Richard Wilmoth

• Doctor of Philosophy
• Principal Investigator at Combustion Research and Flow Technology, Inc.

A study of high-altitude, nonequilibrium flows about an Orion Command Module (CM) is conducted using the collision-based chemical kinetics approach introduced by Bird in 2008. DSMC simulations are performed for Earth entry flow conditions and show significant differences in molecular dissociation in the shock layer from those obtained using traditi...

A hybrid modeling capability has been developed by coupling a CFD code to a DSMC code using a one-way coupled interface surface. Using a unique toolkit, named AEGIS, these surfaces can be generated for complex, arbitrary 3D shapes. The status of this technology development is described. Modifications that include non-equilibrium internal energy mod...

*† ‡ § ** A hybrid continuum/non-continuum methodology is being developed to model flows in which rarefaction effects are important while the continuum approach is accurate for parts of the flowfield. Depending on altitude, rocket and missile exhaust plume expansion can be modeled using a combination of continuum, direct simulation Monte Carlo (DSM...

Rarefied flow conditions can have an impact on performance and radiation characterization at not only high-altitudes conditions but also for separated flow regions either behind blunt bodies or around divert jets at high continuum altitudes. Under rarefied flow conditions, standard continuum CFD methodologies fall short. More appropriate are Direct...

A modified source flow model was used to calculate the plume flowfield from a Mars Odyssey thruster during aerobraking. The source flow model results compared well with previous detailed computational-fluid-dynamics results for a Mars Global Surveyor thruster. Using an isodensity surface for the Odyssey plume, direct simulation Monte Carlo simulati...

During the Mars 2001 Odyssey aerobraking mission, NASA Langley Research Center performed six-degree-of-freedom simulations to model rotational motion of the spacecraft. The main objective of this study was to assess the reaction-control-system models and their effects on the atmospheric flight of Odyssey. Based on these models, a comparison was mad...

Trailing Ballute Aerocapture offers the potential to obtain orbit insertion around a planetary body at a fraction of the mass of traditional methods. This allows for lower costs for launch, faster flight times and additional mass available for science payloads. The technique involves an inflated ballute (balloon-parachute) that provides aerodynamic...

On January 11 of 2002, Mars Odyssey successfully completed the aerobraking phase of the mission, becoming the second successful planetary mission designed specifically to utilize aerobraking as a primary means of achieving its mission objectives. Direct simulation Monte Carlo and free‐molecular analyses were used to provide aerothermodynamic charac...

Aeroheating wind-tunnel tests were conducted on a 0.028 scale model of an orbiter concept considered for a possible Mars sample return mission. The primary experimental objectives were to characterize hypersonic near wake closure and determine if shear layer impingement would occur on the proposed orbiter afterbody at incidence angles necessary for...

During the Mars 2001 Odyssey aerobraking mission NASA Langley Research Center performed 6 degree of freedom (6-DOF) simulations to determine rotational motion of the spacecraft. The main objective of this study was to assess the reaction control system models and their effects on the atmospheric flight of Odyssey. Based on these models, a compariso...

Direct Simulation Monte Carlo and free-molecular analyses were used to provide aerothermodynamic characteristics of the Mars Odyssey spacecraft. The results of these analyses were used to develop an aerodynamic database that was used extensively for the pre-flight planning and in-flight execution for the aerobraking phase of the Mars Odyssey missio...

Aeroheating wind-tunnel tests were conducted on a 0.028 scale model of an orbiter concept considered for a possible Mars sample return mission. The primary experimental objectives were to characterize hypersonic near wake closure and determine if shear layer impingement would occur on the proposed orbiter afterbody at incidence angles necessary for...

This paper describes a parallel implementation of the direct simulation Monte Carlo method. Runtime library support is used for scheduling and execution of communication between nodes, and domain decomposition is performed dynamically to maintain a favorable load balance. Performance tests are conducted using the code to evaluate various remapping...

