Simulation of High-Altitude Plume Flow Fields Using a Hybrid Continuum CFD/DSMC Approach

This paper demonstrates the newly developed particle method in the Direct Simulation Monte Carlo (DSMC) framework, and we refer to it as the Maxwellian–DSMC (M–DSMC) method. The M–DSMC solver is utilized for various fluid flow problems with different length scales varied from continuum to transitional regime. The hypersonic flow of argon over a cylinder simulation is carried out using M–DSMC and the same simulation results are compared with the regular DSMC method. The present work shows that the M–DSMC results are quite a good match with the DSMC results. The computational cost is significantly reduced by using M–DSMC method and the same is compared with regular DSMC method.

When vehicles reenter the Earth’s atmosphere from space, the hypersonic conditions are sufficiently energetic to generate ionizing reactions. The production of a thin plasma layer around a hypersonic vehicle can block radio waves sent to and from the vehicle, leading to communications blackout. For Earth entry from orbit, the maximum energy involved in molecular collisions requires only associative ionization of air-species to be considered. In the present study, the modeling of such reactions is considered in detail using the direct simulation Monte Carlo (DSMC) method. For typical Earth entry conditions, with a velocity near 8km∕s, it is shown that the average ionizing reaction probabilities are small. Special numerical techniques must therefore be used in the DSMC technique in order to numerically resolve these reactions. Additional simulation problems arise from the relatively small mass of the electrons in comparison to the other atoms and molecules in these flow fields. Artificially increasing the electron mass greatly increases computational efficiency, and the viability of this approach is investigated. Simulation results are presented for conditions corresponding to the RAM-C II hypersonic flight experiment that gathered measurements of electron number density. It is demonstrated that simulation results for electron number density in this energy regime are relatively insensitive to the mass of the electrons. Direct comparison of DSMC results with the RAM-C II measurements for electron number density shows excellent agreement. These satisfactory comparisons represent the first direct verification of the ability of the DSMC technique to successfully predict the weak plasma generated around a hypersonic vehicle.

When vehicles re-enter the earth’s atmosphere from space, the hypersonic conditions are sucien tly energetic to generate ionizing reactions. The production of a thin plasma layer around a hypersonic vehicle can block radio waves sent to and from the vehicle leading to communications blackout. For earth entry from orbit, the maximum energy involved in molecular collisions requires only associative ionization of air{species to be considered. In the present study, the modeling of such reactions is considered in detail using the direct simulation Monte Carlo (DSMC) method. For typical earth entry conditions, with a velocity near 8 km/s, it is shown that the average ionizing reaction probabilities are small. Special numerical techniques must therefore be used in the DSMC technique in order to numerically resolve these reactions. Additional simulation problems arise from the relatively small mass of the electrons in comparison to the other atoms and molecules in these o w elds. Articially increasing the electron mass greatly increases computational eciency , and the viability of this approach is investigated. Simulation results are presented for conditions corresponding to the RAM-C II hypersonic igh t experiment that gathered measurements of electron number density. It is demonstrated that simulation results for electron number density in this energy regime are relatively insensitive to the mass of the electrons. Direct comparison of DSMC results with the RAM-C II measurements for electron number density show excellent agreement.

The first reported planar laser-induced fluorescence (PLIF) images depicting the relative ground-state, neutral density of Si within a pulsed laser ablation (PLA) plume as it expands into vacuum and 10 mTorr Ar are presented. Because of the high flow speeds in PLA plumes and the narrow bandwidth of typical lasers, several images acquired with the laser detuned incrementally must be superimposed to produce images which compensate for Doppler shifts. Density profiles of Si along the axis of symmetry as a plume expands into Ar at pressures of 0-150 mTorr demonstrate the influence of a background gas and substrate on the expanding plume in the interval 0 < t < 5μs. In-flight production of SiO along the contact as an Si plume expands into 1.0 Torr air is demonstrated in the first reported images of a reactive intermediate species produced during PLA. The expansion of Si into both 10 mTorr and 100 mTorr Ar is investigated using the technique of Direct Simulation Monte Carlo (DSMC), with a simple ablation model used to generate the Si plume. As a consequence of the rapid displacement of background gas, i.e. the snowplow effect, the plume divides into two components, one which travels nearly collisionlessly toward the substrate and one which interacts more strongly with the gas. At 10 mTorr, collisions between fast component Si atoms and Ar atoms during the period t < 250 ns are responsible for most of the energy transfer from Si to Ar. This results in the creation of a fast (15 eV), collisionless Ar component which impacts the substrate shortly after the fast Si component arrives, but does not result in thermal heating. At 100 mTorr, the energy transfer from the plume to the Ar gas occurs on a μs time scale and is attributed primarily to thermal heating. The energy of the fast Ar component decreases to about 5 eV. Finally, the DSMC technique is employed to study the expansion of Si into both 10 mTorr and 100 mTorr O2. The chemical reaction Si + O2 → SiO + O is investigated, including rotational and vibrational excitation the diatomic molecules O2 and SiO. At both pressures, SiO first appears along the sides of the expanding plume and, at 100 mTorr, appears only along the contact front between the Si and O2. This result compares well will PLIF data. An examination of the O2 rotational temperature indicates that the background gas is heated along a conical surface whose tip is located at the ablation spot as the plume expands. Collectively, these simulations demonstrate the potential of the DSMC method to provide quantitative information about the reactive expansion of a wide variety of PLA plumes into rarified background gas mixtures, and study the particle flux onto a substrate when parameters such as background gas pressure and composition are varied.

