Browsing by Subject "DSMC"

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Development of a hybrid DSMC/CFD method for hypersonic boundary layer flow over discrete surface roughness
(2012-05) Stephani, Kelly Ann; Varghese, Philip L.; Goldstein, David Benjamin, doctor of aeronautics.; Moser, Robert; Raja, Laxminarayan; Levin, Deborah
This work is focused on the development of a hybrid DSMC/CFD solver to examine hypersonic boundary layer flow over discrete surface roughness. The purpose of these investigations is to identify and quantify the non-equilibrium effects that influence the roughness-induced disturbance field and surface quantities of interest for engineering applications. To this end, a new hybrid framework is developed for high-fidelity hybrid solutions involving five-species air hypersonic boundary layer flow applications. A novel approach is developed for DSMC particle generation at a hybrid interface for gas mixtures with internal degrees of freedom. The appropriate velocity distribution function is formulated in the framework of Generalized Chapman-Enskog Theory, and includes contributions from species mass diffusion, shear stress and heat fluxes (both translational and internal) on the perturbation of the equilibrium distribution function. This formulation introduces new breakdown parameters for use in hybrid DSMC/CFD applications, and the new sampling algorithm allows for the generation of DSMC internal energies from the appropriate non-equilibrium distribution for the first time in the literature. The contribution of the internal heat fluxes to the overall perturbation is found to be of the same order as the stress tensor components, underscoring the importance of DSMC particle generation from the Generalized Chapman-Enskog distribution. A detailed comparison of the transport coefficients is made between the DSMC and CFD solvers, and a general best-fit approach is developed for the consistent treatment of diffusion, viscosity and thermal conductivity for a five-species air gas mixture. The DSMC VHS/VSS model parameters are calibrated through an iterative fitting approach using the Nelder-Mead Simplex Algorithm. The VSS model is found to provide the best fit (within 5% over the temperature range) to the transport models used in the CFD solver. The best-fit five-species air parameters are provided for general use by the DSMC community, either for hybrid applications or to provide improved consistency in general DSMC/CFD applications. This hybrid approach has been applied to examine hypersonic boundary layer flow over discrete surface roughness for a variety of roughness geometries and flow conditions. An (asymmetric) elongated hump geometry and (symmetric) diamond shaped roughness geometry are examined at high and low altitude conditions. Detailed comparisons among the hybrid solution and the CFD no-slip and slip wall solutions were made to examine the differences in surface heating, translational/vibrational non-equilibrium in the flow near the roughness, and the vortex structures in the wake through the Q-criterion. In all cases examined, the hybrid solution predicts a lower peak surface heating to the roughness compared to either CFD solution, and a higher peak surface heating in the wake due to vortex heating. The observed differences in vortex heating are a result of the predicted vortex structures which are highlighted using the Q-criterion. The disturbance field modeled by the hybrid solution organizes into a system of streamwise-oriented vortices which are slightly stronger and have a greater spanwise extent compared to the CFD solutions. As a general trend, it was observed that these differences in the predicted heating by the hybrid and CFD solutions increase with increasing Knudsen number. This trend is found for both peak heating values on the roughness and in the wake.
