Effects of distributed roughness on modal instabilities over a semi-infinite swept wing

An experimental investigation of the impact of distributed surface roughness on boundary-layer transition is pre- sented in this paper, with particular emphasis on modal transition governed by Tollmien–Schlichting waves. The experiment was conducted in a subsonic closed-return wind tunnel over a semi-infinite swept wing for differ- ent angles of attack and Reynolds numbers. In terms of roughness, different subcritical stochastic roughness types with varying average roughness heights were inves- tigated for a fixed roughness extent. The onsets of tran- sition were detected by performing streamwise traverses using hot-wire anemometry, which also enabled spectral analysis, providing more detailed insight into the mech- anisms involved in the upstream shift of transition ob- served for all cases. Complementary linear stability anal- ysis further clarified these mechanisms and allowed the proposal of a semi-empirical ∆N relation for transition prediction in similar configurations, dependent on the di- mensionless average roughness height, Sa/ ¯δ ∗