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Virginia Tech
1.
Glesner, Colin Christopher.
Development of Magnetic Nozzle Simulations for Space Propulsion Applications.
Degree: MS, Aerospace Engineering, 2017, Virginia Tech
URL: http://hdl.handle.net/10919/74947 
► A means of space propulsion using the channeling of plasma by a divergent magnetic field, referred to as a magnetic nozzle has been explored by…
(more)
▼ A means of space propulsion using the channeling of plasma by a divergent magnetic field, referred to as a magnetic nozzle has been explored by a number of research groups. This research develops the capability to apply the high order accurate Runge-Kutta discontinuous Galerkin numerical method to the simulation of magnetic nozzles. The resistive magnetohydrodynamic model of plasma behavior is developed for these simulations. To facilitate this work, several modeling capabilities are developed, including the implementation of appropriate inflow and far-field boundary conditions, the application of a technique for correcting errors that develop in the divergence of the magnetic field, and a split formulation for the magnetic field between the applied and the perturbed component. This model is then applied to perform a scaling study of the performance of magnetic nozzles over a range of Bk and Rm. In addition, the effect of the choice of simulation domain size is
investigated. Finally, recommendations for future work are made.
Advisors/Committee Members: Srinivasan, Bhuvana (committeechair), Scales, Wayne A. (committeechair), Adams, Colin (committee member).
Subjects/Keywords: Plasma; Simulation
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APA (6th Edition):
Glesner, C. C. (2017). Development of Magnetic Nozzle Simulations for Space Propulsion Applications. (Masters Thesis). Virginia Tech. Retrieved from http://hdl.handle.net/10919/74947
Chicago Manual of Style (16th Edition):
Glesner, Colin Christopher. “Development of Magnetic Nozzle Simulations for Space Propulsion Applications.” 2017. Masters Thesis, Virginia Tech. Accessed April 14, 2021.
http://hdl.handle.net/10919/74947.
MLA Handbook (7th Edition):
Glesner, Colin Christopher. “Development of Magnetic Nozzle Simulations for Space Propulsion Applications.” 2017. Web. 14 Apr 2021.
Vancouver:
Glesner CC. Development of Magnetic Nozzle Simulations for Space Propulsion Applications. [Internet] [Masters thesis]. Virginia Tech; 2017. [cited 2021 Apr 14].
Available from: http://hdl.handle.net/10919/74947.
Council of Science Editors:
Glesner CC. Development of Magnetic Nozzle Simulations for Space Propulsion Applications. [Masters Thesis]. Virginia Tech; 2017. Available from: http://hdl.handle.net/10919/74947
Virginia Tech
2.
Cagas, Petr.
Continuum Kinetic Simulations of Plasma Sheaths and Instabilities.
Degree: PhD, Aerospace Engineering, 2018, Virginia Tech
URL: http://hdl.handle.net/10919/84979
► A careful study of plasma-material interactions is essential to understand and improve the operation of devices where plasma contacts a wall such as plasma thrusters,…
(more)
▼ A careful study of plasma-material interactions is essential to understand and improve the operation of devices where plasma contacts a wall such as plasma thrusters, fusion devices, spacecraft-environment interactions, to name a few. This work aims to advance our understanding of fundamental plasma processes pertaining to plasma-material interactions, sheath physics, and kinetic instabilities through theory and novel numerical simulations. Key contributions of this work include (i) novel continuum kinetic algorithms with novel boundary conditions that directly discretize the Vlasov/Boltzmann equation using the discontinuous Galerkin method, (ii) fundamental studies of plasma sheath physics with collisions, ionization, and physics-based wall emission, and (iii) theoretical and numerical studies of the linear growth and nonlinear saturation of the kinetic Weibel instability, including its role in plasma sheaths.
The continuum kinetic algorithm has been shown to compare well with theoretical predictions of Landau damping of Langmuir waves and the two-stream instability. Benchmarks are also performed using the electromagnetic Weibel instability and excellent agreement is found between theory and simulation. The role of the electric field is significant during nonlinear saturation of the Weibel instability, something that was not noted in previous studies of the Weibel instability. For some plasma parameters, the electric field energy can approach magnitudes of the magnetic field energy during the nonlinear phase of the Weibel instability.
A significant focus is put on understanding plasma sheath physics which is essential for studying plasma-material interactions. Initial simulations are performed using a baseline collisionless kinetic model to match classical sheath theory and the Bohm criterion. Following this, a collision operator and volumetric physics-based source terms are introduced and effects of heat flux are briefly discussed. Novel boundary conditions are developed and included in a general manner with the continuum kinetic algorithm for bounded plasma simulations. A physics-based wall emission model based on first principles from quantum mechanics is self-consistently implemented and demonstrated to significantly impact sheath physics. These are the first continuum kinetic simulations using self-consistent, wall emission boundary conditions with broad applicability across a variety of regimes.
Advisors/Committee Members: Srinivasan, Bhuvana (committeechair), Adams, Colin (committee member), Scales, Wayne A. (committee member), Warburton, Timothy (committee member).
Subjects/Keywords: plasma sheath; discontinuous Galerkin; continuum kinetic method; plasma instabilities; plasma-material interactions
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APA (6th Edition):
Cagas, P. (2018). Continuum Kinetic Simulations of Plasma Sheaths and Instabilities. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/84979
Chicago Manual of Style (16th Edition):
Cagas, Petr. “Continuum Kinetic Simulations of Plasma Sheaths and Instabilities.” 2018. Doctoral Dissertation, Virginia Tech. Accessed April 14, 2021.
http://hdl.handle.net/10919/84979.
MLA Handbook (7th Edition):
Cagas, Petr. “Continuum Kinetic Simulations of Plasma Sheaths and Instabilities.” 2018. Web. 14 Apr 2021.
Vancouver:
Cagas P. Continuum Kinetic Simulations of Plasma Sheaths and Instabilities. [Internet] [Doctoral dissertation]. Virginia Tech; 2018. [cited 2021 Apr 14].
Available from: http://hdl.handle.net/10919/84979.
Council of Science Editors:
Cagas P. Continuum Kinetic Simulations of Plasma Sheaths and Instabilities. [Doctoral Dissertation]. Virginia Tech; 2018. Available from: http://hdl.handle.net/10919/84979
Virginia Tech
3.
