Abstract

This talk will present an overview of the understanding our group has gained in the last 5-10 years on the interaction of surface texture in general with wall turbulence. I will argue that this interaction occurs essentially through two physical mechanisms. In the first, surface texture imposes a set of effective, homogeneous boundary conditions on the overlying, texture-incoherent background turbulence. The effect of inertia and the lack of separation of scales between the texture and the turbulent eddies are then key features. When the resulting transpiration effect is small, slip dominates, turbulence remains smooth-wall-like and the effective boundary conditions give rise to Paolo Luchini’s `protrusion-height’ regime. When the transpiration effect is large, it can account on its own for the changes in the overlying turbulence, as is the case for high-permeability porous substrates. The second mechanism is the non-linear interaction of the texture-coherent flow with the overlying, texture-incoherent turbulence, which acts on the latter as a forcing term altering the momentum equations. This term can be added back into texture-less simulations, and is sufficient to capture the effect of the texture granularity on the overlying turbulence.