Influence of local forcing on a turbulent boundary layer

Abstract

For the last decades, turbulent boundary layer controls for drag reduction have intensively studied. Recently, the importance of these researches has was highlighted due to skyrocketing oil prices. Experimental studies were carried out to investigate the effect of local forcing on a turbulent boundary layer. When local forcing is imposed in flow field, the flow structures are modified. PIV(particle image velocimetry) was used to obtain quantitative measurements of the instantaneous flow structure and evolving dynamics of turbulent flow. Two types of local forcing were used. the first is the periodic blowing and suction through spanwise thin slot and the second is the ultrasonic forcing which generates an enormous number of microbubbles in the flow field. The periodic blowing and suction was introduced to the boundary layer via a sinusoidally-oscillating jet issuing from a thin spanwise slot. Three forcing frequencies $(f^+= 0.044, 0.066 and 0.088)$ with a fixed forcing amplitude $(A^+=0.6)$ were employed at Reθ=690. The effect of three different forcing angles (α=60°, 90° and 120°) 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 phase-averaged velocity profiles revealed that spanwise large-scale vortices are generated in the downstream of the slot and persist further downstream. The phase averaged flow fields show that the location of strong vortices generated by the local forcing is almost the same as that of the retarded velocity region. This suggests that the vortex generated by the local forcing reduces the skin friction. The spatial fraction of the vortices was examined to analyze the skin friction reduction. The spatial fraction is proportional to the square root of the forcing frequency and inversely proportional to the convection velocity of vortex generated by the local forcing. This suggests that, as the forcing fre...