Browsing by Author "Wang, Joseph J."
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- At The Water's Edge: The Grid in Coastal ConstructionWaltz, Christopher S. (Virginia Tech, 1999-07-29)The Outer Banks are a special place at the end of land and the beginning of water, and yet, most of the houses built on these islands seem to take no notice. A drive down the beach road reveals house after house that are essentially suburban dwellings raised above flood level on piles. This project proposes that oceanfront homes should not be pale imitations of inland housing, but rather as unique as the environment in which they exist. The architect must design for the long term needs of the client and use elements dictated by the environment as integral parts of the design to create a building that is both responsive to and reflective of the condition that occurs at the water's edge.
- Bilinear Immersed Finite Elements For Interface ProblemsHe, Xiaoming (Virginia Tech, 2009-04-20)In this dissertation we discuss bilinear immersed finite elements (IFE) for solving interface problems. The related research works can be categorized into three aspects: (1) the construction of the bilinear immersed finite element spaces; (2) numerical methods based on these IFE spaces for solving interface problems; and (3) the corresponding error analysis. All of these together form a solid foundation for the bilinear IFEs. The research on immersed finite elements is motivated by many real world applications, in which a simulation domain is often formed by several materials separated from each other by curves or surfaces while a mesh independent of interface instead of a body-fitting mesh is preferred. The bilinear IFE spaces are nonconforming finite element spaces and the mesh can be independent of interface. The error estimates for the interpolation of a Sobolev function in a bilinear IFE space indicate that this space has the usual approximation capability expected from bilinear polynomials, which is O(h²) in L² norm and O(h) in H¹ norm. Then the immersed spaces are applied in Galerkin, finite volume element (FVE) and discontinuous Galerkin (DG) methods for solving interface problems. Numerical examples show that these methods based on the bilinear IFE spaces have the same optimal convergence rates as those based on the standard bilinear finite element for solutions with certain smoothness. For the symmetric selective immersed discontinuous Galerkin method based on bilinear IFE, we have established its optimal convergence rate. For the Galerkin method based on bilinear IFE, we have also established its convergence. One of the important advantages of the discontinuous Galerkin method is its flexibility for both p and h mesh refinement. Because IFEs can use a mesh independent of interface, such as a structured mesh, the combination of a DG method and IFEs allows a flexible adaptive mesh independent of interface to be used for solving interface problems. That is, a mesh independent of interface can be refined wherever needed, such as around the interface and the singular source. We also develop an efficient selective immersed discontinuous Galerkin method. It uses the sophisticated discontinuous Galerkin formulation only around the locations needed, but uses the simpler Galerkin formulation everywhere else. This selective formulation leads to an algebraic system with far less unknowns than the immersed DG method without scarifying the accuracy; hence it is far more efficient than the conventional discontinuous Galerkin formulations.
- Development of a Parallel Electrostatic PIC Code for Modeling Electric PropulsionPierru, Julien (Virginia Tech, 2005-06-02)This thesis presents the parallel version of Coliseum, the Air Force Research Laboratory plasma simulation framework. The parallel code was designed to run large simulations on the world fastest supercomputers as well as home mode clusters. Plasma simulations are extremely computationally intensive as they require tracking millions of particles and solving field equations over large domains. This new parallel version will allow Coliseum to run simulations of spacecraft-plasma interactions in domain large enough to reproduce space conditions. The parallel code ran on two of the world fastest supercomputers, the NASA JPL Cosmos supercomputer ranked 37th on the TOP500 list and Virginia Tech's System X, ranked 7th. DRACO, the Virginia Tech PIC module to Coliseum, was modified with parallel algorithms to create a full parallel PIC code. A parallel solver was added to DRACO. It uses a Gauss-Seidel method with SOR acceleration on a Red-Black checkerboard scheme. Timing results were obtained on JPL Cosmos supercomputer to determine the efficiency of the parallel code. Although the communication overhead limits the code's parallel efficiency, the speed up obtained greatly decreases the time required to run the simulations. A speed up of 51 was reached on 128 processors. The parallel code was also used to simulate the plume expansion of an ion thruster array composed of three NSTAR thrusters. Results showed that the multiple beams merge to form a single plume similar to the plume created by a single ion thruster.