Jain, Prachi Lalit.
Characterization of a Low Current LaB6 Heaterless Hollow Cathode with Krypton Propellant.
Degree: MS, Aerospace Engineering, 2020, Virginia Tech
URL: http://hdl.handle.net/10919/99141
► Recent years have seen rapid growth in the development of both stand-alone satellites and satellite constellations. A critical component of these satellites is the on-board…
(more)
▼ Recent years have seen rapid growth in the development of both stand-alone satellites and satellite constellations. A critical component of these satellites is the on-board propulsion system, which is responsible for controlling their orientation with respect to the object of interest and keeping the spacecraft in the assigned orbit. Generally, electric propulsion systems are used for this purpose. These types of propulsion systems use electrical power to change the velocity of satellite, providing a small thrust for a long duration of time as compared to chemical propulsion systems.
Certain types of electric thrusters utilize a hollow cathode device as an electron source to start-off and support the thruster operation. In this research, a non-conventional hollow cathode for low power applications is developed and tested. The main characteristic of the developed cathode is the heaterless configuration, which eliminates the heater module used in conventional cathodes to enable the cathode to reach its operational temperature. The absence of a heater reduces the complexity of the cathode and the electrical power system. The cathode utilizes an electron emitter material which is insensitive to impurities and air exposure. Additionally, unlike typical electric thrusters which use xenon as the fuel, this cathode uses krypton which is similar to xenon but is less expensive.
The presented work includes an overview of electric propulsion and the hollow cathode operation, followed by a detailed discussion of the heaterless hollow cathode design, the experimental setup and the test results. Several noteworthy findings regarding cathode operation are included as well. This research shows that the non-conventional heaterless hollow cathode and its operation with krypton have the potential to improve the overall thruster performance by reducing the weight and the cost, thus contributing to an integral aspect of satellite on-board propulsion.
Advisors/Committee Members: Adams, Colin (committeechair), Earle, Gregory D. (committee member), Srinivasan, Bhuvana (committee member).
Subjects/Keywords: Electric Propulsion; Hollow Cathode; Krypton; Lanthanum hexaboride
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APA (6th Edition):
Jain, P. L. (2020). Characterization of a Low Current LaB6 Heaterless Hollow Cathode with Krypton Propellant. (Masters Thesis). Virginia Tech. Retrieved from http://hdl.handle.net/10919/99141
Chicago Manual of Style (16th Edition):
Jain, Prachi Lalit. “Characterization of a Low Current LaB6 Heaterless Hollow Cathode with Krypton Propellant.” 2020. Masters Thesis, Virginia Tech. Accessed April 14, 2021.
http://hdl.handle.net/10919/99141.
MLA Handbook (7th Edition):
Jain, Prachi Lalit. “Characterization of a Low Current LaB6 Heaterless Hollow Cathode with Krypton Propellant.” 2020. Web. 14 Apr 2021.
Vancouver:
Jain PL. Characterization of a Low Current LaB6 Heaterless Hollow Cathode with Krypton Propellant. [Internet] [Masters thesis]. Virginia Tech; 2020. [cited 2021 Apr 14].
Available from: http://hdl.handle.net/10919/99141.
Council of Science Editors:
Jain PL. Characterization of a Low Current LaB6 Heaterless Hollow Cathode with Krypton Propellant. [Masters Thesis]. Virginia Tech; 2020. Available from: http://hdl.handle.net/10919/99141
4.
Nikrant, Alex Warner.
Development and Modelling of a Low Current LaB6 Heaterless Hollow Cathode.
Degree: MS, Aerospace Engineering, 2019, Virginia Tech
URL: http://hdl.handle.net/10919/93959
► In recent years, the space industry has seen rapidly accelerating growth due to the continuing advancement of technology. A critical area of spacecraft technology is…
(more)
▼ In recent years, the space industry has seen rapidly accelerating growth due to the continuing advancement of technology. A critical area of spacecraft technology is the spacecraft’s propulsion system, which allows the vehicle to achieve and maintain its desired orbit or trajectory through space. One class of propulsion systems known as “electric propulsion” uses electrical power to accelerate the fuel of the spacecraft. These types of propulsion systems are far more efficient than traditional propulsion systems, which use chemical reactions to create thrust. One of the main components of certain types of electric propulsion systems is the hollow cathode, which initiates and sustains the thruster operation. In this research, a hollow cathode with several non-conventional characteristics is developed and tested. First of all, standard hollow cathodes use a heater to bring the cathode up to operational temperature, but this design is heaterless which offers several benefits to the cathode and electrical power system designs. Secondly, the cathode uses a non-conventional choice of material for the “emitter”, which emits electrons when heated and allows the cathode to operate. Lastly, while typical electric propulsion systems use xenon for fuel, this cathode uses argon which has several benefits over xenon including cost. An overview of electric propulsion is presented, as well as a new physics-based model of this type of cathode that allows useful calculations based on simple measurements. The design and test results of the cathode are discussed in detail, with several interesting and insightful behaviors that were noted during testing. Heaterless cathodes have the potential to improve the efficiency, cost, and weight of electric propulsion systems, and this research therefore contributes to an important field for the future of space exploration.
Advisors/Committee Members: Adams, Colin (committeechair), Scales, Wayne A. (committee member), Srinivasan, Bhuvana (committee member).
Subjects/Keywords: heaterless hollow cathode; electric propulsion; plasma dynamics; plume mode
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APA (6th Edition):
Nikrant, A. W. (2019). Development and Modelling of a Low Current LaB6 Heaterless Hollow Cathode. (Masters Thesis). Virginia Tech. Retrieved from http://hdl.handle.net/10919/93959
Chicago Manual of Style (16th Edition):
Nikrant, Alex Warner. “Development and Modelling of a Low Current LaB6 Heaterless Hollow Cathode.” 2019. Masters Thesis, Virginia Tech. Accessed April 14, 2021.
http://hdl.handle.net/10919/93959.
MLA Handbook (7th Edition):
Nikrant, Alex Warner. “Development and Modelling of a Low Current LaB6 Heaterless Hollow Cathode.” 2019. Web. 14 Apr 2021.
