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In 2009, a series of boundary layer transition experiments were designed and flown as part of the STS-119, STS-125 and STS-128 missions (Figure 1).  As part of these flight experiments, a discrete protuberance was integrated into the shuttle tile material, and this tile was affixed to the port side wing on the Orbiter belly.  The surrounding tiles were instrumented with thermocouples in efforts to characterize the onset of boundary layer transition, and to measure the heating augmentation to the shuttle surface as a results of this event. A priori calculations made by high-fidelity Navier-Stokes (CFD) solvers indicate peak heating on the protuberance that was four times higher than the peak heating measured during the flight experiments.

This work seeks to examine the importance of non-equilibrium effects using a hybrid DSMC/CFD approach to model hypersonic boundary layer flow over discrete roughness.  The purpose of these investigations is to identify and quantify the non-equilibrium effects that influence the roughness-induced disturbance field and surface quantities of interest for engineering applications.  To this end, a new hybrid framework is developed for high-fidelity hybrid solutions involving five-species air hypersonic boundary layer flow applications. A novel approach based on Generalized Chapman-Enskog Theory is developed for DSMC particle generation at a hybrid interface for gas mixtures with internal degrees of freedom (Figure 2). Additionally, a general best-fit approach is developed for the consistent treatment of diffusion, viscosity and thermal conductivity for a five-species air mixture.

This hybrid approach is applied to examine the roughness-induced disturbance field and surface quantities for a variety of flow conditions and roughness configurations (Figure 3).  In all cases examined, the hybrid solution predicts a lower peak surface heating on the roughness compared to the CFD solution, and a higher peak surface heating in the wake due to vortex heating (Figure 4). The observed differences in vortex heating are a result of stronger streamwise vortices (highlighted using the Q-criterion) predicted by the hybrid solution (Figure 5).

Funding for this research was provided by NASA and AFOSR.