DSMC simulations of volatile transport in a transient lunar atmosphere and ice deposition in cold traps after a comet impact
(2017-05) Prem, Parvathy; Goldstein, David Benjamin, doctor of aeronautics; Varghese, Philip L.; Trafton, Laurence M.; Raja, Laxminarayan L; Elphic, Richard C
Over the years, a number of missions have detected signs of water and other volatiles in cold, permanently shadowed regions near the lunar poles, where temperatures are sufficiently low that volatile ices can remain stable over geological timescales. Several observations suggest that comet impacts may have played a role in delivering these cold-trapped volatiles. In this work, I use Direct Simulation Monte Carlo (DSMC) simulations to investigate the transport and sequestration of water in the aftermath of a lunar comet impact, focusing on developing a broad understanding of the physical processes that govern the fate of impact-delivered volatiles (particularly water), in order to better interpret remote sensing data. The sheer amount of vapor generated by a volatile-rich impact can transform the Moon’s tenuous, surface-bound exosphere into a collisional, transient atmosphere with characteristic gas dynamic features that influence the redistribution of impact-delivered volatiles. Notably, the simulations indicate that reconvergence of vapor antipodal to the point of impact may result in preferential redistribution of water in the vicinity of the antipode; in some circumstances, water may be distributed non-uniformly between different cold traps. It is also found that atmospheric self-shielding from photodestruction significantly increases the amount of water that reaches the shelter of cold traps. Volatile transport in an impact-generated atmosphere is also influenced by gas phase interactions with solar radiation and the lunar surface. The Moon has a distinctive surface thermal environment, characterized by large gradients in temperature over very small scales. In this work, I develop a stochastic rough surface temperature model that is then coupled to volatile transport simulations. It is found that surface roughness reduces the mobility of water at high latitudes, while also increasing the concentration of atmospheric/exospheric water molecules around the poles. I also implement a coupled DSMC-photon Monte Carlo method to model radiative heat transfer in the evolving, three-dimensional, rarefied atmosphere. The trapping of radiation within the optically thick gas slows the rate of cooling of the expanding vapor cloud, and also affects near-field atmospheric structure and winds. Ultimately, the fate of impact-delivered water is determined by the interplay between these factors.
Modeling reactive rarefied systems using a novel quasi-particle Boltzmann solver
(2020-12-04) Poondla, Yasvanth Kumar; Varghese, Philip L.; Goldstein, David Benjamin, doctor of aeronautics; Raja, Laxminarayan; Liechty, Derek; Moore, Christopher
The goal of this work is to build up the capability of Quasi-Particle Simulation (QuiPS), a novel flow solver, such that it can adequately model the rarefied portion of an atmospheric reentry trajectory. Direct Simulation Monte Carlo (DSMC) is the conventional solver for such conditions, but struggles to resolve transient flows, trace species, and high level internal energy states due to stochastic noise. Quasi-Particle Simulation (QuiPS) is a novel Boltzmann solver that describes a system with a discretized, truncated velocity distribution function. The resulting fixed-velocity, variable weight quasi-particles enable smooth variation of macroscopic properties. The distribution function description enables use of a variance reduced collision model, greatly minimizing expense near equilibrium. Improvements made to the method in this work include parallelization of the collision integral routine, modification of the velocity space definition to improve performance and resolution of the distribution function, and the addition of a neutral chemistry model. Chemistry's dependence on the tail of a distribution function necessitates accurate resolution of said tail, a computationally challenging proposition. The effects of these additions are verified and studied through a number of 0D calculations, including simulations for which analytic solutions exist and model simulations intended to capture relevant physics present in more complicated problems. The explicit representation of internal distributions in QuiPS reveals some of the flaws in existing physics models. Variance reduction, a key feature of QuiPS can greatly reduce expense of multi-dimensional calculations, but is only cheaper when the gas composition is near chemical equilibrium.
