Topographical effects of roughness on turbulence statistics in roughness sublayer

Single-point turbulence statistics are compared in the roughness sublayer (RSL) of turbulent open-channel flows over smooth wall and wall roughness with different textures using direct numerical simulations (DNS). The goal is to identify how the range of scales contained in a roughness topography affects the drag generation, momentum transfer, and energy balance. The presence of large surface wavelengths is shown to reduce the overall surface slope, leading to a sparser distribution of roughness-wake regions. This may provide a physical explanation why a wider scale surface tends to produce lower friction coefficient and higher Reynolds stress anisotropy as observed previously. In addition, despite recent observations of negligible form-induced turbulent kinetic energy (TKE) productions over narrow-scale surface such as sand grains and gravel bed, it is shown that these productions can be significant over a multiscale surface. We also identify several factors crucial for these productions; they include the roughness drag coefficient, the roughness geometry function, and both micro- and macroscopic surface scales.