Major elements of an experiment called the Infrared Sensing Aeroheating Flight Experiment are discussed. The primary experiment goal is to provide reentry global temperature images from infrared measurements to define the characteristics of hypersonic boundary-layer transition during flight. Specifically, the experiment is to identify, monitor, and...

Major elements of an experiment called the Infrared Sensing Aeroheating Flight Experiment are discussed. The primary experiment goal is to provide reentry global temperature images from infrared measurements to define the characteristics of hypersonic boundary-layer transition during flight. Specifically, the experiment is to identify, monitor, and...

On July 4, 1997, after traveling close to 5±108 km, the Pathfinder spacecraft successfully completed entry, descent, and landing at Mars. The hypersonic rarefied portion of Pathfinder's atmospheric entry, where flight measurements, aerodynamic calculations, and atmospheric modeling tools are used to extract Pathfinder's attitude and the freestream...

The hypersonic transitional flow aerodynamics of the Mars Microprobe capsules are simulated with the direct simulation Monte Carlo method. Calculations of axial, normal, and static pitching coefficients were obtained over an angle-of-attack range comparable to actual flight requirements. Comparisons are made with modified Newtonian and free-molecul...

This paper describes a parallel implementation of the direct simulation Monte Carlo (DSMC) method. Runtime library support is used for scheduling and execution of communication between nodes, and domain decomposition is performed dynamically to maintain a good load balance. Performance tests are conducted using the code to evaluate various remappin...

this paper uses rarefied aerodynamic predictions (Knudsen number, Kn1 0:001) for Viking, Pathfinder, Microprobe, Mars'01 Orbiter, and Stardust vehicles under the nominal entry conditions shown in Fig. 1. These vehicles have spherically-blunted cone forebodies with conic or biconic afterbodies. Viking, Pathfinder, and Mars'01 Orbiter have 70-deg 21s...

Direct simulation Monte Carlo and free-molecular methods are used to provide aerothermodynamic characteristics for the Mars Global Surveyor spacecraft. These characteristics have been used for spacecraft design, mission planning, flight operations, and atmospheric reconstruction. Rarefied transitional flow effects on both heating and aerodynamic dr...

The Mars Global Surveyor (MGS) z-axis accelerometer has obtained over 200 vertical structures of thermospheric density, temperature, and pressure, ranging from 110 to 170 kilometers, compared to only three previous such vertical structures. In November 1997, a regional dust storm in the Southern Hemisphere triggered an unexpectedly large thermosphe...

On July 4, 1997, after traveling close to 500 million km, the Pathfinder spacecraft successfully completed entry, descent, and landing at Mars. In the present paper, the focus is on the hypersonic rarefied portion of Pathfinder's atmospheric entry where the synergy of flight measurements, aerodynamic calculations, and atmospheric modeling tools are...

The normal-to-axial force coefficient ratio of the Viking spherically blunted, 70-deg half-angle cone entry vehicle has been extracted from flight data in the transition from the hypersonic continuum into the free-molecule flow transition regime. Results from simulations of the Viking vehicle with a three-dimensional DSMC code show an excellent mat...

The aerodynamics of the Stardust Sample Return Capsule are analyzed in the low-density, transitional flow regime using free-molecular, direct simulation Monte Carlo, Navier-Stokes, and Newtonian methods to provide inputs for constructing a transitional flow bridging relation. The accuracy of this bridging relation in reconstructing the aerodynamic...

This paper examines spatial-discretization approaches for the direct simulation Monte Carlo method for axisymmetric and three-dimensional flows. Computations using three schemes, 1) a uniform Cartesian grid, 2) a structured body-fitted grid, and 3) an unstructured tetrahedral grid, are presented for the hypersonic flow past a blunted cone for two a...

Successful return of interstellar dust and cometary material by the Stardust Sample Return Capsule requires an accurate description of the Earth entry vehicle's aerodynamics. This description must span the hypersonic-rarefied, hypersonic-continuum, supersonic, transonic, and subsonic flow regimes. Data from numerous sources are compiled to accompli...

