Turbulent boundary-layer control by means of spanwise wall oscillation

An investigation into the changes in the turbulent boundary-layer structure with a spanwise-wall oscillation was carried out in a wind tunnel using hot-wire anemometry and Bow visualization. The main purpose of this study is to experimentally confirm the results of recent direct numerical simulations suggesting that the turbulent skin-friction drag can be reduced by a wall oscillation. The present results clearly indicate that the logarithmic velocity profiles are shifted upward and turbulence intensities reduced by the spanwise-wall oscillation. When the wall oscillation was optimized with a nondimensional wall speed, skin-friction reductions of as much as 45% were observed within five boundary-layer thicknesses downstream of the start of wall oscillation. The mechanism of drag reduction seems to strongly relate to the spanwise vorticity generated by the periodic Stokes layer over the oscillating wall, which affects the boundary-layer profile by reducing the mean velocity gradient within the viscous sublayer. The longitudinal vortices in the near-wall region are also realigned into the spanwise direction, reducing the intensity of streamwise vorticity fluctuations across the boundary layer.