Resolving turbulence and drag over textured surfaces using texture-less simulations: the case of slip/no-slip textures

We study the effect of surface texture on an overlying turbulent flow for the case of textures made of an alternating slip/no-slip pattern, a common model for superhydrophobic surfaces but also a particularly simple form of texture. For texture sizes L^+ ≲ 20, we have previously reported that turbulence remained smooth-wall-like, other than experiencing an apparent origin offset for different flow components. For slip/no-slip textures, this effect reduced to the flow experiencing slip conditions in the streamwise and spanwise directions and zero transpiration at the surface. The overlying turbulence effectively perceived such boundary conditions at least up to texture sizes L^+ ≈ 50. However, beyond L^+ ≈ 20 the texture interacted with the overlying turbulence in a non-homogeneous fashion, additional Reynolds stresses arose and turbulence was no longer smooth-wall-like. This is the typical effect of surface texture observed for rough surfaces, and results in an increase in drag relative to smooth-wall flows. In this paper, we argue that this occurs because the texture modifies the overlying turbulence through non-linear, cross-advective terms between the background turbulence and the texture-coherent flow directly induced by the surface topology. To verify this, we conduct homogeneous-slip-length simulations where we introduce additional, forcing terms in the Navier-Stokes equations to capture the effect of this non-linear interaction on the background turbulence. The interaction can then be accounted for without the need to resolve the surface texture. We show that the additional terms quantitatively capture the changes in the flow up to texture sizes L^+ ≈ 70–100, including not just the roughness function but also the flow statistics and structure.