Direct Simulation Monte Carlo (DSMC) solves the Boltzmann equation with large Knudsen number. The Boltzmann equation generally consists of three terms: the force term, the diffusion term and the collision term. While the first two terms of the Boltzmann equation can be discretized by numerical methods such as the finite volume method, the third term can be approximated by DSMC, and DSMC simulates the physical behaviors of gas molecules. However, because of the low sampling efficiency of Monte Carlo Simulation in DSMC, this part usually occupies large portion of computational costs to solve the Boltzmann equation. In this paper, by Markov Chain Monte Carlo (MCMC) and multicore programming, we develop Direct Simulation Multi-Chain Markov Chain Monte Carlo (DSMC3): a fast solver to calculate the numerical solution for the Boltzmann equation. Computational results show that DSMC3 is significantly faster than the conventional method DSMC.

Aerodynamic heating in hypersonic flow presents one of the most serious challenges in the design of space vehicles. In the present study, heat flux reduction mechanism of the pulsed counterflowing jet from the nose of a blunt body is numerically investigated. Flow field around the hemisphere model is calculated in hypersonic free stream of Mach number 10 and Knudsen number 0.4. Considering that the external flow field is highly rarefied, recent works have demonstrated the direct simulation Monte Carlo (DSMC) technique to accurately predict the flow field under steady-state conditions. For supersonic flow with a counterflowing jet, the mean free path could vary by orders of magnitude within the computation domain and the computational mesh size in DSMC should be of one third of local mean free path (MFP) of the gas due to the requirement of selecting colliding partners among particles. Therefore, the open-source SPARTA code with octree Cartesian mesh is utilized in our research with Adaptive Mesh Refinement (AMR), which has been widely used in modern DSMC codes. AMR is crucial for efficient solutions of multi-scale problems characterized by large gradients of parameters in localized areas of the computational domain. Currently, tree-based methods have being developed for extremely large scale AMR simulations with billions of cells. As a conclusion, significant decreases of surface heat flux are obtained and counterflowing jet is proved to be effective on heat flux reduction in the vicinity of the stagnation point.

This paper models a specific example of spacecraft glow from the Atmospheric Explorer Satellite, a small clean satellite, where in-situ measurements were made in the visible spectral region. The direct simulation Monte Carlo (DSMC) technique was used to model the formation of NO in the flowfield of a 1-meter nose radius satellite from altitudes of 130 to 200 km. This species has been suggested as the precursor to the formation of NC>2, the molecular system that produces the spacecraft glow. A surface model was used in conjunction with the DSMC calculations to model the conversion of NO to NO2 and radiation. The satellite orbit-dependent radiation was determined and compared with data. The results show that the surface model can explain both the absolute magnitude as well as most of the altitude dependence of the glow data.

Flow and thermal field investigation of rarefied gas in a trapezoidal micro/nano-cavity using DSMC

The study of the nuclear source term requires the computation of aerosol dynamics. Solutions to the aerosol general dynamic equation (GDE) are difficult to obtain by analytical or numerical methods when more realistic problems are considered. The direct simulation Monte Carlo (DSMC) technique is capable of simulating aerosol evolution reducing simplifications in the implementation of the aerosol GDE. In this work we present a DSMC program for the simulation of multi-component polydisperse aerosol evolution, with the successful integration of the following processes: deposition, electrostatic dispersion, coagulation (considering charge effects) and condensation, assuming a spatially homogeneous medium and spherical particles. Two problems with different particle compositions were simulated to obtain information about the interactions through the different processes and the interacting particles as well as particle number and mass distributions with discrimination of charge levels. This information allowed us to explore the synergistic nature of these processes. It was found that the problem with denser particles had an overall stronger activity in coagulation and initially a stronger activity in deposition compared to the problem with less dense particles. Experiments to generate two-component aerosols by spark discharge in the study of aerosol coagulation with characterization of particle composition are proposed. Moreover, the use of particle weights in the DSMC method is explored.

