Effects of Continuum Breakdown on the Surface Properties of a Hypersonic Sphere

Abstract

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.