Experimental assessment of drag reduction by traveling waves in a turbulent pipe flow

TL;DR

This study experimentally evaluates an active control technique using traveling spanwise wall waves to reduce turbulence-induced drag in pipe flow, confirming prior DNS results and demonstrating up to 33% drag reduction.

Contribution

It provides the first experimental validation of traveling wave-based drag reduction in turbulent pipe flow, confirming the effectiveness of the method.

Findings

Up to 33% drag reduction achieved.

Backward-traveling waves always reduce drag.

Forward-traveling waves reduce drag when phase speed is low.

Abstract

We experimentally assess the capabilities of an active, open-loop technique for drag reduction in turbulent wall flows recently introduced by Quadrio et al. [J. Fluid Mech., v.627, 161, (2009)]. The technique consists in generating streamwise-modulated waves of spanwise velocity at the wall, that travel in the streamwise direction. A proof-of-principle experiment has been devised to measure the reduction of turbulent friction in a pipe flow, in which the wall is subdivided into thin slabs that rotate independently in the azimuthal direction. Different speeds of nearby slabs provide, although in a discrete setting, the desired streamwise variation of transverse velocity. Our experiment confirms the available DNS results, and in particular demonstrates the possibility of achieving large reductions of friction in the turbulent regime. Reductions up to 33% are obtained for slowly forward-traveling waves; backward-traveling waves invariably yield drag reduction, whereas a substantial drop of drag reduction occurs for waves traveling forward with a phase speed comparable to the convection speed of near-wall turbulent structures. A Fourier analysis is employed to show that the first harmonics introduced by the discrete spatial waveform that approximates the sinusoidal wave are responsible for significant effects that are indeed observed in the experimental measurements. Practical issues related to the physical implementation of this control scheme and its energetic efficiency are briefly discussed.