Numerical investigation of the effects of continuum breakdown on hypersonic vehicle surface properties

A hypersonic vehicle crosses many regimes, from rarefied to continuum, as the vehicle descends through the atmosphere. This variation makes it difficult to simulate the flow since the physical accuracy of computational fluid dynamics (CFD) can breakdown in rarefied flows and the direct simulation Monte Carlo (DSMC) method is computationally expensive in continuum flows.;This dissertation investigates the effects of continuum breakdown on the surface aerothermodynamic properties of a hypersonic vehicle. The study begins by investigating a sphere in Mach 10, 25, and 45 flow of non-reacting nitrogen gas. The consideration of nitrogen gas allows the study of the effects of thermal nonequilibrium. The next portion of the study investigates a sphere in Mach 25 reacting flows of both nitrogen and air. This adds multi-species flow to the simulation and permits for reacting flow, which allows the study of the effects of thermal and chemical nonequilibrium.;A separate rotational energy equation is employed in the CFD method to be able to simulate rotational nonequilibrium. Since CFD is numerically more efficient than DSMC, slip boundary conditions are included into the CFD method to extend the range where CFD can be accurately utilized. An investigation of other physical models in both numerical methods is conducted to ensure they are equivalent.;In a flow of nitrogen, 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 increases. This causes an increase in the differences between CFD and DSMC. As the Mach number increases, the amount of continuum breakdown observed increases. However, the difference between CFD and DSMC remains relatively constant. The differences in the surface properties between CFD and DSMC increase when the simulation is run axisymmetrically in comparison to two-dimensional. It is also seen that chemically reacting flow causes the integrated drag to increase, while it decreases the peak heating. Reacting flow is also seen to decrease the amount of continuum breakdown in the flow field.