Vancouver:
Nikrant AW. Development and Modelling of a Low Current LaB6 Heaterless Hollow Cathode. [Internet] [Masters thesis]. Virginia Tech; 2019. [cited 2021 Apr 14].
Available from: http://hdl.handle.net/10919/93959.
Council of Science Editors:
Nikrant AW. Development and Modelling of a Low Current LaB6 Heaterless Hollow Cathode. [Masters Thesis]. Virginia Tech; 2019. Available from: http://hdl.handle.net/10919/93959
5.
Napiecek, Andrew Webster.
Spatial Resolution of Equatorial Plasma Depletions Using Variable-Range Time-Delay Integration.
Degree: MS, Aerospace Engineering, 2019, Virginia Tech
URL: http://hdl.handle.net/10919/90221
► Equatorial spread-F, also termed plasma bubbles, is a phenomenon that occurs in the equatorial region of Earth’s ionosphere, the charged region of Earth’s atmosphere. Plumes…
(more)
▼ Equatorial spread-F, also termed plasma bubbles, is a phenomenon that occurs in the equatorial region of Earth’s ionosphere, the charged region of Earth’s atmosphere. Plumes of less dense plasma, the charged material of the Ionosphere, rise through regions of higher density plasma. This causes disturbances to radio signals that travel through this region, which can lead to GPS range errors or loss of signal. ICON is a NASA Explorer mission aimed at, in part, understanding the sources of variability in the ionosphere. One instrument onboard ICON to accomplish this goal is the FarUltraviolet Imager which images airglow in the far-ultraviolet range. During nighttime, the FUV imager can observe plasma bubbles to study the instability and the mechanisms that produce it. This thesis looks at the ability of the variable-range time-delay integration (TDI) algorithm, used to produce images from ICON’s Farultraviolet imager, to spatially resolve the structure and gradients of observed plasma bubbles. However, due to the viewing geometry of ICON’s FUV imager, each pixel across the observed scene experiences a different angular rate of motion blur. The variable-range TDI algorithm removes this non-uniform motion blur by transforming each raw image onto a surface where the spacecraft moves at a constant angular rate with respect to every pixel in the image. Then raw images are integrated together such that the observed scene is not geographically distorted. It was concluded that the TDI process is able to spatially resolve a wide variety of plasma bubbles under various ionospheric conditions and imager configurations.
Advisors/Committee Members: England, Scott L. (committeechair), Black, Jonathan T. (committee member), Srinivasan, Bhuvana (committee member).
Subjects/Keywords: Time-Delay Integration; Plasma Bubbles; Plasma Imaging
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APA (6th Edition):
Napiecek, A. W. (2019). Spatial Resolution of Equatorial Plasma Depletions Using Variable-Range Time-Delay Integration. (Masters Thesis). Virginia Tech. Retrieved from http://hdl.handle.net/10919/90221
Chicago Manual of Style (16th Edition):
Napiecek, Andrew Webster. “Spatial Resolution of Equatorial Plasma Depletions Using Variable-Range Time-Delay Integration.” 2019. Masters Thesis, Virginia Tech. Accessed April 14, 2021.
http://hdl.handle.net/10919/90221.
MLA Handbook (7th Edition):
Napiecek, Andrew Webster. “Spatial Resolution of Equatorial Plasma Depletions Using Variable-Range Time-Delay Integration.” 2019. Web. 14 Apr 2021.
Vancouver:
Napiecek AW. Spatial Resolution of Equatorial Plasma Depletions Using Variable-Range Time-Delay Integration. [Internet] [Masters thesis]. Virginia Tech; 2019. [cited 2021 Apr 14].
Available from: http://hdl.handle.net/10919/90221.
Council of Science Editors:
Napiecek AW. Spatial Resolution of Equatorial Plasma Depletions Using Variable-Range Time-Delay Integration. [Masters Thesis]. Virginia Tech; 2019. Available from: http://hdl.handle.net/10919/90221
Virginia Tech
6.
Robertson, Ellen Faith.
Validation and Characterization of a Laboratory Ion Source for Testing Thermal Space-Plasma Instruments.
Degree: PhD, Electrical Engineering, 2019, Virginia Tech
URL: http://hdl.handle.net/10919/94631
► This dissertation explores ways to improve autonomous navigation in unstructured terrain conditions, with specific applications to unmanned casualty extraction in disaster scenarios. Search and rescue…
(more)
▼ This dissertation explores ways to improve autonomous navigation in unstructured terrain conditions, with specific applications to unmanned casualty extraction in disaster scenarios. Search and rescue applications often put the lives of first responders at risk. Using robotic systems for human rescue in disaster scenarios can keep first responders out of danger. To enable safe robotic casualty extraction, this dissertation proposes a novel rescue robot design concept named SAVER. The proposed design concept consists of several subsystems including a declining stretcher bed, head and neck support system, and robotic arms that conceptually enable safe casualty manipulation and extraction based on high-level commands issued by a remote operator. In order to enable autonomous navigation of the proposed conceptual system in challenging outdoor terrain conditions, this dissertation proposes improvements in planning, trajectory tracking control and terrain estimation. The proposed techniques are able to take into account the dynamic effects of robot-terrain interaction including slip experienced by the vehicle, slope of the terrain and actuator limitations. The proposed techniques have been validated through simulations and experiments in indoor and simple outdoor terrain conditions. The applicability of the above techniques in improving tele-operation of rescue robotic systems in unstructured terrain is also discussed at the end of this dissertation.
Advisors/Committee Members: Earle, Gregory D. (committeechair), Tront, Joseph G. (committee member), Srinivasan, Bhuvana (committee member), Scales, Wayne A. (committee member), Bailey, Scott M. (committee member).
Subjects/Keywords: Space Science; Ion source; Simulations; Plasma; Vacuum Testing; Satellite Instrumentation
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APA (6th Edition):
Robertson, E. F. (2019). Validation and Characterization of a Laboratory Ion Source for Testing Thermal Space-Plasma Instruments. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/94631
Chicago Manual of Style (16th Edition):
Robertson, Ellen Faith. “Validation and Characterization of a Laboratory Ion Source for Testing Thermal Space-Plasma Instruments.” 2019. Doctoral Dissertation, Virginia Tech. Accessed April 14, 2021.
http://hdl.handle.net/10919/94631.