- Development of the DRACO ES-PIC code and Fully-Kinetic Simulation of Ion Beam NeutralizationBrieda, Lubos (Virginia Tech, 2005-06-02)This thesis describes development of the DRACO plasma simulation code. DRACO is an electro-static (ES) code which uses the particle-in-cell (PIC) formulation to track plasma particles through a computational domain, and operates within the Air Force COLISEUM framework. The particles are tracked on a non-standard mesh, which combines the benefits of a Cartesian mesh with the surface-resolving power of an unstructured mesh. DRACO contains its own mesher, called VOLCAR, which is also described in this work. DRACO was applied to a fully kinetic simulation of an ion-beam neutralization. The thruster configuration and running parameters were based on the NASA's 40cm NEXT ion thruster. The neutralization process was divided into three steps. Electron dynamics was studied by assuming an initial beam neutralization, which was accomplished by injecting both electrons and ions from the optics. Performing the simulation on a full-sized domain with cell size much greater than the Debye length resulted in a formation of a virtual anode. Decrease of the cell size to match the Debye length was not feasible, since it would require a million-fold increase in the number of simulation nodes. Instead, a scaling scheme was devised. Simulations were performed on thruster scaled down by a factor of 100, but its operating parameters were also adjusted such that the produced plasma environment did not change. Loss of electrons at the boundary of the finite simulation domain induced a numerical instability. The instability resulted in a strong axial electric field which sucked out electrons from the beam. It was removed by introducing an energy based particle boundary condition. Combination of surface scaling and energy boundary resulted in physically sound simulation results. Comparison were made between the Maxwellian and polytropic temperatures, as well as between simulation electron density and one predicted by the Boltzmann relationship. The cathode was modeled individually from the beam by introducing a positively charged collector plate at a distance corresponding to the beam edge. The local Debye length at the cathode tip was too small to be resolved by the mesh, even if mesh-refinement was incorporated. Since the simulation was not concerned with the near-tip region, two modifications were performed. First, the a limiting value of charge density at the tip was imposed. Second, the cathode potential was allowed to float. These two modifications were necessary to prevent development of a strong potential gradient at the cathode tip. The modified cathode model was combined with ion injection from the optics to model the actual beam neutralization. Three configurations were tested: a single thruster, a 2x2 cluster with individual cathodes and a similar cluster with a single large neutralizer. Neither of the cases achieved neutralization comparable to one in the base-line pre-neutralized case. The reason for the discrepancy is not known, but it does not seem to be due a loss of electrons at the walls. The difference could be due to limited extent of the modeled physics. An additional work is required to answer this question.
- Electric Propulsion Plume Simulations Using Parallel ComputerWang, Joseph J.; Cao, Yong; Kafafy, Raed; Decyk, Viktor (Hindawi, 2007-01-01)A parallel, three-dimensional electrostatic PIC code is developed for large-scale electric propulsion simulations using parallel supercomputers. This code uses a newly developed immersed-finite-element particle-in-cell (IFE-PIC) algorithm designed to handle complex boundary conditions accurately while maintaining the computational speed of the standard PIC code. Domain decomposition is used in both field solve and particle push to divide the computation among processors. Two simulations studies are presented to demonstrate the capability of the code. The first is a full particle simulation of near-thruster plume using real ion to electron mass ratio. The second is a high-resolution simulation of multiple ion thruster plume interactions for a realistic spacecraft using a domain enclosing the entire solar array panel. Performance benchmarks show that the IFE-PIC achieves a high parallel efficiency of ≥ 90%