The hypersonic transitional flow aerodynamics of the Mars Pathfinder and Mars Microprobe capsules are simulated with the direct simulation Monte Carlo method. Calculations of axial, normal, and static pitching coefficients were obtained over an angle of attack range comparable to actual flight requirements. Comparisons are made with modified Newton...

Acceleration data taken from the Orbital Acceleration Research Experiment (OARE) during reentry on STS-62 have been analyzed using calibration factors taken on orbit. This is the first Orbiter mission which collected OARE data during the Orbiter reentry phase. The data examined include the flight regime from orbital altitudes down to about 90 km wh...

Two different grid methodologies are studied for application to DSMC simulations about reusable launch vehicles. One method uses an unstructured, tetrahedral grid while the other uses a structured, variable-resolution Cartesian grid. The relative merits of each method are discussed in terms of accuracy, computational efficiency, and overall ease of...

This paper examines spatial-discretization approaches for the direct simulation Monte Carlo method for axisymmetric and three-dimensional flows. Computations using the three schemes, a uniform Cartesian grid, a structured body-fitted grid, and an unstructured tetrahedral grid, are presented for the hypersonic flow past a blunted cone for two angles...

The behavior of two different models of gas-surface interactions is studied using the Direct Simulation Monte Carlo (DSMC) method. The DSMC calculations examine differences in predictions of aerodynamic forces and heat transfer between the Maxwell and the Cercignani-Lampis-Lord (CLL) models for flat plate configurations at freestream conditions cor...

Direct Simulation Monte Carlo (DSMC) and Navier-Stokes axisymmetric calculations are presented for hypersonic low-density flow about a 70-deg blunt cone afterbody configuration. The flow conditions simulated are those experienced by a space vehicle for an altitude range of 105-75 km during Earth entry. The entry velocity considered is 7 km/s. A ste...

Results of a numerical study are presented for hypersonic low-density flow about a 70-deg blunt cone using direct simulation Monte Carlo (DSMC) and Navier-Stokes calculations. Particular emphasis is given to the effects of chemistry on the near-wake structure and on the surface quantities and the comparison of the DSMC results with the Navier-Stoke...

This study deals with the development of a methodology for numerically simulating the interaction of a reaction control system (RCS) jet with a low-density external flow. A European Space Agency experiment was chosen as a test case, since it provided experimental data that could validate some of the numerical results. The initial approach was to fo...

Direct simulation Monte Carlo (DSMC) solutions are presented for the hypersonic flow behind a blunt body in which the wake region is solved in a zonally decoupled manner. The forebody flow is solved separately using either a DSMC or a Navier-Stokes method, and the forebody exit-plane solution is specified as the inflow condition to the decoupled DS...

Numerical results obtained with direct simulation Monte Carlo and Navier-Stokes methods are presented for a Mach-20 nitrogen flow about a 70-deg blunted cone. The flow conditions simuulated are those that can be obtained in existing low-density hypersonic wind tunnels. Three sets of flow conditions are considered with freestream Knudsen numbers ran...

The nozzle afterbody is one of the main drag-producing components of an aircraft propulsion system. Thus, considerable effort has been devoted to developing techniques for predicting the afterbody flow field and drag. The RAXJET computer program was developed to predict the transonic, axisymmetric flow over nozzle afterbodies with supersonic jet ex...

Numerical results obtained with direct simulation Monte Carlo (DSMC) and Navier-Stokes methods are presented for Mach 20 nitrogen flow about a 70-deg blunted cone. The flow conditions simulated are those that can be obtained in existing low-density hypersonic wind tunnels. Three sets of flow conditions are considered with freestream Knudsen numbers...

This study deals with the development of a methodology for numerically simulating the interaction of a reaction control system (RCS) jet with a low-density external flow. A European Space Agency (ESA) experiment was chosen as a test case since it provided experimental data that could validate some of the numerical results. The initial approach was...