MLA Handbook (7th Edition):
Robertson, Ellen Faith. “Validation and Characterization of a Laboratory Ion Source for Testing Thermal Space-Plasma Instruments.” 2019. Web. 14 Apr 2021.
Vancouver:
Robertson EF. Validation and Characterization of a Laboratory Ion Source for Testing Thermal Space-Plasma Instruments. [Internet] [Doctoral dissertation]. Virginia Tech; 2019. [cited 2021 Apr 14].
Available from: http://hdl.handle.net/10919/94631.
Council of Science Editors:
Robertson EF. Validation and Characterization of a Laboratory Ion Source for Testing Thermal Space-Plasma Instruments. [Doctoral Dissertation]. Virginia Tech; 2019. Available from: http://hdl.handle.net/10919/94631
Virginia Tech
7.
Schneider, Maximilian Kurt.
Characterization of Collisional Shock Structures Induced by the Stagnation of Railgun-driven Multi-ion-species Plasma-jets.
Degree: PhD, Aerospace Engineering, 2020, Virginia Tech
URL: http://hdl.handle.net/10919/96551
► Plasma, the so-called fourth state of matter, is an ionized gas that often behaves like a fluid but can also become magnetized and carry an…
(more)
▼ Plasma, the so-called fourth state of matter, is an ionized gas that often behaves like a fluid but can also become magnetized and carry an electric current. This combination leads to a lot of interesting yet often un-intuitive physics, the study of which is very important for understanding a wide array of topics. One subset of this field is the study of shock-wave induced species separation. Just like the shock-wave a jet aircraft produces when it moves through the air at a speed greater than the speed of sound, a plasma shock is characterized by a large change in parameters like density, temperature, and pressure across a very small region. A shock-wave propagating through a plasma can cause different ion species present to separate out, a phenomenon that is driven by the gradients that are present across a shock front. Understanding how these mechanisms work is important to a number of applications, including fusion energy research and astrophysical events. The first section of this work discusses the design and development of a plasma-armature railgun, a device that can produce and accelerate jets of plasma to high-Mach-number within a vacuum chamber. The next and most substantive section of the work presents results from experimental campaigns to characterize the accelerated plasma jets and then to induce plasma-jet collisions with the hope of producing shock-waves that exist on time and spatial scales that can be readily measured in a laboratory setting. This work is a foundation for future experimental attempts to measure separation induced by a shock-wave in order to better understand these complex phenomena.
Advisors/Committee Members: Adams, Colin (committeechair), Scales, Wayne A. (committee member), Lowe, Kevin T. (committee member), Srinivasan, Bhuvana (committee member).
Subjects/Keywords: plasma-jet; shock-wave; plasma railgun; multi-ion-species plasma
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APA (6th Edition):
Schneider, M. K. (2020). Characterization of Collisional Shock Structures Induced by the Stagnation of Railgun-driven Multi-ion-species Plasma-jets. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/96551
Chicago Manual of Style (16th Edition):
Schneider, Maximilian Kurt. “Characterization of Collisional Shock Structures Induced by the Stagnation of Railgun-driven Multi-ion-species Plasma-jets.” 2020. Doctoral Dissertation, Virginia Tech. Accessed April 14, 2021.
http://hdl.handle.net/10919/96551.
MLA Handbook (7th Edition):
Schneider, Maximilian Kurt. “Characterization of Collisional Shock Structures Induced by the Stagnation of Railgun-driven Multi-ion-species Plasma-jets.” 2020. Web. 14 Apr 2021.
Vancouver:
Schneider MK. Characterization of Collisional Shock Structures Induced by the Stagnation of Railgun-driven Multi-ion-species Plasma-jets. [Internet] [Doctoral dissertation]. Virginia Tech; 2020. [cited 2021 Apr 14].
Available from: http://hdl.handle.net/10919/96551.
Council of Science Editors:
Schneider MK. Characterization of Collisional Shock Structures Induced by the Stagnation of Railgun-driven Multi-ion-species Plasma-jets. [Doctoral Dissertation]. Virginia Tech; 2020. Available from: http://hdl.handle.net/10919/96551
8.
Tyson, William Conrad.
On Numerical Error Estimation for the Finite-Volume Method with an Application to Computational Fluid Dynamics.
Degree: PhD, Aerospace Engineering, 2018, Virginia Tech
URL: http://hdl.handle.net/10919/86193
► Computational fluid dynamics (CFD) is a branch of computational physics at the intersection of fluid mechanics and scientific computing in which the governing equations of…
(more)
▼ Computational fluid dynamics (CFD) is a branch of computational physics at the intersection of fluid mechanics and scientific computing in which the governing equations of fluid motion, such as the Euler and Navier-Stokes equations, are solved numerically on a computer. CFD is utilized in numerous fields including biomedical engineering, meteorology, oceanography, and aerospace engineering. CFD simulations can provide tremendous insight into physical processes and are often preferred over experiments because they can be performed more quickly, are typically more cost-effective, and can provide data in regions where it may be difficult to measure. While CFD can be an extremely powerful tool, CFD simulations are inherently subject to numerical errors. These errors, which are generated when the governing equations of fluid motion are solved on a computer, can have a significant impact on the accuracy of a CFD solution. If numerical errors are not accurately quantified, ill-informed decision-making can lead to poor system performance, increased risk of injury, or even system failure. In this work, research efforts are focused on numerical error estimation for the finite -volume method, arguably the most widely used numerical algorithm for solving CFD problems. The error estimation techniques provided herein target discretization error, the largest
contributor to the overall numerical error in a given simulation. Discretization error can be very difficult to estimate since these errors are generated, convected, and diffused by the same physical processes embedded in the governing equations. In this work, improvements are made to the accuracy and efficiency of existing discretization error estimation techniques. Results are presented for several inviscid and viscous flow problems. To facilitate the study of these error estimators, a new, higher-order finite -volume solver is developed. A detailed description of the code base is provided along with a discussion of best practices for CFD code design.
Advisors/Committee Members: Roy, Christopher John (committeechair), Xiao, Heng (committee member), Borggaard, Jeffrey T. (committee member), Srinivasan, Bhuvana (committee member).
