Compressibility Effects on Roughness-induced Boundary Layer Transition

Direct numerical simulation is used to investigate the effect of compressibility on roughness-induced boundary layer transition. Computations are performed in both the lowand the high-speed regime (at free-stream Mach number Me = 2) for an isolated three-dimensional element with cubic shape and for two-dimensional roughness strips. For each configuration two values of the roughness height Reynolds number (Rek = 315, 505) are considered. In the case of 3D roughness, the main effect of the obstacle is the generation of streamwise and wall-normal vorticity, with the formation of an unstable detached shear-layer. Consistent with previous experimental observations, the unsteady release of hairpin vortices past the disturbing element is observed at sufficiently high Rek for both incompressibile and supersonic flows, which eventually results in the breakdown to turbulence. A major effect of compressibility is observed for two-dimensional roughness, in which case transition is found in the incompressible simulations for both values of Rek, but only for the higher Rek under supersonic conditions.