Effect of Distributed Roughness on Boundary Layer Transition Induced by Free-stream Turbulence

In this work, we investigate the boundary layer transition in presence of freestream turbulence (FST) and surface roughness which has relevance for the flow over gas-turbine and wind-turbine blades. A strip of coarse-grade emery is positioned in the pre-transitional and transitional regions of the FST-induced transition, and measurements are carried out using single-component hotwire anemometry. We find that the presence of roughness accelerates transition which reduces the length of transition zone and increases drag over the plate. Analysis of the second and higher moments of fluctuating velocity suggests that the roughness reduces the intensity of fluctuations for a given streamwise location, making them more evenly spaced and thus less likely to exhibit "extreme" events, such as "hot-streaks" on a gas-turbine blade, which are associated with high heat transfer. Spectral analysis of the velocity signals shows that the distributed roughness introduces high frequency disturbances inside the boundary layer in contrast to FST-induced transition, wherein the boundary layer filters out high frequency disturbances through the "shear-sheltering" effect. Furthermore, the distributed roughness also seems to reduce the energy of streaky structures while promoting the formation of turbulent spots, thus altering the structure of the boundary layer in a non-trivial fashion.