Numerical Investigation of the Effects of Continuum Breakdown on Hypersonic Vehicle Surface Properties

Abstract

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.