- Electron Temperature Enhancement Effects on Plasma Irregularities Associated with Charged Dust in the Earth's MesosphereChen, Chen (Virginia Tech, 2007-11-26)Recently, experimental observations have shown that Polar Mesospheric Summer Echoes PMSE may be modulated by radio wave heating the irregularity source region with a ground-based ionospheric heating facilities. It is clear from these past investigations that the temporal behavior of PMSE during ionospheric heating shows promise as a diagnostic for the associated dust layer. To investigate the temporal behavior of plasma irregularities thought to produce PMSE, this work describes a new model that incorporates both finite diffusion time effects as well as dust charging. The hybrid model utilizes fluid ions described by continuity and momentum equations, electrons whose behavior is determined from quasi-neutrality, and charged dust described by the standard Particle-In-Cell PIC method. The model has been used to investigate the temporal behavior of charged dust associated electron irregularities during electron temperature enhancement associated with radio wave heating. The model predicts that the temporal behavior of the irregularities depends on the ratio of the electron-ion ambipolar diffusion time to the dust particle charging time Td/Tc. The results indicate that typically for Td/Tc << 1, an enhancement in electron irregularity amplitude occurs for a period after turn-off of the radio wave heating. The work also predicts that for Td/Tc >> 1, an enhancement in electron irregularity amplitude occurs for a time period after the turn-on of the radio wave heating. Due to the dependence of Td on irregularity scale-size, these results have important implications for observations of PMSE modification at different radar frequencies. Both continuous and discrete charging model were embedded into this computational model, the results were compared and analyzed. It is evident that significant diagnostic information may be available about the dust layer from the temporal behavior of the electron irregularities during the heating process which modifies the background electron temperature. Particularly interesting and important periods of the temporal behavior are during the turn-on and turn-off of the radio wave heating. Although a number of past theoretical and experimental investigations have considered both these on and off period, this dissertation considers further possibilities for diagnostic information available as well as the underlying physical processes. Approximate analytical models are developed and compared to a more accurate full computational model as a reference. Then from the temporal behavior of the electron irregularities during the turn-on and turn-off of the radio wave heating, the analytical models are used to obtain possible diagnostic information for various charged dust and background plasma quantities. Finally, two experiment campaigns have been performed at HAARP, Gakona, Alaska. Preliminary observation results look promising for the existence of PMSE turn-on overshoot. However, more careful experiments need to be done before firm conclusions can be drawn. The new designed Echotek digital receiver is ready for use now. It will be much superior to the experimental setup used for measurements in the previous campaign.Therefore, future experimental campaigns are planning next year to support the theoretical research.
- Experimental and Modeling Studies of Low-Energy Ion Sputtering for Ion ThrustersNakles, Michael Robert (Virginia Tech, 1999-08-30)This thesis investigates low-energy xenon-molybdenum (Xe+-Mo) sputtering yields for ion energies of 100 eV and less. Sputtering yield data at these energies are important for ion thruster design and lifetime prediction. The basic principles of sputtering phenomena are discussed. An overview of various popular types of experimental sputtering yield methods is presented with an emphasis on the techniques that have been used to find Xe+-Mo sputtering yields in the past. Sputtering yields in this study are found through both models and experiments. Sputtering yields are calculated using the Sigmund, Bohdansky, Yamamura, and Wilhelm formulas. The computed sputtering yields for these models varied widely at low-energy. TRIM (The TRansport of Ions in Matter), a Monte-Carlo simulation program, was adapted to study sputtering yields, and energy and angular distributions of sputtered atoms. Simulations were run at various combinations of ion energy and ion incidence angle. TRIM did not prove to be an adequate model for low-energy sputtering. Experimental measurements of sputtering were made using both Rutherford backscattering spectrometry (RBS) and mass-loss methods. Sputtering was performed in a small vacuum facility using an ion gun. For the RBS technique, sputtered material was collected on aluminum foil substrates. The area density of the deposited Mo film on the substrates was measured using RBS. These measurements enabled calculation of differential sputtering yields, which were integrated to find the total sputtering yield. Sputtering yield was found by the mass-loss technique by simply comparing the mass of the sample both before and after sputtering using a microbalance. Sputtering yields at 100 eV, 90 eV, 80 eV, 70 eV, and 60 eV were found using the RBS technique. The mass-loss technique was only successful in the 80 eV experiment. The experimental results were unexpected. The measured sputtering yields were significantly higher than those reported by other researchers. Also, sputtering yields were found to increase with decreasing ion energy from 90 eV down to 60 eV.
