Impact of Distributed Roughness on Tollmien–Schlichting Instabilities in Airfoil Boundary Layers

This paper presents an experimental study on the effect of stochastically distributed roughness on boundary layer transition. Among the different transition pathways, the amplification of modal instabilities is of particular interest. The experiments were conducted in a subsonic wind tunnel over a two-dimensional wing with varying angles of attack for chord Reynolds numbers between 3.5e5 and 5.5e5. Two types of surface roughness with different average roughness heights were studied for two roughness extents in the streamwise direction. In order to capture the impact on transition and to quantify its effect, hot-wire anemometry was employed. The results indicate that stochastic roughness, characterized by non-uniform spatial distributions, significantly impacts the stability of the boundary layer, exhibiting different behavior depending on the roughness parameter modified. Finally, a semi-empirical ΔN model for transition prediction based on the dimensionless average roughness height, Sa/delta1, is proposed.