+publisher:"Virginia Tech" +contributor:("Masoud, Ziyad N.")

Virginia Tech

1. Nayfeh, Nader Ali. Adaptation of Delayed Position Feedback to the Reduction of Sway of Container Cranes.

Degree: MS, Electrical and Computer Engineering, 2002, Virginia Tech

► Cranes are increasingly used in transportation and construction. increasing demand and faster requirements necessitate better and more efficient controllers to guarantee fast turn-around time and… (more)

▼ Cranes are increasingly used in transportation and construction. increasing demand and faster requirements necessitate better and more efficient controllers to guarantee fast turn-around time and to meet safety requirements. Container cranes are used extensively in ship-to-port and port-to-ship transfer operations. In this work, we will extend the recently developed delayed position feedback controller to container cranes. In contrast with traditional work, which models a crane as a simple pendulum consisting of a hoisting cable and a lumped mass at its end, we have modeled the crane as a four-bar mechanism. The actual configuration of the hoisting mechanism is significantly different from a simple pendulum. It consists typically of a set of four hoisting cables attached to four different points on the trolley and to four points on a spreader bar. The spreader bar is used to lift the containers. Therefore, the dynamics of hoisting assemblies of large container cranes are different from that of a simple pendulum. We found that a controller which treats the system as a four-bar mechanism has an improved response. We developed a controller to meet the following requirements: traverse an 80-ton payload 50 m in 21.5 s, including raising the payload 15 m at the beginning and lowering the payload 15 m at the end of motion, while reducing the sway to 50 mm within 5.0 s at the end of the transfer maneuver. The performance of the controller has been demonstrated theoretically using numerical simulation. Moreover, the performance of the controller has been demonstrated experimentally using a 1/10th scale model. For the 1/10th scale model, the requirements translate into: traverse an 80 kg payload 5 m in 6.8 s, including raising 1.5 m at the beginning and lowering 1.5 m at the end of motion, while reducing the sway to 5 mm in under 1.6 s. The experiments validated the controller. Advisors/Committee Members: Stilwell, Daniel J. (committee member), Baumann, William T. (committeecochair), Masoud, Ziyad N. (committeecochair).

Subjects/Keywords: gantry crane; delayed feedback control; Container cranes; sway reduction

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APA (6^th Edition):

Nayfeh, N. A. (2002). Adaptation of Delayed Position Feedback to the Reduction of Sway of Container Cranes. (Masters Thesis). Virginia Tech. Retrieved from http://hdl.handle.net/10919/9698

Chicago Manual of Style (16^th Edition):

Nayfeh, Nader Ali. “Adaptation of Delayed Position Feedback to the Reduction of Sway of Container Cranes.” 2002. Masters Thesis, Virginia Tech. Accessed April 18, 2021. http://hdl.handle.net/10919/9698.

MLA Handbook (7^th Edition):

Nayfeh, Nader Ali. “Adaptation of Delayed Position Feedback to the Reduction of Sway of Container Cranes.” 2002. Web. 18 Apr 2021.

Vancouver:

Nayfeh NA. Adaptation of Delayed Position Feedback to the Reduction of Sway of Container Cranes. [Internet] [Masters thesis]. Virginia Tech; 2002. [cited 2021 Apr 18]. Available from: http://hdl.handle.net/10919/9698.

Council of Science Editors:

Nayfeh NA. Adaptation of Delayed Position Feedback to the Reduction of Sway of Container Cranes. [Masters Thesis]. Virginia Tech; 2002. Available from: http://hdl.handle.net/10919/9698

Virginia Tech

2. Daqaq, Mohammed F. Virtual Reality Simulation of Ships and Ship-Mounted Cranes.

Degree: MS, Engineering Science and Mechanics, 2003, Virginia Tech

URL: http://hdl.handle.net/10919/32305

► We present a virtual simulation of ships and ship-mounted cranes. The simulation is carried out in a Cave Automated Virtual Environment (CAVE). This simulation serves… (more)

Subjects/Keywords: OpenGL; CAVE; Virtual Reality; Crane; Ship; Motion Base.; DIVERSE

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APA (6^th Edition):

Daqaq, M. F. (2003). Virtual Reality Simulation of Ships and Ship-Mounted Cranes. (Masters Thesis). Virginia Tech. Retrieved from http://hdl.handle.net/10919/32305

Chicago Manual of Style (16^th Edition):

Daqaq, Mohammed F. “Virtual Reality Simulation of Ships and Ship-Mounted Cranes.” 2003. Masters Thesis, Virginia Tech. Accessed April 18, 2021. http://hdl.handle.net/10919/32305.