- Experimental Studies of Spacecraft Plasma Interactions: Facility Characterization and Initial MeasurementsSawyer, Samuel Thomas (Virginia Tech, 2009-06-09)The objectives of this thesis are to characterize the plasma environment of a new vacuum chamber facility at Virginia Tech and to perform initial measurements of plasma flow field for studying spacecraft-plasma interactions using this facility. An argon plasma environment was created using a hot filament cathode plasma source. Flange plates attached to the sides of the vacuum chamber were modified in order to attach various feedthroughs both now and in the future such that a probe array DAQ system could be used to expedite measurement and analysis. A Langmuir probe array was used to measure 3-D plasma flow field in the chamber. A Matlab code was developed for automatic evaluation of the Langmuir probe traces. Two sets of measurements were preformed. The first measurement characterizes the plasma produced by the hot filament cathode in the chamber. Langmuir Probes were used to characterize the plasma environment yielding the following average characteristics: Plasma Potential = 5.5486V, Electron Saturation Current = 0.003421A, Electron Temperature = 1.505eV, and the Plasma Density = 6.806*10^14 m^-3. It was found that for both the spherical and cylindrical probes used in the test facility Rs > Debye length and thus were analyzed under the thin sheath condition. The second measurement attempts to measure the 3-D plasma flow field for plasma flow over a structure composed of 4-inch biased Al box sitting on a biased Al plate. The results show signs of the the generation of the expansion pre-sheath structure at the leading edge of the plate and the box upper surface predicted by numerical models. However, the current diagnostics system does not have the spatial resolution and range as well as the data accuracy required to reach a definitive measurement of plasma presheath and plasma wake.
- Immersed Finite Element Particle-In-Cell Modeling of Surface Charging in Rarefied PlasmasWang, Pu (Virginia Tech, 2010-01-25)Surface charging is a fundamental interaction process in space plasma engineering. A three-dimensional Immersed Finite Element Particle-In-Cell (IFE-PIC) method is developed to model surface charging involving complex boundary conditions. This method extends the previous IFE-PIC algorithm to explicitly include charge deposition on a dielectric surface for charging calculations. Three simulation studies are carried out using the new algorithm to model current collection and charging in both the orbital motion limited (OML) and space charge limited regime. The first one is a full particle simulation of the charging process of single small sphere and clusters of multiple small spheres in plasma. We find that while single sphere charging agrees well with the predictions of the OML theory, the charging of a sphere in a cluster is significantly, indicating that the often used OML charging model is not an accurate one to model charging in dusty plasma. The second one concerns a secondary electron emission experiment. The simulation includes detailed experimental setup in a vacuum chamber and the results are compared against experimental data. The simulation is used to determine the facility error in experiments. The third one is a full particle simulation of charging on lunar surface. The simulation concerns both flat and non-flat surface, and spacecraft on lunar surface, in the lunar polar region. The surface sees a mesothermal solar wind plasma flow and the emission of photoelectrons and secondary electrons. At a small sun elevation angle, the surface landscape generates a complex plasma flow field and local differential charging on surface. The results will be useful for further study of charging and levitation of lunar dust.
- Immersed Finite Element Particle-In-Cell Simulations of Ion PropulsionKafafy, Raed (Virginia Tech, 2005-09-01)A new particle-in-cell algorithm was developed for plasma simulations involving complex boundary conditions. The new algorithm is based on the three-dimensional immersed finite element method which is developed in this thesis, and a modified legacy particle-in-cell code. The model also applies a new meshing technique that separates the field solution mesh from the particle pushing mesh in order to increase the computational eciency of the model. The new simulation model is used in two applications of great importance to the development of ion propulsion technology: the ion optics performance and the interaction between spacecraft and the ion thruster. The first application is ion optics simulations. Simulations are performed to investigate ion optics plasma flow for a whole subscale NEXT ion optics. The operating conditions modeled cover the entire cross-over to perveance limit range. The results of the ion optics simulations demonstrated good agreement with the available experimental data. The second application is ion thruster plume simulations. Simulations are performed to investigate ion thruster plume - spacecraft interactions for the Dawn spacecraft. Plume induced contaminations on the solar array are studied for a variety of ion thruster configurations including multiple thruster firings.