Monte Carlo sensitivity analyses of DSMC parameters for ionizing hypersonic flows
(2018-10-09) Higdon, Kyle J.; Goldstein, David Benjamin, doctor of aeronautics; Varghese, Philip L.; Liechty, Derek S; Cruden, Brett A; Raja, Laxminarayan L
This work focuses on the development and sensitivity analyses of a direct simulation Monte Carlo (DSMC) code to understand the complex physical processes that occur during hypersonic entry into a rarefied atmosphere. Simulations are performed on 1-dimensional hypersonic shock scenarios that mimic the conditions of high altitude atmospheric entry to Earth and Saturn with the Computation of Hypersonic Ionizing Particles in Shocks (CHIPS) code. To model hypersonic entry problems accurately, the CHIPS code must resolve nonequilibrium flows and account for a number of complex gas dynamics processes at the molecular level. In this thesis, several high temperature models are added to the CHIPS code including charged particle models and electronic excitation. These models are refined using preliminary sensitivity analyses resulting in improved electronic excitation models and a new backward chemical reaction model. The CHIPS simulations completed in this work reproduce rarefied hypersonic shock tube experiments performed in the Electric Arc Shock Tube (EAST) at NASA Ames Research Center. The CHIPS results are post-processed by the NEQAIR line-by-line radiative solver to compare directly to spectra measured experimentally in EAST. The DSMC techniques used to model hypersonic phenomena require numerous experimentally calibrated parameters. Many of these parameters are inferred from lower temperature experiments, resulting in an unknown amount of uncertainty in the simulated results at the extreme conditions of hypersonic flow. A global Monte Carlo sensitivity analysis is performed by simultaneously varying the CHIPS input parameter values to understand the sensitivity of experimentally measured quantities simulated by the CHIPS and NEQAIR codes. The sensitivity of several of these output quantities is used to rank the input parameters, identifying the most important parameters for the simulation of the hypersonic scenario. It was concluded that experimentally measured radiation intensity is most sensitive to the following key processes: N+e⁻⇌N⁺+e⁻+e⁻, NO+N⁺⇌N+NO⁺, N₂+N⇌N+N+N, N+O⇌NO⁺+e⁻, N+N⇌N₂⁺+e⁻, and Z [subscript elec] for N, O, and N₂⁺. In the future, this ranking can be used to identify which input parameters should be experimentally investigated, where model improvements could be beneficial, and aid in reducing the parameter space for DSMC calibrations to experimental data.
Numerical simulations of the flow produced by a comet impact on the Moon and its effects on ice deposition in cold traps
(2010-05) Stewart, Bénédicte; Goldstein, David Benjamin, doctor of aeronautics; Varghese, Philip; Trafton, Laurence; Raman, Venkatramanan; Hurley, Dana
The primary purpose of this study is to model the water vapor flow produced by a comet impact on the Moon using the Direct Simulation Monte Carlo (DSMC) method. Toward that end, our DSMC solver was modified in order to model the cometary water from the time of impact until it is either destroyed due to escape or photodestruction processes or captured inside one of the lunar polar cold traps. In order to model the complex flow induced by a comet impact, a 3D spherical parallel version of the DSMC method was implemented. The DSMC solver was also modified to take as input the solution from the SOVA hydrocode for the impact event at a fixed interface. An unsteady multi-domain approach and a collision limiting scheme were also added to the previous implementation in order to follow the water from the continuum regions near the point of impact to the much later rarefied atmospheric flow around the Moon. The present implementation was tested on a simple unsteady hemispherical expansion flow into a vacuum. For these simulations, the data at the interface were provided by a 1D analytical model instead of the SOVA solution. Good results were obtained downstream of the interface for density, temperature and radial velocity. Freezing of the vibrational modes was also observed in the transitional regime as the flow became collisionless. The 45° oblique impact of a 1 km radius ice sphere at 30 km/s was simulated up to several months after impact. Most of the water crosses the interface under 5 s moving mostly directly downstream of the interface. Most of the water escapes the gravity well of the Moon within the first few hours after impact. For such a comet impact, only ~3% of the comet mass remains on the Moon after impact. As the Moon rotates, the molecules begin to migrate until they are destroyed or captured in a cold trap. Of the 3% of the water remaining on the Moon after impact, only a small fraction, ~0.14% of the comet mass, actually reaches the cold traps; nearly all of the rest is photo-destroyed. Based on the surface area of the cold traps used in the present simulations, ~1 mm of ice would have accumulated in the polar cold traps after such an impact. Estimates for the total mass of water accumulated in the polar cold traps over one billion years are consistent with recent observations.
