Impact of Distributed Roughness on Tollmien–Schlichting Instabilities over an Airfoil

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 at varying angles of attack and for chord Reynolds numbers between [Formula: see text] and [Formula: see text]. 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, hot-wire anemometry was employed. The results indicate that stochastic roughness, characterized by nonuniform spatial distributions, significantly impacts the linear amplification of modal instabilities, exhibiting different behavior depending on the roughness parameter modified. Finally, a semi-empirical [Formula: see text] relation for transition prediction as a function of the dimensionless average roughness height, [Formula: see text], is proposed based on the current data set.