MLA Handbook (7^th Edition):

Daqaq, Mohammed F. “Virtual Reality Simulation of Ships and Ship-Mounted Cranes.” 2003. Web. 18 Apr 2021.

Vancouver:

Daqaq MF. Virtual Reality Simulation of Ships and Ship-Mounted Cranes. [Internet] [Masters thesis]. Virginia Tech; 2003. [cited 2021 Apr 18]. Available from: http://hdl.handle.net/10919/32305.

Council of Science Editors:

Daqaq MF. Virtual Reality Simulation of Ships and Ship-Mounted Cranes. [Masters Thesis]. Virginia Tech; 2003. Available from: http://hdl.handle.net/10919/32305

Virginia Tech

3. Elsayed, Mohamed Aly Khamis. Large Eddy Simulation and Wavelet Analysis of the Flow Field around a Surface Mounted Prism.

Degree: PhD, Engineering Science and Mechanics, 2005, Virginia Tech

URL: http://hdl.handle.net/10919/27811

► Unsteady large-scale vortices, formed by the roll-up of free shear layers separating along sharp edges, are the dominant flow characteristics of the turbulent flow over… (more)

▼ Unsteady large-scale vortices, formed by the roll-up of free shear layers separating along sharp edges, are the dominant flow characteristics of the turbulent flow over buildings. These vortical structures interact with each other and with the building surface resulting in secondary separation and severe pressure fluctuations. Moreover, the interaction of the large-scale vortices with the multiplicity of turbulence scales in the incoming wind exacerbates their unsteady motion and hence significantly affects the pressure fluctuations spectra experienced by the building. Large-eddy simulations are conducted to study the interaction of homogeneous turbulence in the incident flow with a surface-mounted prism. A compact fifth-order upwind difference scheme is used to effectively and accurately perform the simulations. Three cases of incident flow are considered. In one case, the prism is placed in a smooth uniform flow. In the second case, homogeneous isotropic turbulence with von Karman spectrum is superimposed on the uniform flow at the inflow boundary. The integral length scale is one-half the prism height. In the third case, the integral length scale is equal to the prism height. The numerical results are compared with experimental measurements reported by Tieleman et al. (2002). The results show that the highest negative mean value of the pressure coefficient on the roof and the sides is about 30% larger in case two of turbulent inflow and takes place closer to the windward edge of the prism. Moreover, the pressure coefficients on the roof and sides of the prism in the case of turbulent inflow show a higher level of variations in comparison with the case of smooth inflow conditions. The predicted mean characteristics of the pressure coefficients in the turbulent case match the experimental values in terms of both magnitude and location on the roof of the prism reported in Tieleman et al. (1998) and Tieleman et al. (2002). As for the peak value, the peak value of -2 obtained in the turbulent inflow case two is about 20% smaller than the values measured experimentally by Tieleman et al. (2002). On the other hand, it is stressed that the peak value in the simulations would increase as the duration of the simulation is increased to match that of the experimental measurement. The results also show that the turbulent case yields a non-exceedence probability for the peak pressure coefficient that is closer to the one obtained from the measured data than the smooth case data. Also, spectral and cross-spectral analysis are carried out using complex Morlet wavelet transform to investigate pressure-velocity relation. The study shows that the nonlinearity in the relationship of velocity-pressure is detected using wavelet bicoherence. Advisors/Committee Members: Renardy, Yuriko Y. (committee member), Masoud, Ziyad N. (committee member), Tieleman, Henry W. (committee member), Thangjitham, Surot (committee member), Ragab, Saad A. (committeecochair), Hajj, Muhammad R. (committeecochair).