Direct simulation Monte Carlo (DSMC) solutions are presented for the hypersonic flow behind a blunt body in which the wake region is solved in a zonally-decoupled manner. The forebody flow is solved separately using either a DSMC or a Navier-Stokes method, and the forebody exit plane solution is specified as the inflow condition to the decoupled DS...

Numerical results obtained with the direct simulation Monte Carlo (DSMC) method are presented for Mach 20 nitrogen flow about a 70-deg blunted cone. The flow conditions simulated are those that can be obtained in existing low-density hypersonic wind tunnels. Three sets of flow conditions are simulated with freestream Knudsen numbers ranging from 0....

The direct simulation Monte Carlo (DSMC) method is used to model the flowfield produced when an incident shock impinges on an inlet cowl lip of 0.1-in. radius for flight conditions of approximately Mach 15 and 35-km altitude. The shock interaction results in a supersonic jet which can impinge on the cowl lip surface and cause extremes in surface he...

Application of a method for performing direct simulation Monte Carlo calculations using parallel processing to several hypersonic, rarefied flow problems is presented. The performance and efficiency of the parallel method are discussed in terms of some simple benchmark problems. The applications described are the now in a channel and the flow about...

Results of a numerical study using the direct simulation Monte Carlo method are presented for hypersonic rarefied flow about a 1,6-m-diam sphere. The flow conditions considered are those experienced by a typical satellite in orbit or by a space vehicle during entry. The altitude range considered is that from 90 to 200 km, which encompasses the near...

A methodology for implementing parallel DSMC codes in a heterogeneous computing environment is described. The methodology involves the use of a common message-passing software library together with recently developed software that handles the actual interprocessor communications in a standard manner across a variety of computing platforms. Benchmar...

A complex shock interaction is calculated with direct simulation Monte Carlo (DSMC). The calculation is performed for the near-continuum flow produced when an incident shock impinges on the bow shock of a 0.1 in. radius cowl lip for freestream conditions of approximately Mach 15 and 35 km altitude. Solutions are presented both for a full finite-rat...

Results of a numerical study using the direct simulation Monte Carlo method are presented for hypersonic rarefied flow past spheres. The flow conditions considered are those corresponding to low density wind tunnel test conditions. The set of the experimental conditions for the calculations encompasses the transitional to near-continuum flow regime...

Results of a numerical study concerning flow past a 70-deg blunted cone in hypersonic low-density flow environments are presented using the direct simulation Monte-Carlo method. The flow conditions simulated are those that can be obtained in existing low-density hypersonic wind tunnels. Results indicate that a stable vortex forms in the near wake a...

A technique is presented for adapting computational meshes used in the G2 version of the direct simulation Monte Carlo method. The physical ideas underlying the technique are discussed, and adaptation formulas are developed for use on solutions generated from an initial mesh. The effect of statistical scatter on adaptation is addressed, and results...

A method for executing a direct simulation Monte Carlo (DSMC) analysis using parallel processing is described. The method is based on using domain decomposition to distribute the work load among multiple processors, and the DSMC analysis is performed completely in parallel. Message passing is used to transfer molecules between processors and to pro...

The direct simulation Monte Carlo (DSMC) method has been used to calculate the molecular velocity and energy distributions of molecules striking a surface after traversing a shock layer in hypersonic transitional flow. The calculations were performed for a 1.6-m-diameter sphere at a nominal velocity for re-entry of 7.5 km/s over an altitude range o...

A new algorithm for 3D direct simulation Monte Carlo (DSMC) is tested and numerical results are compared with wind tunnel data and results obtained earlier with a more traditional DSMC code. The test case is the flowfield around a delta wing at incidence at Knudsen number of 0.016 and Mach number of 20.2. The results are shown to compare favorably...