- Investigations of Injectors for Scramjet EnginesMaddalena, Luca (Virginia Tech, 2007-07-25)An experimental study of an aerodynamic ramp (aeroramp) injector was conducted at Virginia Tech. The aeroramp consisted of an array of two rows with two columns of flush-wall holes that induce vorticity and enhance mixing. For comparison, a single-hole circular injector with the same area angled downstream at 30 degrees was also examined. Test conditions involved sonic injection of helium heated to 313 K, to safely simulate hydrogen into a Mach 4 air cross-stream with average Reynolds number 5.77 e+7 per meter at a jet to freestream momentum flux ratio of 2.1. Sampling probe measurements were utilized to determine the local helium concentration. Pitot and cone-static pressure probes and a diffuser thermocouple probe were employed to document the flow. The main results of this work was that the mixing efficiency value of this aeroramp design which was optimized at Mach 2.4 for hydrocarbon fuel was only slightly higher than that of the single-hole injector at these flow conditions and the mass-averaged total pressure loss parameter showed that the aero-ramp and single-hole injectors had the same overall losses. The natural extension of the investigation was then to look in detail at two major physical phenomena that occurs in a complex injector design such the Aeroramp: the jet-shock interaction and the interaction of the vortical structures produced by the jets injection into a supersonic cross flow. Experimental studies were performed to investigate the effects of impinging shocks on injection of heated helium into a Mach 4 crossflow. It was found that the addition of a shock behind gaseous injection into a Mach 4 crossflow enhances mixing only if the shock is closer to the injection point where the counter-rotating vortex pair (always associated with transverse injection in a crossflow) is not yet formed, and the deposition of baroclinic generated of vorticity is the highest. The final investigation concerned with the interaction of the usual vortex structure produced by jet injection into a supersonic crossflow and an additional axial vortex typical of those that might be produced by the inlet of a scramjet or the forebody of a vehicle to be controlled by jet interaction phenomena. The additional axial vortices were generated by a strut-mounted, diamond cross-section wing mounted upstream of the injection location. The wing was designed to produce a tip vortex of a strength comparable to that of one of the typical counter-rotating vortex pair (CVP) found in the plume of a jet in a crossflow. The profound interaction of supersonic vortices supported by a quantitative description and characterization of the flowfield has been demonstrated.
- Kinetic Simulations of Spacecraft Charging and Plasma Interactions in the Solar WindJeong, Hyunju (Virginia Tech, 2008-09-01)Analytical and numerical studies are carried out to investigate spacecraft charging and plasma interactions in the solar wind. The physics of spacecraft charging in solar wind is determined by the mesothermal flow and the photoelectron sheath. In order to properly resolve both plasma flow and the photoelectron sheath, a 3-D full particle PIC model is applied. In this model, all plasma species (ambient ions and electrons, and photoelectrons) are modeled as macro-particles so the detailed dynamics of each species can be resolved around a charged spacecraft. In order to correctly resolve the mesothermal velocity ratio, PIC simulations are carried out using the real ion to electron mass ratio. A charging model based on the capacitance matrix method is integrated into the PIC model so the floating potential can be calculated self-consistently with the PIC code from charges deposited on the surface. We first investigate the photoelectron sheath in the solar wind. Previous analytical studies of monotonic and non-monotonic sheath profiles in stationary electrons have suggested that there can exist two solutions of the sheath profiles when photoelectron emissions are significant. We extend the previous analytical approach to include the effects of drifting electrons. Full particle PIC simulations suggest that the non-monotonic sheath profile is the stable solution under solar wind conditions. We found that the current balance calculation is not an accurate method to predict the floating potential when photoelectron emissions are significant. We next apply the simulation model to study spacecraft charging under various solar wind conditions. Due to photoelectron emissions, spacecraft charging is typically not a serious problem. The floating potential is ~2.5V under the mean solar wind condition. We also investigate the plasma interactions of a multi-body system consisting of a large platform and a small free flyer in the absence of photoelectron emissions where we set a free flyer at 2*Debye length behind the platform in the wake. For the particular system studied in this dissertation, the simulation shows that wake charging is not severe under both the mean solar wind condition and severe magnetosheath charging condition.
- Lunar Robotic Precursor Missions Using Electric PropulsionWinski, Richard G. (Virginia Tech, 2006-10-23)A trade study is carried out for the design of electric propulsion based lunar robotic precursor missions. The focus is to understand the relationships between payload mass delivered, electric propulsion power, and trip time. The results are compared against a baseline system using chemical propulsion with LOX/H2. The major differences between the chemical propulsion based and electric propulsion based systems are presented in terms of the payload mass and trip time. It is shown that solar electric propulsion offers significant advantage over chemical propulsion in delivering non-time critical payloads to lunar orbit.