Subjects/Keywords: wind loads; bicoherence; wavelet analysis; peak pressures; large eddy simulation

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APA (6^th Edition):

Elsayed, M. A. K. (2005). Large Eddy Simulation and Wavelet Analysis of the Flow Field around a Surface Mounted Prism. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/27811

Chicago Manual of Style (16^th Edition):

Elsayed, Mohamed Aly Khamis. “Large Eddy Simulation and Wavelet Analysis of the Flow Field around a Surface Mounted Prism.” 2005. Doctoral Dissertation, Virginia Tech. Accessed April 18, 2021. http://hdl.handle.net/10919/27811.

MLA Handbook (7^th Edition):

Elsayed, Mohamed Aly Khamis. “Large Eddy Simulation and Wavelet Analysis of the Flow Field around a Surface Mounted Prism.” 2005. Web. 18 Apr 2021.

Vancouver:

Elsayed MAK. Large Eddy Simulation and Wavelet Analysis of the Flow Field around a Surface Mounted Prism. [Internet] [Doctoral dissertation]. Virginia Tech; 2005. [cited 2021 Apr 18]. Available from: http://hdl.handle.net/10919/27811.

Council of Science Editors:

Elsayed MAK. Large Eddy Simulation and Wavelet Analysis of the Flow Field around a Surface Mounted Prism. [Doctoral Dissertation]. Virginia Tech; 2005. Available from: http://hdl.handle.net/10919/27811

Virginia Tech

4. Chevva, Konda Reddy. Practical Challenges in the Method of Controlled Lagrangians.

Degree: PhD, Engineering Science and Mechanics, 2005, Virginia Tech

URL: http://hdl.handle.net/10919/28877

► The method of controlled Lagrangians is an energy shaping control technique for underactuated Lagrangian systems. Energy shaping control design methods are appealing as they retain… (more)

▼ The method of controlled Lagrangians is an energy shaping control technique for underactuated Lagrangian systems. Energy shaping control design methods are appealing as they retain the underlying nonlinear dynamics and can provide stability results that hold over larger domain than can be obtained using linear design and analysis. The objective of this dissertation is to identify the control challenges in applying the method of controlled Lagrangians to practical engineering problems and to suggest ways to enhance the closed-loop performance of the controller. This dissertation describes a procedure for incorporating artificial gyroscopic forces in the method of controlled Lagrangians. Allowing these energy-conserving forces in the closed-loop system provides greater freedom in tuning closed-loop system performance and expands the class of eligible systems. In energy shaping control methods, physical dissipation terms that are neglected in the control design may enter the system in a way that can compromise stability. This is well illustrated through the "ball on a beam" example. The effect of physical dissipation on the closed-loop dynamics is studied in detail and conditions for stability in the presence of natural damping are discussed. The control technique is applied to the classic "inverted pendulum on a cart" system. A nonlinear controller is developed which asymptotically stabilizes the inverted equilibrium at a specific cart position for the conservative dynamic model. The region of attraction contains all states for which the pendulum is elevated above the horizontal plane. Conditions for asymptotic stability in the presence of linear damping are developed. The onlinear controller is validated through experiments. Experimental cart damping is best modeled using static and Coulomb friction. Experiments show that static and Coulomb friction degrades the closed-loop performance and induces limit cycles. A Lyapunov-based switching controller is proposed and successfully implemented to suppress the limit cycle oscillations. The Lyapunov-based controller switches between the energy shaping nonlinear controller, for states away from the equilibrium, and a well-tuned linear controller, for states close to the equilibrium. The method of controlled Lagrangians is applied to vehicle systems with internal moving point mass actuators. Applications of moving mass actuators include certain spacecraft, atmospheric re-entry vehicles, and underwater vehicles. Control design using moving mass actuators is challenging; the system is often underactuated and multibody dynamic models are higher dimensional. We consider two examples to illustrate the application of controlled Lagrangian formulation. The first example is a spinning disk, a simplified, planar version of a spacecraft spin stabilization problem. The second example is a planar, streamlined underwater vehicle. Advisors/Committee Members: Woolsey, Craig A. (committeechair), Masoud, Ziyad N. (committee member), Inman, Daniel J. (committee member), Hendricks, Scott L. (committee member), Nayfeh, Ali H. (committeecochair).

