Modification of vortical structures in a drag-reduced flow created by transverse wall oscillations in a pipe

We present results of Direct Numerical Simulations of drag reduction in a turbulent pipe flow with transverse wall oscillations at Re_tau=720. Drag-reduced flow exhibits turbulence characteristics that are significantly modified with respected to a standard pipe flow without wall oscillations. In particular, this talk focuses on identification of the number and strength of the hairpin vortices in the buffer and log layers of the flow. We present the evidence of a suppression of the number of the vortical structures with wall oscillation, identified using instantaneous snapshots of the swirling-strength parameter. This reduction in the number of vortices is consequently associated with a weakened signature of the vortex packets and large-scale motions in this flow, albeit the very-large-scale motions still persist. A suppression of the near-wall turbulent eddies and their auto-generation mechanisms is believed to cause drag reduction in this flow.