Numerical simulations of axisymmetric flows over reentry configurations at hypersonic conditions using a Navier-Stokes solver are presented. The Navier-Stokes equations are modified using Park's two-temperature model to account for thermochemical nonequilibrium and weak ionization effects. The finite-volume method is used to solve the set of differential equations. The code has the capability to handle any mixture of hexahedra, tetrahedra, prisms and pyramids in 3D or triangles and quadrilaterals in 2D. The results in this paper only use quadrilaterals. Numerical fluxes between the cells are discretized using a modified Steger-Warming Flux Vector Splitting approach which has low dissipation and is appropriate to calculate boundary layers. A point or line implicit method is used to perform the time integration. Pressure, heat transfer rates and electron number density profiles are compared to available experimental and flight measurements.

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Numerical simulations of a spacecraft at reentry conditions are presented. The fluid at such conditions is modeled as a reacting gas in thermal and chemical nonequilibrium. The reacting gas is modeled using a standard finite rate chemistry model for air. A twotemperature model is used to account for the thermal nonequilibrium effects. The finitevolume method is used to solve the set of differential equations on unstructured meshes. Numerical fluxes between the cells are discretized using a modified Steger-Warming Flux Vector Splitting (FVS) which has low dissipation and is appropriate to calculate boundary layers. The scheme switches back to the original Steger-Warming FVS near shock waves. A point implicit method is used to perform the time march. The numerical results for heat transfer are compared to available experimental data. The influence of the method used to switch between schemes on the results is assessed.

Re-entry vehicles designed for space exploration are usually equipped with thermal protection systems made of ablative material. In order to properly model and predict the aerothermal environment of the vehicle, it is imperative to account for the gases produced by ablation processes. In the case of charring ablators, where an inner resin is pyrolyzed at a relatively low temperature, the composition of the gas expelled into the boundary layer is complex and may lead to thermal chemical reactions that cannot be captured with simple flow chemistry models. In order to obtain better predictions, an appropriate gas flow chemistry model needs to be included in the CFD calculations. Using a recently developed chemistry model for ablating carbon-phenolic-in-air species, a CFD calculation of the Stardust re-entry at 71 km is presented. The code used for that purpose has been designed to take advantage of the nature of the problem and therefore remains very efficient when a high number of chemical species are involved. The CFD result demonstrates the need for such chemistry model when modeling the flow field around an ablative material. Modeling of the nonequilibrium radiation spectra is also presented, and compared to the experimental data obtained during Stardust re-entry by the Echelle instrument. The predicted emission from the CN lines compares quite well with the experimental results, demonstrating the validity of the current approach.

A modular particle-continuum (MPC) numerical method is presented which solves the Navier-Stokes (NS) equations in regions of near-equilibrium and uses the direct simulation Monte Carlo (DSMC) method where the flow is in non-equilibrium. The MPC method is designed specifically for steady-state, hypersonic, nonequilibrium flows and couples existing, state-of-the-art DSMC and NS solvers into a single modular code. The MPC method is tested for 2D flow of N 2 at various Mach numbers over a cylinder where the global Knudsen number is 0.01. For these conditions, NS simulations significantly over-predict the local shear-stress, and also over-predict the peak heating rate by 5-10% when compared with full DSMC simulations. DSMC also predicts faster wake closure and 10-15% higher temperatures in the immediate wake region. The MPC code is able to accurately reproduce DSMC flow field results, local velocity distributions, and surface properties up to 2.8 times faster than full DSMC simulations. The computational time saved by the MPC method is directly proportional to the fraction of the flow field which is in near-equilibrium. It is found that particle simulation of the shock interior is not necessary for accurate prediction of surface properties, however particle simulation of the boundary layer and near-wake region is.

A modular particle-continuum (MPC) numerical method is presented which solves the Navier-Stokes (NS) equations in regions of near-equilibrium and uses the direct simulation Monte Carlo (DSMC) method where the flow is in non-equilibrium. The MPC method is designed specifically for steady-state, hypersonic, nonequilibrium flows and couples existing, state-of-the-art DSMC and NS solvers into a single modular code. The MPC method is tested for 2D flow of N 2 at various Mach numbers over a cylinder where the global Knudsen number is 0.01. For these conditions, NS simulations significantly over-predict the local shear-stress, and also over-predict the peak heating rate by 5-10% when compared with full DSMC simulations. DSMC also predicts faster wake closure and 10-15% higher temperatures in the immediate wake region. The MPC code is able to accurately reproduce DSMC flow field results, local velocity distributions, and surface properties up to 2.8 times faster than full DSMC simulations. The computational time saved by the MPC method is directly proportional to the fraction of the flow field which is in near-equilibrium. It is found that particle simulation of the shock interior is not necessary for accurate prediction of surface properties, however particle simulation of the boundary layer and near-wake region is.

Hypersonic vehicles experience different flow regimes during flight due to changes in atmospheric density. Computational fluid dynamics, although relatively computationally inexpensive, is not physically accurate in areas of highly nonequilibrium flows. The direct simulation Monte Carlo method, although physically accurate for all flow regimes, is relatively computationally expensive. In a continuing effort to understand the performance of computational fluid dynamics and direct simulation Monte Carlo in hypersonic flows, the current study investigates the effect of continuum breakdown on surface aerothermodynamic properties (pressure, shear stress, and heat transfer rate) of a cylinder in Mach-10 and Mach-25 flows of argon gas for several different flow regimes, from the continuum to a rarefied gas. Several different velocity-slip and temperature-jump boundary conditions are examined for use with the computational fluid dynamics method. Computational fluid dynamics and direct simulation Monte Carlo solutions are obtained at each condition. Total drag and peak heat transfer rate predictions by computational fluid dynamics remain within about 6% of the direct simulation Monte Carlo predictions for all regimes considered, with the Gökçen-type slip condition giving the best results.

A modular particle-continuum numerical method is used to simulate the flow over a 70 deg blunted cone planetary probe geometry under various steady-state hypersonic conditions. The conditions studied correspond to low global Knudsen number flow where hypersonic velocities induce a large range of temporal and spatial scales that must be modeled. The modular particle-continuum algorithm loosely couples direct simulation Monte Carlo and Navier-Stokes methods, which operate in nonequilibrium and continuum regions, respectively. In addition, particle and continuum regions have different mesh densities and are updated using different sized time steps. Modular particlecontinuum simulations are shown to reproduce the heating rates, velocity slip, and thermal nonequilibrium on the probe surface, as well as the flowfield properties predicted by the direct simulation Monte Carlo method with high accuracy. For the first time, such a hybrid particle-continuum method is shown to achieve high accuracy while achieving an order of magnitude decrease in required computational time and memory compared with pure particle simulation. The modular particle-continuum simulations agree well with Navier-Stokes simulations and experimental measurements in the dense forebody flow. In the rarefied wake of the planetary probe, the modular particle-continuum results are in better agreement with experimental measurements than Navier-Stokes simulations.