Subjects/Keywords: Underactuated Systems; Moving Mass Actuators; Method of Controlled Lagrangians; Energy Shaping Control

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APA (6^th Edition):

Chevva, K. R. (2005). Practical Challenges in the Method of Controlled Lagrangians. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/28877

Chicago Manual of Style (16^th Edition):

Chevva, Konda Reddy. “Practical Challenges in the Method of Controlled Lagrangians.” 2005. Doctoral Dissertation, Virginia Tech. Accessed April 18, 2021. http://hdl.handle.net/10919/28877.

MLA Handbook (7^th Edition):

Chevva, Konda Reddy. “Practical Challenges in the Method of Controlled Lagrangians.” 2005. Web. 18 Apr 2021.

Vancouver:

Chevva KR. Practical Challenges in the Method of Controlled Lagrangians. [Internet] [Doctoral dissertation]. Virginia Tech; 2005. [cited 2021 Apr 18]. Available from: http://hdl.handle.net/10919/28877.

Council of Science Editors:

Chevva KR. Practical Challenges in the Method of Controlled Lagrangians. [Doctoral Dissertation]. Virginia Tech; 2005. Available from: http://hdl.handle.net/10919/28877

Virginia Tech

5. Younis, Mohammad Ibrahim. Modeling and Simulation of Microelectromechanical Systems in Multi-Physics Fields.

Degree: PhD, Engineering Science and Mechanics, 2004, Virginia Tech

URL: http://hdl.handle.net/10919/11202

► The first objective of this dissertation is to present hybrid numerical-analytical approaches and reduced-order models to simulate microelectromechanical systems (MEMS) in multi-physics fields. These include… (more)

▼ The first objective of this dissertation is to present hybrid numerical-analytical approaches and reduced-order models to simulate microelectromechanical systems (MEMS) in multi-physics fields. These include electric actuation (AC and DC), squeeze-film damping, thermoelastic damping, and structural forces. The second objective is to investigate MEMS phenomena, such as squeeze-film damping and dynamic pull-in, and use the latter to design a novel RF-MEMS switch. In the first part of the dissertation, we introduce a new approach to the modeling and simulation of flexible microstructures under the coupled effects of squeeze-film damping, electrostatic actuation, and mechanical forces. The new approach utilizes the compressible Reynolds equation coupled with the equation governing the plate deflection. The model accounts for the slip condition of the flow at very low pressures. Perturbation methods are used to derive an analytical expression for the pressure distribution in terms of the structural mode shapes. This expression is substituted into the plate equation, which is solved in turn using a finite-element method for the structural mode shapes, the pressure distributions, the natural frequencies, and the quality factors. We apply the new approach to a variety of rectangular and circular plates and present the final expressions for the pressure distributions and quality factors. We extend the approach to microplates actuated by large electrostatic forces. For this case, we present a low-order model, which reduces significantly the cost of simulation. The model utilizes the nonlinear Euler-Bernoulli beam equation, the von K´arm´an plate equations, and the compressible Reynolds equation. The second topic of the dissertation is thermoelastic damping. We present a model and analytical expressions for thermoelastic damping in microplates. We solve the heat equation for the thermal flux across the microplate, in terms of the structural mode shapes, and hence decouple the thermal equation from the plate equation. We utilize a perturbation method to derive an analytical expression for the quality factor of a microplate with general boundary conditions under electrostatic loading and residual stresses in terms of its structural mode shapes. We present results for microplates with various boundary conditions. In the final part of the dissertation, we present a dynamic analysis and simulation of MEMS resonators and novel RF MEMS switches employing resonant microbeams. We first study microbeams excited near their fundamental natural frequencies (primary-resonance excitation). We investigate the dynamic pull-in instability and formulate safety criteria for the design of MEMS sensors and RF filters. We also utilize this phenomenon to design a low-voltage RF MEMS switch actuated with a combined DC and AC loading. Then, we simulate the dynamics of microbeams excited near half their fundamental natural frequencies (superharmonic excitation) and twice their fundamental natural… Advisors/Committee Members: Nayfeh, Ali H. (committeechair), Masoud, Ziyad N. (committee member), Ragab, Saad A. (committee member), Leo, Donald J. (committee member), Hendricks, Scott L. (committee member).