- Measurements of the Tip-gap Turbulent Flow Structure in a Low-speed Compressor CascadeTang, Genglin (Virginia Tech, 2003-09-03)This dissertation presents results from a thorough study of the tip-gap turbulent flow structure in a low-speed linear compressor cascade wind tunnel at Virginia Tech that includes a moving belt system to simulate the relative motion between the tip and the casing. The endwall pressure measurements and the surface oil flow visualizations were made on a stationary endwall to obtain the flow features and to determine the measurement profiles of interest. A custom-made miniature 3-orthogonal-velocity-component fiber-optic laser-Doppler velocimetry (LDV) system was used to measure all three components of velocity within a 50 mm spherical measurement volume within the gap between the endwall and the blade tip, mainly for the stationary wall with 1.65% and 3.30% tip gaps as well as some initial experiments for the moving wall. Since all of the vorticity in a flow originates from the surfaces under the action of strong pressure gradient, it was very important to measure the nearest-wall flow on the endwall and around the blade tip. The surface skin friction velocity was measured by using viscous sublayer velocity profiles, which verified the presence of an intense lateral shear layer that was observed from surface oil flow visualizations. All second- and third-order turbulence quantities were measured to provide detailed data for any parallel CFD efforts. The most complete data sets were acquired for 1.65% and 3.30% tip gap/chord ratios in a low-speed linear compressor cascade. This study found that tip gap flows are complex pressure-driven, unsteady three-dimensional turbulent flows. The crossflow velocity normal to the blade chord is nearly uniform in the mid tip-gap and changes substantially from the pressure to suction side. The crossflow velocity relies on the local tip pressure loading that is different from the mid-span pressure loading because of tip leakage vortex influence. The tip gap flow is highly skewed three-dimensional flow throughout the full gap. Normalized circulation within the tip gap is independent of the gap size. The tip gap flow interacts with the primary flow, separates from the endwall, and rolls up on the suction side to form the tip leakage vortex. The tip leakage vortex is unsteady from the observation of the TKE transport vector and oil flow visualizations. The reattachment of tip separation vortex on the pressure side strongly depends on the blade thickness-to-gap height ratio after the origin of tip leakage vortex but is weakly related to it before the origin of tip leakage vortex for a moderate tip gap. Other than the nearest endwall and blade tip regions, the TKE does not vary much in tip gap. The tip leakage vortex produces high turbulence intensities. The tip gap flow correlations of streamwise and wall normal velocity fluctuations decrease significantly from the leading edge to the trailing edge of the blade due to flow skewing. The tip gap flow is a strongly anisotropic turbulent flow. Rapid distortion ideas can not apply to it. A turbulence model based on stress transport equations and experimental data is necessary to reflect the tip gap flow physics. For the moving endwall, relative motion skews the inner region flow and is decorrelated with the outer layer flow. Hence, the TKE and correlations of streamwise and wall normal velocity fluctuations decrease.
- Modeling Differential Charging of Composite Spacecraft Bodies Using the Coliseum FrameworkBarrie, Alexander (Virginia Tech, 2006-08-14)The COLISEUM framework is a tool designed for electric propulsion plume interactions. Virginia Tech has been developing a module for COLISEUM called DRACO, a particle-in-cell based code capable of plume modeling for geometrically complex spacecraft. This work integrates a charging module into DRACO. Charge is collected via particle impingement on the spacecraft surface and converted to potential. Charge can be stored in the surface, or added to a local ground potential. Current can flow through the surface and is governed by the internal electric field in the spacecraft. Several test cases were run to demonstrate the code's capabilities. The first was a plume impingement of a composite spherical probe by a xenon thruster. It was shown that the majority of current conducted will reach the interior of the spacecraft, not other surface elements. A conductive interior will therefore result in a uniform surface potential, even for low surface conductivities. A second test case showed a composite spacecraft exposed to a solar wind. This test showed that when a potential gradient is applied to a conductive body, the ground potential of the spacecraft will lower significantly to compensate and maintain a zero net current. The case that used the semiconductive material showed that the effect of the potential gradient can be lowered in cases such as this, where some charge will always be stuck in the surface. If a dielectric material is used, the gradient will disappear altogether. The final test case showed the effect of charge exchange ion backflow on the potential of a spacecraft similar to the DAWN spacecraft. This case showed that CEX ion distribution is not even along the spacecraft and will result in a transverse potential gradient along the panel.
