Measurement of local forcing on a turbulent boundary layer using PIV

Abstract

An experimental study was carried out to investigate the effect of periodic blowing and suction on a turbulent boundary layer. Particle image velocimetry (PIV) was used to probe the characteristics of the flow. Local forcing was introduced to the boundary layer via a sinusoidally-oscillating jet issuing from a thin spanwise slot. Three forcing frequencies (f(+)=0.44, 0.66 and 0.88) with a fixed forcing amplitude (A(+)=0.6) were employed at Re-theta=690. The effect of three different forcing angles (alpha=60degrees, 90degrees and l20degrees) was investigated under a fixed forcing frequency (f(+)=0.088). The PIV results showed that the wall-region velocity decreases on imposition of the local forcing. Inspection of the phase-averaged velocity profiles revealed that spanwise large-scale vortices are generated downstream of the slot and persist farther downstream. The highest reduction in skin friction was achieved at the highest forcing frequency (f(+)=0.088) and a forcing angle of alpha=120degrees. The spatial fraction of the vortices was examined to analyze the skin friction reduction.