Subjects/Keywords: Computational Fluid Dynamics; Discretization Error Estimation; Truncation Error Estimation; Adjoint Methods; Numerical Error Estimation
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APA (6th Edition):
Tyson, W. C. (2018). On Numerical Error Estimation for the Finite-Volume Method with an Application to Computational Fluid Dynamics. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/86193
Chicago Manual of Style (16th Edition):
Tyson, William Conrad. “On Numerical Error Estimation for the Finite-Volume Method with an Application to Computational Fluid Dynamics.” 2018. Doctoral Dissertation, Virginia Tech. Accessed April 14, 2021.
http://hdl.handle.net/10919/86193.
MLA Handbook (7th Edition):
Tyson, William Conrad. “On Numerical Error Estimation for the Finite-Volume Method with an Application to Computational Fluid Dynamics.” 2018. Web. 14 Apr 2021.
Vancouver:
Tyson WC. On Numerical Error Estimation for the Finite-Volume Method with an Application to Computational Fluid Dynamics. [Internet] [Doctoral dissertation]. Virginia Tech; 2018. [cited 2021 Apr 14].
Available from: http://hdl.handle.net/10919/86193.
Council of Science Editors:
Tyson WC. On Numerical Error Estimation for the Finite-Volume Method with an Application to Computational Fluid Dynamics. [Doctoral Dissertation]. Virginia Tech; 2018. Available from: http://hdl.handle.net/10919/86193
Virginia Tech
9.
Edwards, Thomas Raymond.
Empirical Ionospheric Models: The Road To Conductivity.
Degree: PhD, Electrical Engineering, 2019, Virginia Tech
URL: http://hdl.handle.net/10919/100284
► The Earth's polar ionosphere is a highly dynamic region of the upper atmosphere, and acts as the closure of the greater magnetospheric current system. This…
(more)
▼ The Earth's polar ionosphere is a highly dynamic region of the upper atmosphere, and acts as the closure of the greater magnetospheric current system. This region plays host to many electrodynamic effects that impact terrestrial systems, such as power grids, railroads, and pipelines. These effects are fundamentally related to the currents, electric fields, and conductivity present in the polar ionosphere. Understanding and predicting the electrodynamics of this region is vital to being able to determine the physical impacts on terrestrial systems and provide predictions to government and commercial entities.
Empirical models play a key role in the research and forecasting of the solar wind and interplanetary magnetic field's impact on the polar ionosphere, and is an active area of development and research. Recent interest in polar ionospheric conductivity has led to a community-wide campaign to develop our understanding of this portion of the electrodynamic system.
Characterizing the interactions between the solar wind and the polar ionosphere is a difficult task, as the region of interest is highly data starved in many respects. In particular, satellite based data products are scarce due to being costly and logistically difficult. Recent advancements in data sources (such as the Swarm and CHAMP satellite missions) as well as continued research into the physical relationships between solar wind and interplanetary magnetic field drivers have provided the opportunity to develop new, novel tools to study this region of interest. In this dissertation, two polar ionosphere models are described in Chapters 3 and 4, along with the original research that their construction has produced in Chapter 1. These two models are combined to provide a foundation for future research in this area, which is described in Chapter 5.
Advisors/Committee Members: Weimer, Daniel R. (committeechair), Srinivasan, Bhuvana (committee member), Bailey, Scott M. (committee member), Stilwell, Daniel J. (committee member), Clauer, C. Robert (committee member).
Subjects/Keywords: Ionospheric Electrodynamics; Empirical Modelling; Field-Aligned Currents; Ionosphere Electric Potential
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APA (6th Edition):
Edwards, T. R. (2019). Empirical Ionospheric Models: The Road To Conductivity. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/100284
Chicago Manual of Style (16th Edition):
Edwards, Thomas Raymond. “Empirical Ionospheric Models: The Road To Conductivity.” 2019. Doctoral Dissertation, Virginia Tech. Accessed April 14, 2021.
http://hdl.handle.net/10919/100284.
MLA Handbook (7th Edition):
Edwards, Thomas Raymond. “Empirical Ionospheric Models: The Road To Conductivity.” 2019. Web. 14 Apr 2021.
Vancouver:
Edwards TR. Empirical Ionospheric Models: The Road To Conductivity. [Internet] [Doctoral dissertation]. Virginia Tech; 2019. [cited 2021 Apr 14].
Available from: http://hdl.handle.net/10919/100284.
Council of Science Editors:
Edwards TR. Empirical Ionospheric Models: The Road To Conductivity. [Doctoral Dissertation]. Virginia Tech; 2019. Available from: http://hdl.handle.net/10919/100284
Virginia Tech
10.
Song, Yang.
Unstructured Nodal Discontinuous Galerkin Method for Convection-Diffusion Equations Applied to Neutral Fluids and Plasmas.
Degree: PhD, Aerospace Engineering, 2020, Virginia Tech
URL: http://hdl.handle.net/10919/99291
► High-energy density (HED) plasma science is an important area in studying astrophysical phenomena as well as laboratory phenomena such as those applicable to inertial confinement…
(more)
▼ High-energy density (HED) plasma science is an important area in studying astrophysical phenomena as well as laboratory phenomena such as those applicable to inertial confinement fusion (ICF). ICF plasmas undergo radial compression, with an aim of achieving fusion ignition, and are subject to a number of hydrodynamic instabilities that can significantly alter the implosion and prevent sufficient fusion reactions. An understanding of these instabilities and their mitigation mechanisms is important allow for a stable implosion in ICF experiments. This work aims to provide a high order accurate and robust numerical framework that can be used to study these instabilities through simulations.
The first half of this work aims to provide a detailed description of the numerical framework, texttt{PHORCE}. texttt{PHORCE} is a high order numerical package that can be used in solving convection-diffusion problems in neutral fluids and plasmas. Outstanding challenges exist in simulating high energy density (HED) hydrodynamics, where very large gradients exist in density, temperature, and transport coefficients (such as viscosity), and numerical instabilities arise from these region if there is no intervention. These instabilities may lead to inaccurate results or cause simulations to fail, especially for high-order numerical methods. Substantial work has been done in texttt{PHORCE} to improve its robustness in dealing with numerical instabilities. This includes the implementation and design of several high-order limiters. An novel algorithm is also proposed in this work to solve the diffusion term accurately and efficiently, which further enriches the physics that texttt{PHORCE} can investigate.
