Roughness Effects on Swept-wing Crossflow Transition in Moderate Free-stream Turbulence

An experimental investigation of swept-wing crossflow transition is carried out, focussing on the effects of chord Reynolds number and direct roughness element height (spaced at the critical crossflow wavelength) in a moderate-level disturbance facility. Flow quality measurements performed in the wind tunnel indicate turbulence intensities of urms/U∞ = 0.14% and v ′ rms/U∞ = 0.20%. The Reynolds numbers are Rec = 1.3× 106 and Rec = 1.6×106. Despite the disturbance levels being indicative of travelling wave dominated crossflow, flow visualisation reveals that stationary crossflow is the dominant mode. Increased DRE height is shown through flow visualisation to advance transition, with boundarylayer velocity profiles showing mean-flow deformation and fluctuating-velocity spectra containing a high-frequency mode normally associated with stationary crossflow related secondary instability. Nevertheless, spectral information and unsteady disturbance profiles do indicate the travelling crossflow modes are excited, and that their disturbance amplitude increases with Reynolds number (as expected from linear stability theory) and with roughness height.