- Modeling of Ion Thruster Discharge Chamber Using 3D Particle-In-Cell Monte-Carlo-Collision MethodTran, Binh Phuoc (Virginia Tech, 2005-06-02)This thesis is aimed toward developing a method to simulate ion thruster discharge chambers in a full three dimensional environment and to study the effect of discharge chamber size on ion thruster performance. The study focuses solely on ring-cusped thrusters that make use of Xenon for propellant and discharge cathode assembly for mean of propellant ionization. Commercial software is used in both the setup and analysis phases. Numerical simulation is handled by 3D Particle-In-Cell Monte-Carlo-Collision method. Simulation results are analyzed and compared with other works. It is concluded that the simulation methodology is validated and can be used to simulate different cases. Therefore, different simulation cases of varying chamber sizes are done and the results are used to develop a performance curve. This plot suggests that the most efficient case is the 30 cm thruster. The result further validates the simulation process since the operating parameters used for all of the cases are taken from a 30 cm thruster experiment. One of the obvious applications for such a simulation process is to determine a set of the most efficient operating parameters for a certain size thruster before actual fabrication and laboratory testing.
- Numerical Simulation of Ion-Cyclotron Turbulence Generated by Artificial Plasma Cloud ReleaseChang, Ouliang (Virginia Tech, 2009-07-01)Possibilities of generating plasma turbulence to provide control of space weather processes have been of particular interest in recent years. Such turbulence can be created by chemical released into a magnetized background plasma. The released plasma clouds are heavy ions which have ring velocity distribution and large free energy to drive the turbulence. An electromagnetic hybrid (fluid electrons and particle ions) model incorporating electron inertia is developed to study the generation and nonlinear evolution of this turbulence. Fourier pseudo-spectral methods are combined with finite difference methods to solve the electron momentum equations. Time integration is accomplished by a 4th-order Runge-Kutta scheme or predicator-corrector method. The numerical results show good agreement with theoretical prediction as well as provide further insights on the nonlinear turbulence evolution. Initially the turbulence lies near harmonics of the ring plasma ion cyclotron frequency and propagates nearly perpendicular to the background magnetic field as predicted by the linear theory. If the amplitude of the turbulence is sufficiently large, the quasi-electrostatic short wavelength ion cyclotron waves evolve nonlinearly into electromagnetic obliquely propagating shear Alfven waves with much longer wavelength. The results indicate that ring densities above a few percent of the background plasma density may produce wave amplitudes large enough for such an evolution to occur. The extraction of energy from the ring plasma may be in the range of 10-15% with a generally slight decrease in the magnitude as the ring density is increased from a few percent to several 10's of percent of the background plasma density. Possibilities to model the effects of nonlinear processes on energy extraction by introducing electron anomalous resistivity are also addressed. Suitability of the nonlinearly generated shear Alfven waves for applications to scattering radiation belt particles is discussed.