Subjects/Keywords: Primary and Secondary Excitations; Singular Perturbation; Dynamic Pull-in; Finite Element; RF Switches; Microbeams; Microplates; MEMS; Electrostatic Actuation; Thermoelastic Damping; Squeeze-Film Damping; Resonators

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APA · Chicago · MLA · Vancouver · CSE | Export to Zotero / EndNote / Reference Manager

APA (6^th Edition):

Younis, M. I. (2004). Modeling and Simulation of Microelectromechanical Systems in Multi-Physics Fields. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/11202

Chicago Manual of Style (16^th Edition):

Younis, Mohammad Ibrahim. “Modeling and Simulation of Microelectromechanical Systems in Multi-Physics Fields.” 2004. Doctoral Dissertation, Virginia Tech. Accessed April 18, 2021. http://hdl.handle.net/10919/11202.

MLA Handbook (7^th Edition):

Younis, Mohammad Ibrahim. “Modeling and Simulation of Microelectromechanical Systems in Multi-Physics Fields.” 2004. Web. 18 Apr 2021.

Vancouver:

Younis MI. Modeling and Simulation of Microelectromechanical Systems in Multi-Physics Fields. [Internet] [Doctoral dissertation]. Virginia Tech; 2004. [cited 2021 Apr 18]. Available from: http://hdl.handle.net/10919/11202.

Council of Science Editors:

Younis MI. Modeling and Simulation of Microelectromechanical Systems in Multi-Physics Fields. [Doctoral Dissertation]. Virginia Tech; 2004. Available from: http://hdl.handle.net/10919/11202

Virginia Tech

6. Zhao, Xiaopeng. Modeling and Simulation of MEMS Devices.

Degree: PhD, Engineering Science and Mechanics, 2004, Virginia Tech

URL: http://hdl.handle.net/10919/11227

► The objective of this dissertation is to present a modeling and simulation methodology for MEMS devices and identify and understand the associated nonlinearities due to… (more)

▼ The objective of this dissertation is to present a modeling and simulation methodology for MEMS devices and identify and understand the associated nonlinearities due to large deflections, electric actuation, impacts, and friction. In the first part of the dissertation, we introduce a reduced-order model of flexible microplates under electric excitation. The model utilizes the von Karman plate equations to account for geometric nonlinearities due to large plate deflections. The Galerkin approach is employed to reduce the partial-differential equations of motion and associated boundary conditions into a finite dimensional system of nonlinearly coupled ordinary-differential equations. We use the reduced-order model to analyze the mechanical behavior of a simply supported microplate and a fully clamped microplate. Effect of various design parameters on both the static and dynamic characteristics of microplates is studied. The second part of the dissertation presents comprehensive modeling and simulation tools for impact microactuators. Nonsmooth dynamics due to impacts and friction are studied, combining various approaches, including direct numerical integration, root-finding technique for periodic motions, continuation of grazing periodic orbits, and local analysis of the near grazing dynamics. The transition between nonimpacting and impacting long term motions, referred to as grazing bifurcations, indicates the transition between on and off states of an impact microactuator. Three different on-off switching mechanisms are identified for the Mita microactuator. These mechanisms also generalize to arbitrary impacting systems with a similar nonlinearity. A local map based on the concept of discontinuity mapping provides an effcient and accurate tool for the grazing bifurcation analysis. Nonlinear impacting dynamics of the microactuator are studied in detail to identify various bifurcations and parameter ranges corresponding to chaotic motions. We find that the frequency-response curves of the impacting dynamics are significantly different from those of the nonimpacting dynamics. Advisors/Committee Members: Nayfeh, Ali H. (committeechair), Dankowicz, Harry J. (committee member), Hendricks, Scott L. (committee member), Masoud, Ziyad N. (committee member), Woolsey, Craig A. (committee member).

Subjects/Keywords: Discontinuity Mappings; Nonsmooth Dynamics; Reduced-Order Modeling; Finite Element Method; MEMS

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APA · Chicago · MLA · Vancouver · CSE | Export to Zotero / EndNote / Reference Manager

APA (6^th Edition):

Zhao, X. (2004). Modeling and Simulation of MEMS Devices. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/11227

Chicago Manual of Style (16^th Edition):

Zhao, Xiaopeng. “Modeling and Simulation of MEMS Devices.” 2004. Doctoral Dissertation, Virginia Tech. Accessed April 18, 2021. http://hdl.handle.net/10919/11227.