The second half of this work involves rigorous benchmarks and experimentally relevant simulations of hydrodynamic instabilities. Both advection and diffusion solvers are well verified through convergence studies. Hydrodynamic and plasma models implemented are also validated against results in existing literature. Rayleigh-Taylor instability growth with experimentally relevant parameters are performed on both planar and radially converging domains. Although this work is motivated by physics in HED hydrodynamics, the emphasis is placed on numerical models that are generally applicable across a wide variety of fields and disciplines.
Advisors/Committee Members: Srinivasan, Bhuvana (committeechair), Roy, Christopher John (committee member), Adams, Colin (committee member), Scales, Wayne A. (committee member).
Subjects/Keywords: Nodal Discontinuous Galerkin; Magnetohydrodynamics; Two-Fluid Plasma Model; Plasma Instabilities; Convection-Diffusion; Reconstruction; Mass of Fluid
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APA (6th Edition):
Song, Y. (2020). Unstructured Nodal Discontinuous Galerkin Method for Convection-Diffusion Equations Applied to Neutral Fluids and Plasmas. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/99291
Chicago Manual of Style (16th Edition):
Song, Yang. “Unstructured Nodal Discontinuous Galerkin Method for Convection-Diffusion Equations Applied to Neutral Fluids and Plasmas.” 2020. Doctoral Dissertation, Virginia Tech. Accessed April 14, 2021.
http://hdl.handle.net/10919/99291.
MLA Handbook (7th Edition):
Song, Yang. “Unstructured Nodal Discontinuous Galerkin Method for Convection-Diffusion Equations Applied to Neutral Fluids and Plasmas.” 2020. Web. 14 Apr 2021.
Vancouver:
Song Y. Unstructured Nodal Discontinuous Galerkin Method for Convection-Diffusion Equations Applied to Neutral Fluids and Plasmas. [Internet] [Doctoral dissertation]. Virginia Tech; 2020. [cited 2021 Apr 14].
Available from: http://hdl.handle.net/10919/99291.
Council of Science Editors:
Song Y. Unstructured Nodal Discontinuous Galerkin Method for Convection-Diffusion Equations Applied to Neutral Fluids and Plasmas. [Doctoral Dissertation]. Virginia Tech; 2020. Available from: http://hdl.handle.net/10919/99291
Virginia Tech
11.
Xue, Weicheng.
CPU/GPU Code Acceleration on Heterogeneous Systems and Code Verification for CFD Applications.
Degree: PhD, Aerospace Engineering, 2021, Virginia Tech
URL: http://hdl.handle.net/10919/102073
► Computational Fluid Dynamics (CFD) is a numerical method to solve fluid problems, which usually requires a large amount of computations. A large CFD problem can…
(more)
▼ Computational Fluid Dynamics (CFD) is a numerical method to solve fluid problems, which usually requires a large amount of computations. A large CFD problem can be decomposed into smaller sub-problems which are stored in discrete memory locations and accelerated by a large number of compute units. In addition to code acceleration, it is important to ensure that the code and algorithm are implemented correctly, which is called code verification. This dissertation focuses on the CFD code acceleration as well as the code verification for turbulence model implementation. In this dissertation, multiple Graphic Processing Units (GPUs) are utilized to accelerate two CFD codes, considering that the GPU has high computational power and high memory bandwidth. A variety of optimizations are developed and applied to improve the performance of CFD codes on different parallel computing systems. The program execution time can be reduced significantly especially when multiple GPUs are used. In addition, code-to-code comparisons with some NASA CFD codes and the method of manufactured solutions are utilized to verify the correctness of a research CFD code.
Advisors/Committee Members: Roy, Christopher John (committeechair), Wang, Kevin Guanyuan (committee member), De Sturler, Eric (committee member), Srinivasan, Bhuvana (committee member).
Subjects/Keywords: GPU; OpenACC; MPI; Domain Decomposition; Performance Optimization; GPUDirect; Code Verification; OOA; Discretization Error
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APA ·
Chicago ·
MLA ·
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CSE |
Export
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APA (6th Edition):
Xue, W. (2021). CPU/GPU Code Acceleration on Heterogeneous Systems and Code Verification for CFD Applications. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/102073
Chicago Manual of Style (16th Edition):
Xue, Weicheng. “CPU/GPU Code Acceleration on Heterogeneous Systems and Code Verification for CFD Applications.” 2021. Doctoral Dissertation, Virginia Tech. Accessed April 14, 2021.
http://hdl.handle.net/10919/102073.
MLA Handbook (7th Edition):
Xue, Weicheng. “CPU/GPU Code Acceleration on Heterogeneous Systems and Code Verification for CFD Applications.” 2021. Web. 14 Apr 2021.
Vancouver:
Xue W. CPU/GPU Code Acceleration on Heterogeneous Systems and Code Verification for CFD Applications. [Internet] [Doctoral dissertation]. Virginia Tech; 2021. [cited 2021 Apr 14].
Available from: http://hdl.handle.net/10919/102073.
Council of Science Editors:
Xue W. CPU/GPU Code Acceleration on Heterogeneous Systems and Code Verification for CFD Applications. [Doctoral Dissertation]. Virginia Tech; 2021. Available from: http://hdl.handle.net/10919/102073
Virginia Tech
12.
Shi, Xueling.
Occurrence Statistics and Driving Mechanisms of Ionospheric Ultra-Low Frequency Waves Observed by SuperDARN Radars.