- Robust Kalman Filters Using Generalized Maximum Likelihood-Type EstimatorsGandhi, Mital A. (Virginia Tech, 2009-11-20)Estimation methods such as the Kalman filter identify best state estimates based on certain optimality criteria using a model of the system and the observations. A common assumption underlying the estimation is that the noise is Gaussian. In practical systems though, one quite frequently encounters thick-tailed, non-Gaussian noise. Statistically, contamination by this type of noise can be seen as inducing outliers among the data and leads to significant degradation in the KF. While many nonlinear methods to cope with non-Gaussian noise exist, a filter that is robust in the presence of outliers and maintains high statistical efficiency is desired. To solve this problem, a new robust Kalman filter framework is proposed that bounds the influence of observation, innovation, and structural outliers in a discrete linear system. This filter is designed to process the observations and predictions together, making it very effective in suppressing multiple outliers. In addition, it consists of a new prewhitening method that incorporates a robust multivariate estimator of location and covariance. Furthermore, the filter provides state estimates that are robust to outliers while maintaining a high statistical efficiency at the Gaussian distribution by applying a generalized maximum likelihood-type (GM) estimator. Finally, the filter incorporates the correct error covariance matrix that is derived using the GM-estimator's influence function. This dissertation also addresses robust state estimation for systems that follow a broad class of nonlinear models that possess two or more equilibrium points. Tracking state transitions from one equilibrium point to another rapidly and accurately in such models can be a difficult task, and a computationally simple solution is desirable. To that effect, a new robust extended Kalman filter is developed that exploits observational redundancy and the nonlinear weights of the GM-estimator to track the state transitions rapidly and accurately. Through simulations, the performances of the new filters are analyzed in terms of robustness to multiple outliers and estimation capabilities for the following applications: tracking autonomous systems, enhancing actual speech from cellular phones, and tracking climate transitions. Furthermore, the filters are compared with the state-of-the-art, i.e. the Hâ -filter for tracking an autonomous vehicle and the extended Kalman filter for sensing climate transitions.
- Scramjet Operability Range Studies of an Integrated Aerodynamic-Ramp-Injector/Plasma-Torch Igniter with Hydrogen and Hydrocarbon FuelsBonanos, Aristides Michael (Virginia Tech, 2005-08-22)An integrated aerodynamic-ramp-injector/plasma-torch-igniter of original design was tested in a Mâ = 2, unvitiated, heated flow facility arranged as a diverging duct scramjet combustor. The facility operated at a total temperature of 1000 K and total pressure of 330 kPa. Hydrogen (H2), ethylene (C2H4) and methane (CH4) were used as fuels, and a wide range of global equivalence ratios were tested. The main data obtained were wall static pressure measurements, and the presence of combustion was determined based on the pressure rises obtained. Supersonic and dual-mode combustion were achieved with hydrogen and ethylene fuel, whereas very limited heat release was obtained with the methane. Global operability limits were determined to be 0.07 < Ï < 0.31 for hydrogen, and 0.14 < Ï < 0.48 for ethylene. The hydrogen fuel data for the aeroramp/torch system was compared to data from a physical 10 unswept compression ramp injector and similar performance was found with the two arrangements. With hydrogen and ethylene as fuels and the aeroramp/plasma-torch system, the effect of varying the air total temperature was investigated. Supersonic combustion was achieved with temperatures as low as 530K and 680K for the two fuels, respectively. These temperatures are facility/operational limits, not combustion limits. The pressure profiles were analyzed using the Ramjet Propulsion Analysis (RJPA) code. Results indicate that both supersonic and dual-mode ramjet combustion were achieved. Combustion efficiencies varied with Ï from a high of about 75% to a low of about 45% at the highest Ï . With a theoretical diffuser and nozzle assumed for the configuration and engine, thrust was computed for each fuel. Fuel specific impulse was on average 3000 and 1000 seconds for hydrogen and ethylene respectively, and air specific impulse varied from a low of about 9 sec to a high of about 24 sec (for both fuels) for the To = 1000K test condition. The GASP RANS code was used to numerically simulate the injection and mixing process of the fuels. The results of this study were very useful in determining the suitability of the selected plasma torch locations. Further, this tool can be used to determine whether combustion is theoretically possible or not.
- Three dimensional character of whistler turbulenceGanguli, Gurudas; Rudakov, Leonid; Scales, Wayne A.; Wang, Joseph J.; Mithaiwala, Manish (AIP Publishing, 2010-05-01)It is shown that the dominant nonlinear effect makes the evolution of whistler turbulence essentially three dimensional in character. Induced nonlinear scattering due to slow density perturbation resulting from ponderomotive force triggers energy flux toward lower frequency. Anisotropic wave vector spectrum is generated by large angle scatterings from thermal plasma particles, in which the wave propagation angle is substantially altered but the frequency spectrum changes a little. As a consequence, the wave vector spectrum does not indicate the trajectory of the energy flux. There can be conversion of quasielectrostatic waves into electromagnetic waves with large group velocity, enabling convection of energy away from the region. We use a two-dimensional electromagnetic particle-in-cell model with the ambient magnetic field out of the simulation plane to generate the essential three-dimensional nonlinear effects. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3420245]