Rarefied gas dynamic simulations of planetary atmospheric systems
(2018-02-05) Hoey, William Andrew; Goldstein, David Benjamin, doctor of aeronautics; Varghese, Philip L.; Trafton, Laurence M; Bisetti, Fabrizio; Johnson, Robert E; Tucker, Orenthal J
My doctoral research involves the advanced numerical simulation of rarefied (low-pressure) planetary atmospheres and volcanism with advanced physical modeling, in application of the Direct Simulation Monte Carlo (DSMC) method. This method is the approach of choice for modeling a wide range of continuum-to-rarefied systems - in which the average spacing between molecules in the flow becomes comparable to the flow length scales, and in which traditional means of computing fluid dynamics with the partial differential equations of continuum theory break down. DSMC is a probabilistic technique by which the motions and collisions of representative molecules are computed. Multiple gas species are modelled, along with non-equilibrium radiation, high speed collisions, photochemistry, and a wide range of other relevant physics. Comprehensive atmospheric simulations are computed in parallel on one- and three-dimensional domains that, depending on the scope of a particular project, can span entire atmospheric systems from planetary surface through vacuum. These projects are ongoing efforts in modeling and understanding global-scale atmospheric flows and the processes by which such flows are populated and propagated, and they represent advancements of the state-of-the-art in planetary atmospheric simulation. I have produced and presented research on four distinct topics: 1) simulations of the complete atmosphere of Jupiter's volcanic moon Io including sublimation and plasma-sputtering processes; 2) the creation of a novel neutral density model for Earth's upper-atmosphere in partnership with Los Alamos' ISR division; 3) multi-species simulations of the rarefied gas dynamic, transfer, and escape processes of the Pluto-Charon system; and 4) investigations of the canopy unsteadiness and development of transient filamentary structure as observed by the New Horizons probe at the Ionian Tvashtar plume site. In the course of these projects, and using my research group's existing planetary-science DSMC code as a foundation, I have developed a novel, generalized framework for rarefied atmospheric simulation that enables efficient and thorough construction of entire upper-atmospheric models. My dissertation offers an analysis of the methodology of rarefied gas dynamic planetary atmospheric simulation, in addition to discussion of each project's scientific context, the results of my simulations, and their relevance toward the explanation of various observed phenomena in planetary atmospheric science.
Realistic simulation of Io's Pele plume and its effects on Io's atmosphere
(2015-12) McDoniel, William Joseph; Goldstein, David Benjamin, doctor of aeronautics; Varghese, Philip L.; Trafton, Laurence M; Raja, Laxminarayan L; Spencer, John R
The direct simulation Monte Carlo (DSMC) method is used to investigate gas and dust in Io's giant Pele plume as well as the interaction of giant plumes with other plumes, with Jupiter's plasma torus, and with Io's sublimation atmosphere. Three-dimensional simulations of time-varying systems are performed on up to 2002 processors. Methods for the efficient simulation of interactions between neutral gas and dust particles and between neutral gas and ions (including chemistry) are developed. Simulations are load-balanced dynamically and the grid structure adapts to resolve (potentially moving) regions of high density. Three-dimensional simulations of the Pele plume show how gas erupting from cracks and holes in a lava lake produces the observed plume and deposition pattern. Small-scale features of the lava lake are found to explain the asymmetric shape of the giant deposition ring. Dust particles are included in the simulations and the computed deposition patterns of different sizes of dust are compared with observations in order to obtain a best-fit size distribution for particles in the plume. The interaction of Pele and Pillan is investigated by the simultaneous simulation of both plumes. Giant plumes on Io's equator and north pole are simulated alongside ions which move with Jupiter's magnetic field, and a chemistry model allows for high-energy collisions to produce daughter species. The effect of plasma bombardment on plumes is found to depend on the location of the plumes, and it gives rise to a large, diffuse cloud of neutrals surrounding the plumes. The dense gas in plume canopies also influences the trajectories of ions in the plasma torus, causing ion slip and further asymmetry in the plumes. Dynamic simulations of plumes on Io's equator and at 30 degrees north latitude over an Io day show how plumes interact with Io's sublimation atmosphere. Plume material becomes suspended in the atmosphere, displacing many times the night-side mass of the plume in sublimated material. The total mass of the atmosphere, however, increases by only a fraction of the plume's night-side mass as the surface frost effectively maintains vapor pressure equilibrium.