MLA Handbook (7^th Edition):

Zhao, Xiaopeng. “Modeling and Simulation of MEMS Devices.” 2004. Web. 18 Apr 2021.

Vancouver:

Zhao X. Modeling and Simulation of MEMS Devices. [Internet] [Doctoral dissertation]. Virginia Tech; 2004. [cited 2021 Apr 18]. Available from: http://hdl.handle.net/10919/11227.

Council of Science Editors:

Zhao X. Modeling and Simulation of MEMS Devices. [Doctoral Dissertation]. Virginia Tech; 2004. Available from: http://hdl.handle.net/10919/11227

Virginia Tech

7. El-Okda, Yasser Mohamed. Experimental and Numerical Investigations of the Effects of Incident Turbulence on the Flow Over a Surface-Mounted Prism.

Degree: PhD, Engineering Science and Mechanics, 2005, Virginia Tech

URL: http://hdl.handle.net/10919/26400

► The issue of the effects of free stream turbulence on the flow field over a surface-mounted prism is examined through experimental and numerical investigations. In… (more)

▼ The issue of the effects of free stream turbulence on the flow field over a surface-mounted prism is examined through experimental and numerical investigations. In the experimental studies, particle image velocimetry measurements are conducted in the ESM water tunnel at Reynolds number of 9,600 and under two cases of turbulent inflow conditions. The results show that the mean flow separation, reattachment and parameters such as mean velocity, root mean square, Reynolds stresses and turbulent kinetic energy are affected by the turbulence characteristics of the incident flow. The instantaneous dynamics of the interactions between the separating shear layer and the solid wall and between the shear layer and the turbulence in the incident flow are detailed. In the numerical studies, large eddy simulations of the flow over a surface-mounted prism under two inflow conditions, namely, smooth inflow and isotropic homogeneous turbulence inflow, are performed. The use of a fifth-order scheme (CUD-II-5), which is a member of a family of Compact Upwind Difference schemes, in large eddy simulations of this flow is assessed. The performance of this scheme is validated by comparing the rate of temporal decay of isotropic turbulence with available experimental measurements for grid-generated turbulence. The results show that the spectra are sensitive to the method of flux vector splitting needed for the implementation of the upwind scheme. With van Leer splitting, the CUD-II-5 scheme is found to be too dissipative. On the other hand, using the Lax-Friedrichs vector splitting yields good agreement with experiments by controlling the level of artificial dissipation. This led us to recommend a new procedure, we denote by C6CUD5 scheme, that combines a compact sixth-order scheme with the CUD-II-5 scheme for large eddy simulation of complex flows. The simulation results, including flow patterns, pressure fields and turbulence statistics show that the CUD-II-5 scheme, with Lax-Friedricks flux vector splitting, provides high resolution of local flow structures. The results present new physical aspects of the flow topology over surface-mounted prisms. The effects of the incident homogeneous turbulence on the size of the separation region and suction pressures are determined by pointing out differences in the flow topologies between the two incident flow cases. Advisors/Committee Members: Morris, Don H. (committee member), Masoud, Ziyad N. (committee member), Devenport, William J. (committee member), Ragab, Saad A. (committeecochair), Hajj, Muhammad R. (committeecochair).

Subjects/Keywords: Low-Rise Structures; Surface-Mounted Prism; Compact Upwind Schemes; Large Eddy Simulation; Particle Image Velocimetry

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APA (6^th Edition):

El-Okda, Y. M. (2005). Experimental and Numerical Investigations of the Effects of Incident Turbulence on the Flow Over a Surface-Mounted Prism. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/26400

Chicago Manual of Style (16^th Edition):

El-Okda, Yasser Mohamed. “Experimental and Numerical Investigations of the Effects of Incident Turbulence on the Flow Over a Surface-Mounted Prism.” 2005. Doctoral Dissertation, Virginia Tech. Accessed April 18, 2021. http://hdl.handle.net/10919/26400.

MLA Handbook (7^th Edition):

El-Okda, Yasser Mohamed. “Experimental and Numerical Investigations of the Effects of Incident Turbulence on the Flow Over a Surface-Mounted Prism.” 2005. Web. 18 Apr 2021.

