Roughness-induced transition in high speed flows

Roughness elements in a laminar boundary layer can cause the flow to transition. The effects of discrete and distributed roughness are explored using direct numerical simulation on unstructured grids. Velocity profiles for Mach 8.12 flow past a cylindrical roughness element are compared to experiment. Flow features produced by an isolated hemispherical bump are studied for three Mach numbers [3.37, 5.26, 8.23] whose simulation parameters are chosen to match the experiments performed by Danehyet al. at NASA Langley. The vortices formed upstream of the roughness due to boundary layer separation wrap around the hemisphere generating coherent streamwise vortices. A significant increase in wall skin friction coefficient along with the presence of unsteady structures far downstream of the bump for the lower Mach number cases indicated the transitional/turbulent nature of the flow while M∞=8.23 remained laminar. These observations are consistent with those in experiment. The effect of a sinusoidally varying distributed roughness was also investigated at M∞=2.9. The flow transitioned to a fully developed turbulent boundary layer that shows good quantitative agreement with experimental data. The cumulative effect of multiple roughness elements is to decelerate the near wall fluid, and setup inflexional velocity profiles. For both types of roughness it was seen that: (1) coherent streamwise vortices were produced which broke down far downstream, (2) prominent hairpin shaped structures were observed as the flow transitioned and (3) wall normal and spanwise inflexion points in streamwise velocity are observed.