Simulating water vapor plumes on Europa
(2015-12) Berg, Jared James; Goldstein, David Benjamin, doctor of aeronautics; Varghese, Philip
A computational investigation of water vapor plumes on the Jovian moon Europa was performed with a focus on characteristics relevant to observation and spacecraft mission operations. The Direct Simulation Monte Carlo (DSMC) method was implemented to model the plume expansion. The structure of the plume, including the number density, temperature, and velocity fields was determined. Integrated line of sight column densities of ~1018 H2O molecules/m2 were calculated and compared to observations. The possibility of grain condensation above the vent was considered, but determined to be negligible for the postulated vent size. However, preexisting grains of three diameters (0.1, 1, 50 μm) were included in the simulation and their trajectories examined. A preliminary study of photodissociation of H2O into OH was performed to demonstrate the behavior of daughter species. Different vent parameters were evaluated to determine their effects on the plume, including vent Mach number (Mach 2, 3, 5) and a reduced temperature “Cold” case that was a proxy for energy loss to the region surrounding the vent. Future research pathways are discussed, including higher mass flow cases, unsteady plumes, and accurate modeling of internal energy modes in photodissociation products.
Statistical methods for the analysis of DSMC simulations of hypersonic shocks
(2012-05) Strand, James Stephen; Goldstein, David Benjamin, doctor of aeronautics; Moser, Robert; Varghese, Philip; Ezekoye, Ofodike; Prudencio, Ernesto
In this work, statistical techniques were employed to study the modeling of a hypersonic shock with the Direct Simulation Monte Carlo (DSMC) method, and to gain insight into how the model interacts with a set of physical parameters. Direct Simulation Monte Carlo (DSMC) is a particle based method which is useful for simulating gas dynamics in rarefied and/or highly non-equilibrium flowfields. A DSMC code was written and optimized for use in this research. The code was developed with shock tube simulations in mind, and it includes a number of improvements which allow for the efficient simulation of 1D, hypersonic shocks. Most importantly, a moving sampling region is used to obtain an accurate steady shock profile from an unsteady, moving shock wave. The code is MPI parallel and an adaptive load balancing scheme ensures that the workload is distributed properly between processors over the course of a simulation. Global, Monte Carlo based sensitivity analyses were performed in order to determine which of the parameters examined in this work most strongly affect the simulation results for two scenarios: a 0D relaxation from an initial high temperature state and a hypersonic shock. The 0D relaxation scenario was included in order to examine whether, with appropriate initial conditions, it can be viewed in some regards as a substitute for the 1D shock in a statistical sensitivity analysis. In both analyses sensitivities were calculated based on both the square of the Pearson correlation coefficient and the mutual information. The quantity of interest (QoI) chosen for these analyses was the NO density profile. This vector QoI was broken into a set of scalar QoIs, each representing the density of NO at a specific point in time (for the relaxation) or a specific streamwise location (for the shock), and sensitivities were calculated for each scalar QoI based on both measures of sensitivity. The sensitivities were then integrated over the set of scalar QoIs to determine an overall sensitivity for each parameter. A weighting function was used in the integration in order to emphasize sensitivities in the region of greatest thermal and chemical non-equilibrium. The six parameters which most strongly affect the NO density profile were found to be the same for both scenarios, which provides justification for the claim that a 0D relaxation can in some situations be used as a substitute model for a hypersonic shock. These six parameters are the pre-exponential constants in the Arrhenius rate equations for the N2 dissociation reaction N2 + N ⇄ 3N, the O2 dissociation reaction O2 + O ⇄ 3O, the NO dissociation reactions NO + N ⇄ 2N + O and NO + O ⇄ N + 2O, and the exchange reactions N2 + O ⇄ NO + N and NO + O ⇄ O2 + N. After identification of the most sensitive parameters, a synthetic data calibration was performed to demonstrate that the statistical inverse problem could be solved for the 0D relaxation scenario. The calibration was performed using the QUESO code, developed at the PECOS center at UT Austin, which employs the Delayed Rejection Adaptive Metropolis (DRAM) algorithm. The six parameters identified by the sensitivity analysis were calibrated successfully with respect to a group of synthetic datasets.