A method is described for performing direct simulation Monte Carlo (DSMC) calculations on parallel processors using adaptive domain decomposition to distribute the computational work load. The method has been implemented on a commercially available hypercube and benchmark results are presented which show the performance of the method relative to cu...

Results of a numerical study using the direct simulation Monte Carlo (DSMC) method are presented for hypersonic rarefied flow about a 1.6-m-diameter sphere. The flow conditions considered are those experienced by a typical satellite in orbit or by a space vehicle during entry. The altitude range considered is that from 90 to 200 km, which encompass...

This paper describes a method for doing direct simulation Monte Carlo (DSMC) calculations using parallel processing and presents some results of applying the method to several hypersonic, rarefied flow problems. The performance and efficiency of the parallel method are discussed. The applications described are the flow in a channel and the flow abo...

Aerothermodynamic, aerodynamic, and atmospheric science data acquired between 55 and 150 km has been limited by the lack of vehicles or platforms capable of sustained operation at these altitudes. Tethered satellites, which have been under study for this purpose by NASA, the Italian Space Agency (ASI), and others for more than a decade, are expecte...

ABSTRACT: Aerothermodynamic, aerodynamic, and atmospheric science data acquired between 55 and 150 km has been limited by the lack of vehicles or platforms capable of sustained operation at these altitudes. Tethered satellites, which have been under study for this purpose by NASA, the Italian Space Agency (ASI), and others for more than a decade, a...

A method is presented for executing a direct simulation Monte Carlo (DSMC) analysis using parallel processing. The method is based on using domain decomposition to distribute the work load among multiple processors, and the DSMC analysis is performed completely in parallel. Message passing is used to transfer molecules between processors and to pro...

a modified version of the multiscale turbulence model of K. Hanjalic has been applied to the problem of underexpanded supersonic jets. In particular, the shock-cell decay resulting from shock-mixing layer interactions has been studied for both mildly interacting and strongly resonant jet conditions. A version of the Hanjalic et al. model, which acc...

The Direct Simulatiaon Monte Carlo method is used to study the hypersonic, rarefied flow interference effects on a flat plate caused by nearby surfaces. Calculations focus on shock-boundary-layer and shock-lip interactions in hypersonic inlets. Results are presented for geometries consisting of a flat plate with different leading-edge shapes over a...

A two-dimensional, Navier-Stokes code developed by Imlay based on the implicit, finite-volume method of MacCormack has been applied to the prediction of the flow fields and performance of several nonaxisymmetric, convergent-divergent nozzles with and without thrust vectoring. Comparisons of predictions with experiment show that the Navier-Stokes co...

A modified version of the multiscale turbulence model of Hanjalic has been applied to the problem of supersonic jets exhausting into still air. In particular, the problem of shock-cell decay through turbulent interaction with the mixing layer has been studied for both mildly interacting and strongly resonant jet conditions. The modified Hanjalic mo...

The first NASA Space Shuttle flight (STS-1) produced an overpressure wave that exceeded preflight predictions by as much as 5 to 1. This second overpressure wave occurred just after the solid rocket booster (SRB) igniter wave. To understand this overpressure phenomenon, a numerical simulation effort was undertaken. Both the SRB static firing test a...

Prediction methods for several nozzle/afterbody flow problems at subsonic and transonic speeds are presented. These methods range from viscous-inviscid interaction methods to solutions for the Navier-Stokes equations in two and three dimensions. The problems addressed are the flow around isolated axisymmetric nozzles, isolated nonaxisymmetric nozzl...

A GENERAL user-oriented computer program called VNAP2 has been developed to calculate high Reynolds number, internal/external flows. VNAP2 solves the twodimensional, time-dependent Navier-Stokes equations. Turbulence is modeled with mixing-length and transport equation models. Interior grid points are computed using the explicit MacCormack scheme w...

