Recent advances in the numerical simulation of rough-wall boundary layers

Turbulent flows over rough surfaces are ubiquitous in nature and in man-made devices. They present many unique aspects, as roughness alters the turbulence structure and the momentum and energy transfer mechanisms. Numerical simulations that resolve the roughness sublayer are computationally demanding: for an accurate calculation of a flow in realistic conditions, such as the flow in a hydraulic turbine, or over terrain, the roughness should be much smaller than the boundary-layer thickness, and the Reynolds number should be high enough to achieve the fully rough regime. While simulations that relaxed these constraints have highlighted new physical phenomena, more advanced numerical techniques and increased computational power make it possible to perform simulations of rough-wall flows in more realistic conditions. This paper reviews some recent work carried out at the Turbulence Simulation and Modelling Laboratory at Queen's University, highlighting the unique information that can be obtained from numerical simulations.