Recently, as aerodynamics was applied to flying vehicles with very high speed and flying at high altitude, the numerical simulation based on the Navier–Stokes (NS) equations was found that cannot correctly predict certain aero-thermo-dynamic properties in a certain range of velocity and altitude while the Knudsen number indicates that the flow is still in the continuum regime. As first noted by Zhou and Zhang (Science in China, 2015), the invalidity of NS equations for such flows might be attributed to an non-equilibrium effect originating from the combined effects of gas rarefaction and strong shear in the boundary-layer flows. In this paper, we present the scope, physical concept, mathematical model of this shear non-equilibrium effect in hypersonic flows, as well as the way of considering this effect in conventional computational fluid mechanics (CFD) for engineering applications. Several hypersonic flows over sharp bodies and blunt bodies are analyzed by the proposed new continuum model, named direct simulation Monte Carlo (DSMC) data-improved Navier–Stokes (DiNS) model. Recently, as aerodynamics was applied to flying vehicles with very high speed and flying at high altitude, the numerical simulation based on the Navier–Stokes (NS) equations was found that cannot correctly predict certain aero-thermo-dynamic properties in a certain range of velocity and altitude while the Knudsen number indicates that the flow is still in the continuum regime. As first noted by Zhou and Zhang (Science in China, 2015), the invalidity of NS equations for such flows might be attributed to an non-equilibrium effect originating from the combined effects of gas rarefaction and strong shear in the boundary-layer flows. In this paper, we present the scope, physical concept, mathematical model of this shear non-equilibrium effect in hypersonic flows, as well as the way of considering this effect in conventional computational fluid mechanics (CFD) for engineering applications. Several hypersonic flows over sharp bodies and blunt bodies are analyzed by the proposed new continuum model, named direct simulation Monte Carlo (DSMC) data-improved Navier–Stokes (DiNS) model..

Establishment of particle motion model and study of particle volume fraction distribution in the turbo air classifier based on DSMC

Three-dimensional plume impingement flows are simulated using the direct simulation Monte Carlo (DSMC) technique. Two impingement problems are computed. The impact of a jet of nitrogen on an inclined flat plate is considered. A free molecular model of the plume is shown to provide reasonable estimates of impingement quantities. The plume of a hydrazine control thruster firing in a model satellite configuration is simulated. Surface quantities and net impingement effects are calculated. Free molecular analysis provides less accurate modeling of a multispecies flow with boundary-layer effects

This paper aims to settle disputes in the literature about the spatial variation of the molecular mean free path (MFP) in confined geometries. The MFP of a gas is determined by using both molecular dynamics (MD) and the direct simulation Monte Carlo (DSMC) technique. In spatially-homogeneous cases, the numerical results exactly recover the kinetic theory predictions of a constant MFP. However, in microchannels, the MFP is found to vary near to the bounding walls and reduce at the surfaces to half of its bulk value as long as collisions between gas molecules and wall atoms are taken into account in the calculation of the MFP.

A study of ‘thermophoresis-like’ force on a heated trapezoidal nano-object confined between parallel plates

We study the dynamics of a two-dimensional driven inelastic gas, by means of direct simulation Monte Carlo techniques, i.e., under the assumption of molecular chaos. Under the effect of a uniform stochastic driving in the form of a white noise plus a friction term, the gas is kept in a nonequilibrium steady state characterized by fractal density correlations and non-Gaussian distributions of velocities; the mean-squared velocity, that is the so-called granular temperature, is lower than the bath temperature. We observe that a modified form of the Kubo relation, which relates the autocorrelation and the linear response for the dynamics of a system at equilibrium, still holds for the off equilibrium, though stationary, dynamics of the systems under investigation. Interestingly, the only needed modification to the equilibrium Kubo relation is the replacement of the equilibrium temperature with an effective temperature, which results equal to the global granular temperature. We present two independent numerical experiments, where two different observables are studied: (a) the staggered density current, whose response to an impulsive shear is proportional to its autocorrelation in the unperturbed system and (b) the response of a tracer to a small constant force, switched on at time t(w), which is proportional to the mean-square displacement in the unperturbed system. Both measures confirm the validity of Kubo's formula, provided that the granular temperature is used as the proportionality factor between response and autocorrelation, at least for not too large inelasticities.