Spanwise wall oscillation as a drag reduction technique

Abstract

Reduction of skin-friction drag over a fully developed canonical zero-pressure gradient turbulent boundary layer (ZPGTBL) subjected to spanwise oscillation is measured using planar particle image velocimetry (PIV). The experiments are conducted at Re_θ of 1000 and 1800, the chosen range of spanwise oscillations amplitude and frequency are around the optimum reported in literature studies (T⁺_osc = [100-700], A⁺_osc= [50, 150]). A high-resolution planar PIV apparatus is employed to measure the drag reduction directly by wall shear stress estimates on the oscillating wall. The measurement uncertainty of the drag estimates from PIV measurements is examined. The results show drag reduction in the order of 15% after 6 boundary layer thicknesses from the beginning of the oscillating section. Variations of the drag reduction follow the trends reported in literature. The PIV measurements enable the analysis in terms of Reynolds shear stresses, turbulence production and allow visualising vortex packets by vorticity. A pronounced drop of turbulence production in the range y⁺ = [5, 30] is observed. The vorticity analysis indicates a distortion from the well-known hairpin-packet arrangement, suggesting that the mechanism of hairpin auto-generation may be inhibited by spanwise wall oscillations