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. LDV measurements allowed the computation of the wall shear stress and the BL statistical moments at selected spatial positions. On the other hand, PIV data are used to characterize the riblet effects on turbulent spot nucleation and transition: the higher the non-dimensional riblet height, the more the breakup events occur upstream with respect to the smooth case. Momentum losses significantly increase when the non-dimensional riblet spacing becomes larger than 30 wall-units. Otherwise, beneficial effects are obtained for smaller riblet spacing, regardless of their streamwise extension and position with respect to the BL transition region.