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

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