Turbulent Boundary-Layer Control by Means of Spanwise-Wall Oscillation

Abstract

An experimental investigation into the changes in turbulence structure of the boundary layer over a wall oscillating in spanwise direction was carried out in a wind tunnel using hot-wire anemometry and flow visualisation. The main purpose of this investigation is to confirm recent numerical results which seem to indicate that the turbulent skinfriction drag can be reduced by up to 40 percent over the oscillating wall. The results from the present investigation clearly indicate that the logarithmic velocity profiles are shifted upwards and turbulence intensities reduced by the spanwise-wall oscillation, confirming the basic conclusions of recent direct numerical simulation. Also, the skinfriction reductions as much as 45% are observed in the present experiment at an optimum speed of wall oscillation. The flow-visualisation study indicates that the longitudinal vortices in the near-wall region of the boundary layer are twisted towards the direction of spanwise-wall motion with oscillation. As a result, the vortices are realigned into the cross-flow direction, resulting in a reduction of turbulent wall-skin friction by spanwise-wall oscillation. A conceptual model for a turbulent boundary layer over an oscillating wall is proposed to examine the mechanism of turbulent drag reduction by spanwise-wall oscillation.