Analysis of Stationary Crossflow Instability on HIFiRE-5 Using Direct Numerical Simulation

Abstract

Direct numerical simulation is performed on a 38.1% scale HIFiRE-5 forebody to study stationary crossflow instability. Computations use the US3D Navier-Stokes solver to simulate Mach 6 flow at Reynolds numbers of 8.1e6 /m and 11.8e6$ /m, which are conditions used by quiet tunnel experiments at Purdue University. Distributed roughness with point-to-point height variation on the computational grid and maximum heights of 0.5-4.0 microns is used with the intent to emulate smooth-body transition and excite the naturally-occurring most unstable disturbance wavenumber. Cases at the low Reynolds number condition use three grid sizes, and hence three different roughness patterns of varying wavelength, and demonstrate that the final flow solution is extremely dependent on the particular roughness pattern. The same roughness pattern is interpolated onto each grid which yields similar solutions, indicating grid convergence. At the high Reynolds number condition, a steady physical mechanism is introduced which explains sharp increases seen in the wall heat flux for both computations and experiment. Namely, the sharp increase is caused by large streamwise velocity disturbances impinging on the wall. Evolution of disturbance spanwise wavelength is computed, and it is found that this wavelength is more sensitive to Reynolds number than roughness, indicating that the disturbance wavelength is primarily flow--selected for these cases. The calculation of disturbance growth rates shows the region over which crossflow disturbances behave linearly and where nonlinear effects become important. The effect of roughness height and nose sharpness are considered, and both were found to have a large effect on the resulting heating pattern. Crossflow vortex coalescence is observed and a possible cause is discussed.