Development of an Unstructured 3-D Direct Simulation Monte Carlo/Particle-in-Cell Code and the Simulation of Microthruster Flows

This work is part of an effort to develop an unstructured, three-dimensional, direct simulation Monte Carlo/particle-in-cell (DSMC/PIC) code for the simulation of non-ionized, fully ionized and partially-ionized flows in micropropulsion devices. Flows in microthrusters are often in the transitional to rarefied regimes, requiring numerical techniques based on the kinetic description of the gaseous or plasma propellants. The code is implemented on unstructured tetrahedral grids to allow discretization of arbitrary surface geometries and includes an adaptation capability. In this study, an existing 3D DSMC code for rarefied gasdynamics is improved with the addition of the variable hard sphere model for elastic collisions and a vibrational relaxation model based on discrete harmonic oscillators. In addition the existing unstructured grid generation module of the code is enhanced with grid-quality algorithms. The unstructured DSMC code is validated with simulation of several gaseous micronozzles and comparisons with previous experimental and numerical results. Rothe s 5-mm diameter micronozzle operating at 80 Pa is simulated and results are compared favorably with the experiments. The Gravity Probe-B micronozzle is simulated in a domain that includes the injection chamber and plume region. Stagnation conditions include a pressure of 7 Pa and mass flow rate of 0.012 mg/s. The simulation examines the role of injection conditions in micronozzle simulations and results are compared with previous Monte Carlo simulations. The code is also applied to the simulation of a parabolic planar micronozzle with a 15.4-micron throat and results are compared with previous 2D Monte Carlo simulations. Finally, the code is applied to the simulation of a 34-micron throat MEMS-fabricated micronozzle. The micronozzle is planar in profile with sidewalls binding the upper and lower surfaces. The stagnation pressure is set at 3.447 kPa and represents an order of magnitude lower pressure than used in previous experiments. The simulation demonstrates the formation of large viscous boundary layers in the sidewalls. A particle-in-cell model for the simulation of electrostatic plasmas is added to the DSMC code. Solution to Poisson\u27s equation on unstructured grids is obtained with a finite volume implementation. The Poisson solver is validated by comparing results with analytic solutions. The integration of the ionized particle equations of motion is performed via the leapfrog method. Particle gather and scatter operations use volume weighting with linear Lagrange polynomial to obtain an acceptable level of accuracy. Several methods are investigated and implemented to calculate the electric field on unstructured meshes. Boundary conditions are discussed and include a formulation of plasma in bounded domains with external circuits. The unstructured PIC code is validated with the simulation of a high voltage sheath formation

TCP-Carson: A loss-event based Adaptive AIMD algorithm for Long-lived Flows

The diversity of network applications over the Internet has propelled researchers to rethink the strategies in the transport layer protocols. Current applications either use UDP without end-to-end congestion control mechanisms or, more commonly, use TCP. TCP continuously probes for bandwidth even at network steady state and thereby causes variation in the transmission rate and losses. This thesis proposes TCP Carson, a modification of the window-scaling approach of TCP Reno to suit long-lived flows using loss-events as indicators of congestion. We analyzed and evaluated TCP Carson using NS-2 over a wide range of test conditions. We show that TCP Carson reduces loss, improves throughput and reduces window-size variance. We believe that this adaptive approach will improve both network and application performance

Aeronautical engineering: A continuing bibliography with indexes (supplement 277)

This bibliography lists 467 reports, articles, and other documents introduced into the NASA scientific and technical information system in Mar. 1992. Subject coverage includes: the engineering and theoretical aspects of design, construction, evaluation, testing, operation, and performance of aircraft (including aircraft engines); and associated aircraft components, equipment, and systems. It also includes research and development in ground support systems, theoretical and applied aspects of aerodynamics, and general fluid dynamics

Modeling of shock boundary layer interactions and stability analysis using particle approaches

