This study investigates the effects of continuum breakdown on the surface aerothermodynamic properties (pressure, shear, heat transfer rate) of a sphere in Mach 10, 25, and 45 flows of nitrogen gas in regimes varying from continuum to rarefied gas. A rotational energy relaxation model is employed in the computational fluid dynamics code and is tested to confirm its accuracy. As the global Knudsen number is increased, from continuum flow to a rarefied gas, the amount of continuum breakdown seen in the flow and on the surface is increased. This increase in continuum breakdown affects the surface properties, such that an increase in the differences between computational fluid dynamics and direct simulation Monte Carlo method is observed. As the Mach number is increased, the amount of continuum breakdown observed in the flow is increased, but the gradient length local Knudsen number stays approximately constant. Even though the amount of continuum breakdown has increased, the difference between computational fluid dynamics and direct simulation Monte Carlo method remains relatively constant. The last part of this study compares the results of the sphere with that of the analogous cylinder case. At the same global Knudsen number, the differences in the surface properties between computational fluid dynamics and direct simulation Monte Carlo method increase when the simulation is run axisymmetrically.

A modular particle-continuum method is extended to include thermal nonequilibrium between translational and rotational energy modes to simulate hypersonic steady-state flows that exhibit small regions of collisional nonequilibrium in a mainly continuum flowfield. This method loosely couples an existing direct-simulation Monte Carlo code to a Navier-Stokes solver (computational fluid dynamics) while allowing both time step and cell size to be completely decoupled between each method. By limiting the size of the direct-simulation Monte Carlo region to only areas in collisional nonequilibrium, the modular particle-continuum method is able to reproduce full direct-simulation Monte Carlo results for flows with global Knudsen numbers of 0.01 and 0.002 while decreasing the computational time required by factors of 2.94 and 28.1, respectively. The goal of the present study is to include consistent models that separate rotational and translational modes in both flow modules. Inclusion of rotational relaxation decreases the computational cost of the modular particle-continuum method.

A hypersonic vehicle crosses many regimes from rarefied to continuum due to the change in density with altitude during the course of its trajectory through a planet's atmosphere. This variation makes it difficult to simulate the flow since the physical accuracy of computational fluid dynamics (CFD) can break down in rarefied flows and the direct simulation Monte Carlo (DSMC) method is computationally expensive in continuum flows. This study investigates the effects of continuum break down on the surface aerothermodynamic properties (pressure, shear, heat transfer rate) of a sphere in Mach 10, 25, and 45 flow of nitrogen gas in regimes varying from continuum flow to rarefied gas flow. A rotational energy relaxation model is employed in the CFD code and is tested to confirm its accuracy. As the global Knudsen number is increased, from continuum flow to a rarefied gas, the amount of continuum breakdown seen in the flow and on the surface is increased. This increase in continuum breakdown affects the surface properties, such that an increase in the differences between CFD and DSMC is observed. As the Mach number is increased, the amount of continuum breakdown observed in the flow is increased, but the gradient length local Knudsen number stays approximately constant. Even though the amount of continuum breakdown has increased, the difference between CFD and DSMC remains relatively constant. The last part of this study compares the results of the sphere with that of the analogous cylinder case. At the same global Knudsen number, the differences in the surface properties between CFD and DSMC increase when the simulation is run axisymmetrically.

This study investigates the effects of continuum breakdown on the surface aerothermodynamic properties ͑pressure, stress, and heat transfer rate͒ of a sphere in a Mach 25 flow of reacting air in regimes varying from continuum to a rarefied gas. Results are generated using both continuum ͓computational fluid dynamics ͑CFD͔͒ and particle ͓direct simulation Monte Carlo ͑DSMC͔͒ approaches. The DSMC method utilizes a chemistry model that calculates the backward rates from an equilibrium constant. A preferential dissociation model is modified in the CFD method to better compare with the vibrationally favored dissociation model that is utilized in the DSMC method. Tests of these models are performed to confirm their validity and to compare the chemistry models in both numerical methods. This study examines the effect of reacting air flow on continuum breakdown and the surface properties of the sphere. As the global Knudsen number increases, the amount of continuum breakdown in the flow and on the surface increases. This increase in continuum breakdown significantly affects the surface properties, causing an increase in the differences between CFD and DSMC. Explanations are provided for the trends observed.

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