Vancouver:

El-Okda YM. Experimental and Numerical Investigations of the Effects of Incident Turbulence on the Flow Over a Surface-Mounted Prism. [Internet] [Doctoral dissertation]. Virginia Tech; 2005. [cited 2021 Apr 18]. Available from: http://hdl.handle.net/10919/26400.

Council of Science Editors:

El-Okda YM. Experimental and Numerical Investigations of the Effects of Incident Turbulence on the Flow Over a Surface-Mounted Prism. [Doctoral Dissertation]. Virginia Tech; 2005. Available from: http://hdl.handle.net/10919/26400

Virginia Tech

8. Alhazza, Khaled. Nonlinear Vibrations of Doubly Curved Cross-PLy Shallow Shells.

Degree: PhD, Mechanical Engineering, 2002, Virginia Tech

URL: http://hdl.handle.net/10919/30042

► The objective of this work is to study the local and global nonlinear vibrations of isotropic single-layered and multi-layered cross-ply doubly curved shallow shells with… (more)

▼ The objective of this work is to study the local and global nonlinear vibrations of isotropic single-layered and multi-layered cross-ply doubly curved shallow shells with simply supported boundary conditions. The study is based-on the full nonlinear partial-differential equations of motion for shells. These equations of motion are based-on the von Kármán-type geometric nonlinear theory and the first-order shear-deformation theory, they are developed by using a variational approach. Many approximate shell theories are presented. We used two approaches to study the responses of shells to a primary resonance: a direct approach and a discretization approach. In the discretization approach, the nonlinear partial-differential equations are discretized using the Galerkin procedure to reduce them to an infinite system of nonlinearly coupled second-order ordinary-differential equations. An approximate solution of this set is then obtained by using the method of multiple scales for the case of primary resonance. The resulting equations describing the modulations of the amplitude and phase of the excited mode are used to generate frequency- and force-response curves. The effect of the number of modes retained in the approximation on the predicted responses is discussed and the shortcomings of using low-order discretization models are demonstrated. In the direct approach, the method of multiple scales is applied directly to the nonlinear partial-differential equations of motion and associated boundary conditions for the same cases treated using the discretization approach. The results obtained from these two approaches are compared. For the global analysis, a finite number of equations are integrated numerically to calculate the limit cycles and their stability, and hence their bifurcations, using Floquet theory. The use of this theory requires integrating 2n+(2n)² nonlinear first-order ordinary-differential equations simultaneously, where n is the number of modes retained in the discretization. A convergence study is conducted to determine the number of modes needed to obtain robust results. The discretized system of equation are used to study the nonlinear vibrations of shells to subharmonic resonances of order one-half. The effect of the number of modes retained in the approximation is presented. Also, the effect of the number of layers on the shell parameters is shown. Modal interaction between the first and second modes in the case of a two-to-one internal resonance is investigated. We use the method of multiple scales to determine the modulation equations that govern the slow dynamics of the response. A pseudo-arclength scheme is used to determine the fixed points of the modulation equations and the stability of these fixed points is investigated. In some cases, the fixed points undergo Hopf bifurcations, which result in dynamic solutions. A combination of a long-time integration and Floquet theory is used to determine the detailed solution branches and chaotic solutions and their stability. The… Advisors/Committee Members: Masoud, Ziyad N. (committee member), Inman, Daniel J. (committee member), Leo, Donald J. (committee member), Nayfeh, Ali H. (committeecochair), Ahmadian, Mehdi (committeecochair).

Subjects/Keywords: Shallow Shells; Resonance; Nonlinear Vibrations; Galerkin Discretization; Modal Interaction

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APA (6^th Edition):

Alhazza, K. (2002). Nonlinear Vibrations of Doubly Curved Cross-PLy Shallow Shells. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/30042

Chicago Manual of Style (16^th Edition):

Alhazza, Khaled. “Nonlinear Vibrations of Doubly Curved Cross-PLy Shallow Shells.” 2002. Doctoral Dissertation, Virginia Tech. Accessed April 18, 2021. http://hdl.handle.net/10919/30042.

MLA Handbook (7^th Edition):

Alhazza, Khaled. “Nonlinear Vibrations of Doubly Curved Cross-PLy Shallow Shells.” 2002. Web. 18 Apr 2021.

