Wall-Modelled Large-Eddy Simulations of flows with non-uniform roughness

Wall-Modelled Large-Eddy Simulations (WMLES) and hybrid RANS/LES simulations of wall-bounded flows with non-homogeneous roughness were carried out. The flow under consideration is an open channel, with alternating rough- and smooth-wall strips oriented transverse to the flow. Results were compared to turbulent boundary-layer experiments in a similar configuration. Despite the fact that all models considered rely on an equilibrium assumption (production=dissipation), and are designed to yield the logarithmic law-of-the-wall at the inner/outer-layer interface, they predict the readjustment of the flow to the new surface conditions fairly accurately. They compare well to the experiments, both in terms of mean velocity and wall-shear stress. The hybrid method predicts the sharp variation of the skin friction coefficient after the roughness transition more accurately, at the expense of a higher computational cost. The implications and limitations of the equilibrium assumption will be discussed. Research supported by the Natural Sciences and Engineering Council of Canada (NSERC).