Modification of near-wall turbulence due to spanwise wall oscillations

An experimental investigation of a turbulent boundary layer modified by spanwise wall oscillations is conducted in a water channel by means of the hydrogen-bubble technique. The purpose is to study the dynamics of near-wall turbulent structures to shed new light on the physical mechanisms characterizing the boundary layer perturbed by the wall motion and to comprehend how these changes cause a wall-shear stress reduction. It is likely that flow visualizations conducted at the highest values of maximum wall velocity describe the spatial transient evolution of the flow to the new modified state because of the limited extension of the oscillating wall section. When the oscillatory motion is imposed, the low-speed streaks shift laterally, and cyclically incline to an angle with respect to the streamwise direction. Flow visualizations from the end of the water channel distinctly show that the interaction between these low-velocity pockets and the overriding longitudinal vortices is strongly altered, the latter being only slightly disturbed by the spanwise Stokes layer which develops due to the wall movement. We also notice that the ejection phenomena of low-speed fluid from the viscous sublayer to higher regions of the boundary layer and the sweeping motions of high-speed fluid towards the wall are both significantly weakened. During the first instants of the wall oscillation, the near-wall turbulent structures are already remarkably affected after one oscillating cycle. The drag-reducing flow pattern downstream of the moving wall is also described.