The importance of aerodynamic interactions associated with external flow-field effects on turbulent jet exhaust plume structure is discussed. A viscous/inviscid prediction technique is presented which combines the overlaid mixing and inviscid plume components of the JANNAF Standardized Plume Flow-Field (SPF) model with inviscid external flow and bo...

A viscous-inviscid interaction method to calculate the subsonic and transonic flow over nozzle afterbodies with supersonic jet exhausts was developed. The method iteratively combines a relaxation solution of the full potential equation for the inviscid external flow, a shock capturing-shock fitting inviscid jet solution, an integral boundary layer...

A viscous-inviscid interaction model for predicting jet entrainment effects on axisymmetric, nozzle afterbodies at subsonic speeds is presented. The model is based on a displacement thickness correction to the inviscid jet boundary that accounts for mixing-induced streamline deflections in the inviscid region. The displacement correction is shown t...

A viscous-inviscid interaction model was developed to account for jet entrainment effects in the prediction of the subsonic flow over nozzle afterbodies. The model is based on the concept of a weakly interacting shear layer in which the local streamline deflections due to entrainment are accounted for by a displacement-thickness type of correction...

The development of a computational model (BOAT) for calculating near-field jet entrainment, and its application to the prediction of nozzle boattail pressures, is discussed. BOAT accounts for the detailed turbulence and thermochemical processes occurring in the near-field shear layers of exhaust plumes, while interfacing with the inviscid exhaust a...

A viscous-inviscid interaction model has been developed which accounts for jet entrainment effects in the prediction of the subsonic flow over nozzle afterbodies. The jet entrainment model is based on the concept of a weakly interacting shear layer in which the local streamline deflections due to entrainment are accounted for by a displacement-thic...

The development of a computational model (BOAT) for calculating nearfield jet entrainment, and its application to the prediction of nozzle boattail pressures, is discussed. BOAT accounts for the detailed turbulence and thermochemical processes occurring in the nearfield shear layers of jet engine (and rocket) exhaust plumes while interfacing with t...

A pulsed molecular beam apparatus is used to measure mean stay times for gases physisorbing on cooled surfaces. Most of the data are for Xe on nickel surfaces. Data are also presented for Kr and CO2 on nickel, Xe on copper, and Xe on ion-sputter-cleaned nickel. All targets are polycrystalline. Surface temperatures range from 92 to 125 K and measure...

During the first few seconds of the space shuttle trajectory, the solid rocket boosters will be in the proximity of the launch pad. Because of the launch pad structures and the surface of the earth, the turbulent mixing experienced by the exhaust gases will be greatly increased over that for the free flight situation. In addition, a system will be...

An analytical study has been made of the effects of viscous/inviscid interactions on the transonic flow over boattail nozzles. The theoretical method couples a relaxation solution of the full potential transonic flow equations with a conventional boundary-layer solution to account for displacement-thickness effects. Surface pressures calculated on...

The pulse height response of a right angle scattering optical particle counter has been investigated using monodisperse aerosols of polystyrene latex spheres, di-octyl phthalate and methylene blue. The results confirm previous measurements for the variation of mean pulse height as a function of particle diameter and show good agreement with the rel...

Stay-time distributions have been obtained for Xe physisorbing on polycrystalline nickel as a function of the target temperature using a pulsed molecular-beam technique. Some interesting effects due to ion bombardment of the surface using He, Ar, and Xe ions are presented. Measured detector signal shapes are found to deviate from those predicted fo...

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A general, user oriented computer program, called VNAP2, has been developed to calculate high Reynolds number, internal/external flows. VNAP2 solves the two-dimensional, time-dependent Navier-Stokes equations. The turbulence is modeled with either a mixing-length, a one transport equation, or a two transport equation model. Interior grid points are...

High-intensity, nearly monoenergetic, nozzle-type molecular beams together with a sensitive time-of-flight detection system have been employed to study the scattering of argon and nitrogen from polycrystalline nickel and stainless-steel surfaces. The apparatus has allowed the precise measurement of essentially complete flux and speed distributions...