Hypersonic flow separation and laminar shock wave boundary layer interactions (SWBLIs) have received considerable attention since these interactions can lead to laminar to turbulent transition, unsteadiness, and localized high pressure and heating regions. Accurate predictions of these phenomena, particularly when thermochemical nonequilibrium is present, play a crucial role for design purposes. In this regard, many experimental, theoretical, and numerical works have been conducted over the decades. In this work, numerical investigations of SWBLIs for hypersonic flows over a double wedge and cone and ”tick-shaped” model configurations have been conducted to investigate the origin of SWBLIs and to compare with measurements in the Hypervelocity Expansion Tube (HET), Calspan-University at Buffalo Research Center (CUBRC), and T-ADFA free-piston shock tunnel facilities using particle approaches to model the Boltzmann equation. The Boltzmann transport equation is the most general formulation of binary gas flows for a wide Knudsen number spectrum including rarefied, slip, continuum regimes. The direct simulation Monte Carlo (DSMC) method, a well-known stochastic approach to solving the Boltzmann equation, provides high-fidelity molecular transport and thermal nonequilibrium, commonly seen in strong shock-shock interactions and inherently captures rarefaction effects such as velocity slip and temperature jump without a priori specific model. Therefore, the DSMC method has been applied to SWBLIs in order to take all these effects into account. However, DSMC becomes prohibitively expensive for calculations in the continuum regime. In order to potentially reduce the computational costs related to DSMC computations for low Knudsen number flows, the ellipsoidal statistical Bhatnagar-Gross-Krook (ES-BGK) model of the Boltzmann equation was developed and applied to shock dominated flows. The DSMC method has been used for modeling shock dominated separated hypersonic flows at Mach 7 for a unit Reynolds number of 4.15 × 105 m−1 previously studied in the HET over a double wedge configuration to investigate the impacts of thermochemical effects on SWBLIs by changing the chemical composition. The DSMC simulations are found to reproduce many of the classical features of Edney Type IV strong shock interactions. A comparison of simulated heat flux with measurements reveals that the calculated surface heating profiles were found to be time-dependent and in disagreement with experiments at later flow times, especially for the 2D wedge model. Further investigations using a three-dimensional model, taking the pressure relief into account, indicate that the simulated 3D heat fluxes, shock structure, and triple point movement were found to be in fair agreement with the experimental heat flux values, especially in the aft part of the wedge, and the shock tracking measurements. Nonetheless, both the 2D and 3D cases do not reach steady state for the duration of the experiment. To reduce 3D effects and to investigate time-dependency more closely, shock-dominated hypersonic lam- inar flows over a double cone are investigated using time-accurate DSMC combined with the residuals algorithm (RA) for unit Reynolds numbers gradually increasing from 9.35×104 to 3.74×105 m−1 at a Mach number of about 16. The main flow features, such as the strong bow-shock, location of the separation shock, the triple point, and the entire laminar separated region show a time-dependent behavior. As the Reynolds number is increased, larger pressure values in the under-expanded jet region due to strong shock interactions form more prominent λ-shocklets in the supersonic region between two contact surfaces. A Kelvin-Helmholtz instability arising at the shear layer results in an unsteady flow for the highest Reynolds number. These findings suggest that consideration of experimental measurement times is important when it comes to deter- mining the steady state surface parameters even for a relatively simple double cone geometry at moderately large Reynolds numbers. Further studies have been conducted to analyze the unsteadiness of the double cone flows using a com- bination of DSMC calculations, linear global instability analysis and momentum potential theory (MPT). Close to steady state linear analysis reveals the spatial structure of the underlying temporally stable global modes. Application of the MPT (valid for both linear and nonlinear signals) to the highest Reynolds number DSMC results shows that large acoustic and thermal potential variations exist in the vicinity of the sepa- ration shock, the λ-shock patterns, and the shear layers. It is further shown that the motion of the bow shock system is highly affected by non-uniformities in the acoustic field. At the highest Reynolds number considered here, the unsteadiness is characterized by Strouhal numbers in the shear layer and bow-shock regions. Lastly, a modal analysis with window proper orthogonal decomposition (WPOD) has been applied to hypersonic separated flows with different chemical composition over the double wedge near steady state in order to correlate POD modes with global modes, to predict future states without running computationally demanding simulations, and to eliminate statistical noise inherent to the DSMC method. Thermochemical nonequilibrium effects are found to change the shock structures, the size of the separation region, and the required time to reach steady state. The temporal analysis of POD modes shows that the decay rate of the least damped eigenmode for the chemically reacting air case is found to be smaller in comparison to the non-reacting air case. For the first time, steady state solutions for an unsteady, chemically reacting hypersonic flow are predicted using the WPOD method

Development of an Unstructured CFD Solver for External Aerothermodynamics and Nano/Micro Flows

