periodic Dynamics; Floquet Analysis; Optimal Control
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APA (6th Edition):
Hussein, A. A. E. A. (2018). Dynamical System Representation and Analysis of Unsteady Flow and Fluid-Structure Interactions. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/85626
Chicago Manual of Style (16th Edition):
Hussein, Ahmed Abd Elmonem Ahmed. “Dynamical System Representation and Analysis of Unsteady Flow and Fluid-Structure Interactions.” 2018. Doctoral Dissertation, Virginia Tech. Accessed October 19, 2019.
http://hdl.handle.net/10919/85626.
MLA Handbook (7th Edition):
Hussein, Ahmed Abd Elmonem Ahmed. “Dynamical System Representation and Analysis of Unsteady Flow and Fluid-Structure Interactions.” 2018. Web. 19 Oct 2019.
Vancouver:
Hussein AAEA. Dynamical System Representation and Analysis of Unsteady Flow and Fluid-Structure Interactions. [Internet] [Doctoral dissertation]. Virginia Tech; 2018. [cited 2019 Oct 19].
Available from: http://hdl.handle.net/10919/85626.
Council of Science Editors:
Hussein AAEA. Dynamical System Representation and Analysis of Unsteady Flow and Fluid-Structure Interactions. [Doctoral Dissertation]. Virginia Tech; 2018. Available from: http://hdl.handle.net/10919/85626
29.
Chatterjee, Krishnashis.
Analytical and Experimental Investigation of Insect Respiratory System Inspired Microfluidics.
Degree: PhD, Engineering Science and Mechanics, 2018, Virginia Tech
URL: http://hdl.handle.net/10919/85688
► Microfluidics has been the focal point of research in various disciplines due to its advantages of portability and cost effectiveness, and the ability to perform…
(more)
▼ Microfluidics has been the focal point of research in various disciplines due to its advantages of portability and cost effectiveness, and the ability to perform complex tasks with precision. In the past two decades microfluidic technology has been used to cool integrated circuits, for exoplanetary chemical analysis, for mimicking cellular environments, and in the design of specialized organ-on-a-chip devices. While there have been considerable advances in the complexity and miniaturization of microfluidic devices, particularly with the advent of microfluidic large-scale integration (mLSI) and microfluidic very-large-scale-integration (mVLSI), in which there are hundreds of thousands of flow channels per square centimeter on a microfluidic chip, there remains an actuation overhead problem: these small, complex microfluidic devices are tethered to extensive off-chip actuation machinery that limit their portability and efficiency. Insects, in contrast, actively and efficiently handle their respiratory air flows in complex networks consisting of thousands of microscale tracheal pathways. This work analytically and experimentally investigates the viability of incorporating some of the essential kinematics and actuation strategies of insect respiratory systems in microfluidic devices. Mathematical models of simplified individual tracheal pathways were derived and analyzed, and insect-mimetic PDMS-based valveless microfluidic devices were fabricated and tested. It was found that not only are these devices are capable of pumping fluids very efficiently using insect-mimetic actuation techniques, but also that the fluid flow direction and magnitude could be controlled via the actuation frequency alone, a feature never before realized in microfluidic devices. These results suggest that insect-mimicry may be a promising direction for designing more efficient microfluidic devices.
Advisors/Committee Members: Staples, Anne E. (committeechair), Davalos, Rafael V. (committee member), Socha, John (committee member), Ragab, Saad A. (committee member), Untaroiu, Alexandrina (committee member).
Subjects/Keywords: Microfluidics; insect-inspired; microscale pumping; slip boundary conditions; single actuation; frequency dependent flow control
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APA (6th Edition):
Chatterjee, K. (2018). Analytical and Experimental Investigation of Insect Respiratory System Inspired Microfluidics. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/85688
Chicago Manual of Style (16th Edition):
Chatterjee, Krishnashis. “Analytical and Experimental Investigation of Insect Respiratory System Inspired Microfluidics.” 2018. Doctoral Dissertation, Virginia Tech. Accessed October 19, 2019.
http://hdl.handle.net/10919/85688.
MLA Handbook (7th Edition):
Chatterjee, Krishnashis. “Analytical and Experimental Investigation of Insect Respiratory System Inspired Microfluidics.” 2018. Web. 19 Oct 2019.
Vancouver:
Chatterjee K. Analytical and Experimental Investigation of Insect Respiratory System Inspired Microfluidics. [Internet] [Doctoral dissertation]. Virginia Tech; 2018. [cited 2019 Oct 19].
