Transition of hypersonic flow past flat plate with roughness elements

Roughness elements in high speed flows can cause laminar-turbulent transition leading to higher heating rates and drag. Transition of flow past a hemispherical bump placed on a flat plate is explored in this paper for three Mach numbers [3.37, 5.26, 8.23] using direct numerical simulation on unstructured grids. The simulation parameters are chosen to match the experiments carried out by Danehy et al. The wall is at a constant temperature of 300K. For flow conditions corresponding to Ma=3.37 and 5.26, unsteady flow structures were observed while for Ma=8.23, the flow remained laminar downstream of the trip. The location of transition was closer to the trip for the lowest Mach number. Qualitative comparison between the computation and experiment show good agreement. Based on the computed skin friction coefficient values, Ma=3.37 appeared to become turbulent in nature, Ma=5.26 was transitional and Ma=8.23 was laminar. Quantitative comparisons of flow profiles downstream of the trip were made with theoretical turbulent profiles to confirm transition to turbulence. The effect of distributed roughness on transition was also studied at Ma=2.9. A laminar boundary layer at Ma=2.9 was observed to transition to a turbulent boundary layer that shows good quantitative agreement with experimental data. The freestream Mach number and roughness amplitude were seen to strongly influence whether or not the flow transitions. A local Reynolds number based on bump/roughness amplitude is seen to correlate the tendency to transition for both single bump and distributed roughness cases.