Computational aerothermodynamics is the branch of science which focuses on the computation of the effect of thermodynamic and transport models on aerodynamics and heating. They are widely used for external ow cases. On the other hand, the computation of heat and stress in the design of Nano/Micro Electronic Mechanical Systems from the point of view of a Fluid mechanics engineer is also an important area of study. A generalized computational tool which can simulate the low and high speed flows at both the macro and micro levels is desirable from the perspective of industry, academics and research. For a developing nation, it is extremely important to have such a solver developed indigenously to create self-sufficiency and self-reliance. In this work, a robust three-dimensional density-based general purpose computational fluid dynamics solver was developed in house by our research group. The cell centred finite volume discretization method is used on an unstructured grid, which is more desirable for computation on a complex geometry from the perspective of pre-processing (meshing). Compressible ow solutions obtained from density-based solvers usually do not work well at low speeds where the ow is close to incompressible, unless special schemes and/or special treatments are used. An all-speed algorithm was incorporated using two different methods: (a) preconditioning of the governing equations or (b) through the use of the recently developed SLAU2 all speed convective scheme. The time-stepping discretization is done implicitly, using the lower-upper symmetric-Gauss-Seidel method, which allows us to take a high CFL number during computations. Throughout this work, we have used a second-order accurate reconstruction with limiters to accurately capture the shocks without dispersive error. Turbulence modelling is done using Favre- and Reynolds- Averaged Navier-Stokes equations using the Spalart Allmaras turbulence model. The developed solver is used to solve external ow problems at low and high speeds (hypersonic regimes). In these problems, the thesis focus is on the implementation and testing of an automatic wall function treatment for the Spalart-Allmaras turbulence model. The applicability of the solver is extended to rarefied gas ow regimes in the following manner. Thermal non-equilibrium which exists in the rarefied ow regime is tackled using non-equilibrium boundary conditions in the slip ow regime. The use of non-equilibrium boundary conditions allows the applicability of the Navier-Stokes equation to be extended beyond the continuum to the slip regime. This approach is used to solve problems of hypersonic rarefied flows and nano/micro flows; and for testing and validation of several recently proposed boundary conditions for several problems in the slip ow regime. The main focus of this work is in developing newer numerical methods and on testing and improving other recently proposed numerical techniques that are used for solving the problems covered in this thesis. In the following paragraphs we present the major outcomes of the thesis. The Spalart-Allmaras (SA) is one of the most popular turbulence models in the aerospace CFD community. In its original (low-Reynolds number) formulation it requires a very tight grid (with y+ ' 1) spacing near the wall to resolve the high ow gradients. The use of _ne grids increases the computational cost of the solutions. However, the use of wall functions with an automatic feature of switching from the wall function to the low-Reynolds number approach is an effective solution to this problem. We have extended Menter's automatic wall treatment (AWT), devised for the

ICASE

This report summarizes research conducted at the Institute for Computer Applications in Science and Engineering in the areas of (1) applied and numerical mathematics, including numerical analysis and algorithm development; (2) theoretical and computational research in fluid mechanics in selected areas of interest, including acoustics and combustion; (3) experimental research in transition and turbulence and aerodynamics involving Langley facilities and scientists; and (4) computer science

Ab initio quantum-chemistry database for N2 (v, J) + N in a state-to-state implementation of the DSMC method

In this work, the implementation within DSMC of a coarse-grain model for nitrogen is presented. The main contribution of this thesis is the development of a methodology by which a detailed state-to-state reaction mechanism for internal energy exchange and molecular dissociation can be reduced to a manageable size and incorporated into a DSMC code. The feasibility of using this model to simulate problems with realistic 2D/3D geometries and conditions relevant for atmospheric entry applications is demonstrated

CHAOS: A multi-GPU PIC-DSMC solver for modeling gas and plasma flows

Numerical modeling of gas and plasma-surface interactions is critical to understanding the complex kinetic processes that dominate the extreme environments of planetary entry and in-space propulsion. However, simulations of these systems that evolve over multiple length- and time-scales is computationally expensive. Until recently, approximations were used to keep computational costs tenable, which in turn, increased the uncertainty in predictions and offered limited insights into the micro-scale flow properties and electron kinetics that dominate the macroscale processes. The need to perform high-fidelity physics-based gas and plasma simulations has led to the development of a three-dimensional, multi-GPU, Particle-in-cell (PIC)-direct simulation Monte Carlo (DSMC) solver called Cuda-based Hybrid Approach for Octree Simulations (CHAOS) that is presented in this work. This computational tool has been applied to candidate PICA-like TPS materials that consist of an irregular porous network of fibers to allow high-temperature boundary layer gases as well as pyrolysis by-products to penetrate in and flow out of the material. Quantifying bulk transport properties of these materials is essential for accurate prediction of the macroscopic ablation rate. The second application that CHAOS is being used with is the modeling of ion thruster plumes that consist of fast beam ions and slow neutrals that undergo charge-exchange (CEX) reactions to produce slow ions and fast neutrals. These slow CEX ions are strongly influenced by the electric field induced between the ion plume and the thruster surface, resulting in a backflow of ions towards the critical solar panel and thruster surfaces. Three backflow quantities, namely, ion flux, incidence angle, and incidence energy affect the macroscopic sputtering rate of the solar panel surfaces over extended operational times and are predicted from the PIC-DSMC simulations