Experimental and numerical study of coherent structures in a roughness induced transition boundary layer at Mach 5

Abstract

A wind tunnel experiment and large eddy simulation are carried out for the investigations of a Mach 5 boundary layer transition flow induced by distributed ramp shaped roughness elements. The wind tunnel experiment is carried out using the nano-tracer planar laser scattering technique to capture the coherent structures, whereas implicit large eddy simulation is carried out in the same flow conditions. Dominant unstable modes are analyzed. Comparisons show that the major frequencies obtained from the experiment and the computation generally agree with each other, although transition occurs much earlier in the wind tunnel. Coherent structures play a significant role in the transition process. Their formation and development are analyzed. The roughness generates a pair of counter-rotating streamwise vortices in its wake. Two high vorticity regions, i.e., the detached curved shear layer formed by streamwise counter-rotating vortices and the near-wall boundary layer, contribute to a majority of the transition process. In the early transition stage, hairpin-like vortices are generated as a consequence of the Kelvin-Helmholtz instability of the curved shear layer. Meanwhile, vertical convection is formed, which enhances the disturbance growth in the near wall boundary layer and finally accelerates the transition.