Degree: PhD, Electrical Engineering, 2019, Virginia Tech
URL: http://hdl.handle.net/10919/100904
► Ultra-low frequency (ULF; 1 mHz - 1 Hz) waves are known to play an important role in the transfer of energy from the solar wind…
(more)
▼ Ultra-low frequency (ULF; 1 mHz - 1 Hz) waves are known to play an important role in the transfer of energy from the solar wind to Earth's magnetosphere and ionosphere. The Super Dual Auroral Radar Network (SuperDARN) is an international network consisting of 35 low-power high frequency (HF: 3-30 MHz) coherent scatter radars at middle to polar latitudes that look into Earth's upper atmosphere and ionosphere. In this study, we use Doppler velocity measurements obtained by the SuperDARN radars and coordinated spacecraft observations to investigate the occurrence statistics and driving mechanisms of ionospheric ULF waves. We begin in Chapter 2 with a case study of Pi2 pulsations which are short-duration (5-15 min) damped geomagnetic field oscillations with periods of 40-150 s. Simultaneous observations of Pi2 pulsations from THEMIS spacecraft, midlatitude SuperDARN radars, and ground magnetometers, together with analysis of their longitudinal polarization pattern and azimuthal phase propagation, confirmed that they are consistent with a plasmaspheric virtual resonance excited by a longitudinally localized source near midnight. In Chapter 3, to further investigate the overall occurrence of ionospheric ULF signatures, a comprehensive statistical study was conducted using an automated detection algorithm to identify ionospheric signatures of Pc3-4 and Pc5 waves over 7 years of high time resolution SuperDARN radar data. Specifically, we have investigated their spatial occurrence, frequency characteristics, seasonal factors, and dependence on solar wind and geomagnetic conditions. We note two particular findings: (i) an internal wave-particle interaction source is most likely responsible for Pc4 waves at high latitudes in the duskside ionosphere; and, (ii) a source associated with magnetotail dynamics during active geomagnetic times is suggested for Pc3-4/Pi2 waves at midlatitudes in the nightside ionosphere. These findings are further expanded in Chapter 4 which investigates the hypothesis that internal wave-particle interactions are an important source for generation of these waves. A case study of long-lasting poloidal waves was conducted using coordinated observations with the GOES and THEMIS satellites to examine the generation and propagation of waves observed in the dayside ionosphere by multiple SuperDARN radars. The source of wave excitation is suggested to be bump-on-tail ion distributions at 1-3 keV. Collectively, these research findings provide better constraints on where and when ionospheric ULF waves occur, their source mechanisms, and how they might affect magnetospheric and ionospheric dynamics.
Advisors/Committee Members: Ruohoniemi, John Michael (committeechair), Baker, Joseph B. H. (committeechair), Srinivasan, Bhuvana (committee member), Buehrer, R. Michael (committee member), Scales, Wayne A. (committee member), Clauer, C. Robert (committee member).
Subjects/Keywords: ULF waves; geomagnetic pulsations; SuperDARN; ionosphere; magnetosphere
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APA ·
Chicago ·
MLA ·
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CSE |
Export
to Zotero / EndNote / Reference
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APA (6th Edition):
Shi, X. (2019). Occurrence Statistics and Driving Mechanisms of Ionospheric Ultra-Low Frequency Waves Observed by SuperDARN Radars. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/100904
Chicago Manual of Style (16th Edition):
Shi, Xueling. “Occurrence Statistics and Driving Mechanisms of Ionospheric Ultra-Low Frequency Waves Observed by SuperDARN Radars.” 2019. Doctoral Dissertation, Virginia Tech. Accessed April 14, 2021.
http://hdl.handle.net/10919/100904.
MLA Handbook (7th Edition):
Shi, Xueling. “Occurrence Statistics and Driving Mechanisms of Ionospheric Ultra-Low Frequency Waves Observed by SuperDARN Radars.” 2019. Web. 14 Apr 2021.
Vancouver:
Shi X. Occurrence Statistics and Driving Mechanisms of Ionospheric Ultra-Low Frequency Waves Observed by SuperDARN Radars. [Internet] [Doctoral dissertation]. Virginia Tech; 2019. [cited 2021 Apr 14].
Available from: http://hdl.handle.net/10919/100904.
Council of Science Editors:
Shi X. Occurrence Statistics and Driving Mechanisms of Ionospheric Ultra-Low Frequency Waves Observed by SuperDARN Radars. [Doctoral Dissertation]. Virginia Tech; 2019. Available from: http://hdl.handle.net/10919/100904
Virginia Tech
13.
Lin, Dong.
Solar Wind-Magnetosphere-Ionosphere Coupling: Multiscale Study with Computational Models.
Degree: PhD, Electrical Engineering, 2019, Virginia Tech
URL: http://hdl.handle.net/10919/100903
► Solar wind-magnetosphere-ionosphere (SW-M-I) coupling is investigated with three different computational models that characterize space plasma dynamics on distinct spatial/temporal scales. These models are used to…
(more)
▼ Solar wind-magnetosphere-ionosphere (SW-M-I) coupling is investigated with three different computational models that characterize space plasma dynamics on distinct spatial/temporal scales. These models are used to explore three important aspects of SW-M-I coupling. A particle-in-cell (PIC) model has been developed to explore the kinetic scale dynamics associated with the magnetotail dipolarization front (DF), which is generated as a result of magnetotail reconnection. The PIC study demonstrates that the electron-ion hybrid (EIH) instability could relax the velocity shear within the DF via emitting lower hybrid waves. The velocity inhomogeneity driven instability is highlighted as an important mechanism for energy conversion and wave emission during the solar wind-magnetosphere coupling, which has been long neglected before. The Lyon-Fedder-Mobbary (LFM) global magnetohydrodynamic (MHD) model is used to explore the fluid scale electrodynamic response of the magnetosphere-ionosphere to the interplanetary electric field (IEF). It is found that the cross polar cap potential (CPCP) varies linearly with very large IEF if the solar wind density is high enough. With controlled experiments of global MHD modeling driven by observed parameters, the linearity was interpreted as a result of the magnetosheath force balance theory. This study highlights the role of solar wind density in the electrodynamic SW-M-I coupling under extreme driving conditions. The LFM-TIEGCM-RCM (LTR) model, which is the Coupled-Magnetosphere-Ionosphere-Thermosphere (CMIT) model with Ring Current extension, is used to explore the integrated SW-M-I system. The LTR simulation study focuses on the subauroral polarization streams (SAPS), which involve both MHD and non-MHD processes and three-way coupling in the SW-M-I system. The global structure and dynamic evolution of SAPS are illustrated with state-of-the-art first-principle models for the first time. This study has successfully utilized multiscale models to characterize the forefront issues in the space plasma dynamics, which is required by the facts that plasmas have both particle and fluid featured properties and those properties are vastly different across geospace regions. It is highlighted that SW-M-I coupling could be significantly influenced by both microscopic and macroscopic processes. In order for a comprehensive understanding of the SW-M-I coupling, multiscale models and integrated framework of their combinations are critical.
