Effects of streamwise oriented riblets on spot nucleation in free-stream turbulence induced transition

The effects due to streamwise oriented riblets on the free-stream turbulence (FST) induced transition of a flat plate boundary layer (BL) are studied by means of Particle Image Velocimetry (PIV) and Laser Doppler Velocimetry (LDV). The experiments are performed on a flat plate installed between adjustable endwalls providing an adverse pressure gradient typical of turbine blades for aeronautical applications. Four different ribbed surfaces are compared with a smooth reference case to test the effects of riblet dimension and location on turbulent spot nucleation. Riblets are installed in the laminar, the transitional, and the fully turbulent BL. Their effects are studied for three Reynolds numbers (Re=70000, 150000, 220000) under fixed pressure gradient and free-stream turbulence intensity (Tu=5%). Varying the flow Reynolds number leads to different non-dimensional riblet height and spacing values, providing an overall test matrix of 13 different conditions.