Wall-oscillation conditions for drag reduction in turbulent channel flow

TL;DR

This paper investigates how spanwise sinusoidal wall oscillations can reduce drag in turbulent channel flows, using direct numerical simulations and a scaling parameter to predict optimal conditions and effects.

Contribution

It introduces a linear relation between drag reduction and a new scaling parameter based on wall oscillation characteristics, providing predictive insight into optimal oscillation conditions.

Findings

Maximum drag reduction occurs at specific oscillation periods.

A minimum wall forcing intensity is needed for drag reduction.

Drag reduction effectiveness depends on Reynolds number.

Abstract

The drag reduction properties of a turbulent channel flow modified by spanwise sinusoidal oscillations of the walls are investigated by direct numerical simulations. The work is based on the linear relation between the drag reduction and the parameter $S$, function of the maximum wall velocity and the period of the oscillation. This quantity has been found through physical arguments pertaining to the action of the oscillating Stokes layer on the near-wall turbulence dynamics. The predictive potential of the scaling parameter is exploited to gain insight into the drag-reducing effects of the oscillating wall technique. The period of oscillation which guarantees the maximum drag reduction for a given maximum wall displacement is studied for the first time. The issue of the minimum intensity of wall forcing required to produce a non-zero drag reduction effect and the dependence of the drag reduction on the Reynolds number are also addressed. The drag reduction data available in the literature are compared with the prediction given by the scaling parameter, thus attaining a comprehensive view of the state of the art.