Advisors/Committee Members: Scales, Wayne A. (committeechair), Zhu, Yizheng (committee member), Ruohoniemi, John Michael (committee member), Baker, Joseph B. H. (committee member), Srinivasan, Bhuvana (committee member), Clauer, C. Robert (committee member).
Subjects/Keywords: SW-M-I coupling; Particle-in-cell model; Global MHD model; LTR model
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APA ·
Chicago ·
MLA ·
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CSE |
Export
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Manager
APA (6th Edition):
Lin, D. (2019). Solar Wind-Magnetosphere-Ionosphere Coupling: Multiscale Study with Computational Models. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/100903
Chicago Manual of Style (16th Edition):
Lin, Dong. “Solar Wind-Magnetosphere-Ionosphere Coupling: Multiscale Study with Computational Models.” 2019. Doctoral Dissertation, Virginia Tech. Accessed April 14, 2021.
http://hdl.handle.net/10919/100903.
MLA Handbook (7th Edition):
Lin, Dong. “Solar Wind-Magnetosphere-Ionosphere Coupling: Multiscale Study with Computational Models.” 2019. Web. 14 Apr 2021.
Vancouver:
Lin D. Solar Wind-Magnetosphere-Ionosphere Coupling: Multiscale Study with Computational Models. [Internet] [Doctoral dissertation]. Virginia Tech; 2019. [cited 2021 Apr 14].
Available from: http://hdl.handle.net/10919/100903.
Council of Science Editors:
Lin D. Solar Wind-Magnetosphere-Ionosphere Coupling: Multiscale Study with Computational Models. [Doctoral Dissertation]. Virginia Tech; 2019. Available from: http://hdl.handle.net/10919/100903
14.
Venkataramani, Karthik.
Modeling the Energetics of the Upper Atmosphere.
Degree: PhD, Electrical Engineering, 2018, Virginia Tech
URL: http://hdl.handle.net/10919/84400
► Nitric oxide (NO) is a minor species in the Earth’s atmosphere whose densities have been measured to closely reflect solar energy deposition above 100 km.…
(more)
▼ Nitric oxide (NO) is a minor species in the Earth’s atmosphere whose densities have been measured to closely reflect solar energy deposition above 100 km. It is an efficient emitter in the infrared where the thermosphere is optically thin, and serves as an important source of radiative cooling between 100 - 200 km. The primary mechanism of this cooling involves the conversion of kinetic energy from the background atmosphere into vibrational energy in NO, followed by the radiative de-excitation of the NO molecule. This results in the production of a 5.3 µm photon which escapes the thermosphere and results in a net cooling of the region. While this process causes the excitation of ground state NO to its first vibrational level, nascent vibrational excitation to the (v≥ 1) levels may also occur from the reactions that produce NO in the thermosphere. The NO(v≥ 1) molecules produced from this secondary process can undergo a radiative cascade and emit multiple photons, thus forming a significant fraction of the 5.3 µm emission from NO in the thermosphere.
Existing thermospheric models consider the collisional excitation of NO to be the only source of the 5.3 µm emission and assume the contribution from nascent excitation to be negligible. These models also tend to use a rate coefficient for the collisional excitation that is significantly larger than the values suggested in literature in order to obtain a temperature profile that is in agreement with empirical data. We address these discrepancies by presenting an updated calculation of the chemically produced emission by accounting for the v ≤ 10 level populations. By incorporating this process into a three dimensional global upper atmospheric model, it is shown that the additional emission contributes between 5 − 40% of the daytime emission from nitric oxide under quiet solar conditions, and is a significant source of energy loss during periods of enhanced solar energy deposition. Accounting for this process however does not resolve the model-data discrepancy seen with regards to the recovery times of thermospheric densities following geomagnetic storms, suggesting that an improved treatment of nitric oxide chemistry is required to resolve this issue.
In order to improve our understanding of the thermospheric energy budget, we also develop the Atmospheric Chemistry and Energetics (ACE) 1D model using up-to-date aeronomic results. The model self-consistently solves the 1D momentum and energy equations to produce a global average profile of the coupled thermosphere and ionosphere system in terms of its constituent densities and temperatures. The model calculations of neutral densities and exospheric temperatures are found to be in good agreement with empirical data for a wide range of solar activity.
It is concluded from the present work that while the magnitude of the chemically produced emission from nitric oxide has previously been underestimated, its effect on the thermospheric energy budget is relatively small. Including the secondary emission in thermospheric…
Advisors/Committee Members: Bailey, Scott M. (committeechair), Zhu, Yizheng (committee member), Baker, Joseph B. H. (committee member), Earle, Gregory D. (committee member), Srinivasan, Bhuvana (committee member).
Subjects/Keywords: Thermosphere; Thermospheric Modeling; Nitric Oxide; Infrared emissions
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APA ·
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MLA ·
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Export
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APA (6th Edition):
Venkataramani, K. (2018). Modeling the Energetics of the Upper Atmosphere. (Doctoral Dissertation). Virginia Tech. Retrieved from http://hdl.handle.net/10919/84400
Chicago Manual of Style (16th Edition):
Venkataramani, Karthik. “Modeling the Energetics of the Upper Atmosphere.” 2018. Doctoral Dissertation, Virginia Tech. Accessed April 14, 2021.
http://hdl.handle.net/10919/84400.
MLA Handbook (7th Edition):
Venkataramani, Karthik. “Modeling the Energetics of the Upper Atmosphere.” 2018. Web. 14 Apr 2021.
Vancouver:
Venkataramani K. Modeling the Energetics of the Upper Atmosphere. [Internet] [Doctoral dissertation]. Virginia Tech; 2018. [cited 2021 Apr 14].
Available from: http://hdl.handle.net/10919/84400.
Council of Science Editors:
Venkataramani K. Modeling the Energetics of the Upper Atmosphere. [Doctoral Dissertation]. Virginia Tech; 2018. Available from: http://hdl.handle.net/10919/84400
. 
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