Available from: http://hdl.handle.net/10919/85688.
Council of Science Editors:
Chatterjee K. Analytical and Experimental Investigation of Insect Respiratory System Inspired Microfluidics. [Doctoral Dissertation]. Virginia Tech; 2018. Available from: http://hdl.handle.net/10919/85688
Virginia Tech
30.
Aidi, Bilel.
Experimental and Numerical Analysis of Damage in Notched Composites.
Degree: PhD, Engineering Science and Mechanics, 2016, Virginia Tech
URL: http://hdl.handle.net/10919/82559
► This dissertation contains the results from an experimental study, numerical, and analytical study conducted on quasi-isotropic carbon fiber laminates containing a center hole (notch) subjected…
(more)
▼ This dissertation contains the results from an experimental study, numerical, and analytical study conducted on quasi-isotropic carbon fiber laminates containing a center hole (notch) subjected to constant amplitude tension-tension fatigue loading in order to investigate the effect of fatigue damage development on the residual properties. Quasi-static tests were initially performed on notched composites using digital image correlation (DIC) to determine the strain profiles at selected transverse sections of the outer ply of the laminates and the static strength of the laminates. Subsequently, tension-tension fatigue tests were carried out at 70%, 75% and 80% of the nominal static failure load. A finite element model was developed using Abaqus and Digimat in which Digimat was used to implement the damage evolution model via a user-defined material subroutine. Damage initiation has been assessed using Hashin's failure criteria and the Matzenmiller model was adopted for damage evolution. A second finite element model was developed using Abaqus and Autodesk Simulation Composite Analysis (ASCA) in which ASCA was used to implement the user-material subroutine. The subroutine includes a failure initiation criterion based upon multi-continuum theory (MCT) and an energy-based damage evolution law. Numerical and experimental strain results were presented and compared for different section lines on the outer surface of the laminate at different loading stages. Additionally, the experimentally measured notched composite strength was compared with the predictions from the finite element solutions. These results are used as baseline for subsequent comparison with strain profiles obtained using DIC for specimens fatigued at different stress levels and fatigue lifetime fractions. The results showed a significant effect of fatigue damage development on strain redistribution even at early stages of fatigue. The results also showed the capability of DIC technique to identify damage initiation and its location. Furthermore, X-ray computed tomography (CT) was used to examine the sequence of damage development throughout the fatigue lifetime and to connect the observed damage mechanisms with the occurred change of strain profiles.
Experimental vibrational modal analysis tests were also conducted to assess the effect of fatigue damage on the residual frequency responses (RFRs). Vibrational measurements were initially performed on pristine notched composites. The results are used as baseline for subsequent comparison with strain profiles obtained with DIC. Finite element models based on the classical plate theory (Kirchhoff) and the shear deformable theory (Mindlin) within the framework of equivalent single-layer and layer-wise concepts as well as the three-dimensional theory of elasticity are developed to predict the natural frequencies of non-fatigued specimen. These models are implemented using the finite element software, Abaqus, to determine the natural frequencies and the corresponding mode shapes. In addition, an analytical model…
Advisors/Committee Members: Case, Scott W. (committeechair), Ragab, Saad A. (committee member), Lattimer, Brian Y. (committee member), Cramer, Mark S. (committee member), De Vita, Raffaella (committee member).
Subjects/Keywords: Carbon fiber; notched composite; fatigue; residual properties; digital image correlation (DIC); damage monitoring; vibration; Finite Element Method
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APA (6th Edition):
Aidi, B. (2016). Experimental and Numerical Analysis of Damage in Notched Composites. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/82559
Chicago Manual of Style (16th Edition):
Aidi, Bilel. “Experimental and Numerical Analysis of Damage in Notched Composites.” 2016. Doctoral Dissertation, Virginia Tech. Accessed October 19, 2019.
http://hdl.handle.net/10919/82559.
MLA Handbook (7th Edition):
Aidi, Bilel. “Experimental and Numerical Analysis of Damage in Notched Composites.” 2016. Web. 19 Oct 2019.
Vancouver:
Aidi B. Experimental and Numerical Analysis of Damage in Notched Composites. [Internet] [Doctoral dissertation]. Virginia Tech; 2016. [cited 2019 Oct 19].
Available from: http://hdl.handle.net/10919/82559.
Council of Science Editors:
Aidi B. Experimental and Numerical Analysis of Damage in Notched Composites. [Doctoral Dissertation]. Virginia Tech; 2016. Available from: http://hdl.handle.net/10919/82559
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Open Access Theses and Dissertations