Vancouver:

Alhazza K. Nonlinear Vibrations of Doubly Curved Cross-PLy Shallow Shells. [Internet] [Doctoral dissertation]. Virginia Tech; 2002. [cited 2021 Apr 18]. Available from: http://hdl.handle.net/10919/30042.

Council of Science Editors:

Alhazza K. Nonlinear Vibrations of Doubly Curved Cross-PLy Shallow Shells. [Doctoral Dissertation]. Virginia Tech; 2002. Available from: http://hdl.handle.net/10919/30042

Virginia Tech

9. Ge, Zhongfu. Analysis of surface pressure and velocity fluctuations in the flow over surface-mounted prisms.

Degree: PhD, Engineering Science and Mechanics, 2004, Virginia Tech

URL: http://hdl.handle.net/10919/25965

► The full-scale value of the Reynolds number associated with wind loads on structures is of the order of 10⁷. This is further complicated by the… (more)

▼ The full-scale value of the Reynolds number associated with wind loads on structures is of the order of 10⁷. This is further complicated by the high levels of turbulence fluctuations associated with strong winds. On the other hand, numerical and wind tunnel simulations are usually carried out at smaller values of Re. Consequently, the validation of these simulations should only be based on physical phenomena derived with tools capable of their identification. In this work, two physical aspects related to extreme wind loads on low-rise structures are examined. The first includes the statistical properties and prediction of pressure peaks. The second involves the identification of linear and nonlinear relations between pressure peaks and associated velocity fluctuations. The first part of this thesis is concerned with the statistical properties of surface pressure time series and their variations under different incident flow conditions. Various statistical tools, including space-time correlation, conditional sampling, the probability plot and the probability plot correlation coefficient, are used to characterize pressure peaks measured on the top surface of a surface-mounted prism. The results show that the Gamma distribution provides generally the best statistical description for the pressure time series, and that the method of moments is sufficient for determining its parameters. Additionally, the shape parameter of the Gamma distribution can be directly related to the incident flow conditions. As for prediction of pressure peaks, the results show that the probability of non-exceedence can best be derived from the Gumbel distribution. Two approaches for peak prediction, based on analysis of the parent pressure time series and of observed peaks, are presented. The prediction based on the parent time series yields more conservative estimates of the probability of non-exceedence. The second part of this thesis is concerned with determining the linear and nonlinear relations between pressure peaks and the velocity field. Validated by analytical test signals, the wavelet-based analysis is proven to be effective and accurate in detecting intermittent linear and nonlinear relations between the pressure and velocity fluctuations. In particular, intermittent linear and nonlinear velocity pressure relations are observed over the nondimensional frequency range fH/U<0.32. These results provide the basis for flow parameters and characteristics required in the simulation of the wind loads on structures. Advisors/Committee Members: Hajj, Muhammad R. (committeechair), Kohler, Werner E. (committee member), Masoud, Ziyad N. (committee member), Henneke, Edmund G. II (committee member), Ragab, Saad A. (committee member), Tieleman, Henry W. (committee member).

Subjects/Keywords: velocity and pressure fluctuations; higher-order spectra; wavelet; statistics; pressure coefficient; Low-rise structures; wind loads

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APA (6^th Edition):

Ge, Z. (2004). Analysis of surface pressure and velocity fluctuations in the flow over surface-mounted prisms. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/25965

Chicago Manual of Style (16^th Edition):

Ge, Zhongfu. “Analysis of surface pressure and velocity fluctuations in the flow over surface-mounted prisms.” 2004. Doctoral Dissertation, Virginia Tech. Accessed April 18, 2021. http://hdl.handle.net/10919/25965.

MLA Handbook (7^th Edition):

Ge, Zhongfu. “Analysis of surface pressure and velocity fluctuations in the flow over surface-mounted prisms.” 2004. Web. 18 Apr 2021.

Vancouver:

Ge Z. Analysis of surface pressure and velocity fluctuations in the flow over surface-mounted prisms. [Internet] [Doctoral dissertation]. Virginia Tech; 2004. [cited 2021 Apr 18]. Available from: http://hdl.handle.net/10919/25965.

Council of Science Editors:

Ge Z. Analysis of surface pressure and velocity fluctuations in the flow over surface-mounted prisms. [Doctoral Dissertation]. Virginia Tech; 2004. Available from: http://hdl.handle.net/10919/25965

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