Turbulent channel flow over an anisotropic porous wall - Drag increase and reduction

TL;DR

This study investigates how anisotropic porous walls influence turbulent channel flow, demonstrating that permeability directionality can significantly reduce or increase drag, with potential for scalable flow control.

Contribution

It reveals how permeability anisotropy in porous walls can be tuned to control drag in turbulent flows, a novel approach for flow management.

Findings

Drag can be reduced or increased by over 20% through permeability adjustments.

Vertical permeability lower than horizontal promotes drag reduction and turbulence coherence.

High wall-normal permeability increases fluctuations and triggers spanwise-structured flow.

Abstract

The effect of the variations of the permeability tensor on the close-to-the-wall behaviour of a turbulent channel flow bounded by porous walls is explored using a set of direct numerical simulations. It is found that the total drag can be either reduced or increased by more than $20\%$ by adjusting the permeability directional properties. Drag reduction is achieved for the case of materials with permeability in the vertical direction lower than the one in the wall-parallel planes. This configuration limits the wall normal velocity at the interface while promoting an increase of the tangential slip velocity leading to an almost "one-component" turbulence where the low- and high-speed streaks coherence is strongly enhanced. On the other hand, strong drag increase is found when a high wall-normal and low wall-parallel permeabilities are prescribed. In this condition, the enhancement of the wall-normal fluctuations due to the reduced wall-blocking effect triggers the onset of structures which are strongly correlated in the spanwise direction, a phenomenon observed by other authors in flows over isotropic porous layers or over ribletted walls with large protrusion heights. The use of anisotropic porous walls for drag reduction is particularly attractive since equal gains can be achieved at different Reynolds numbers by rescaling the magnitude of the permeability only.