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

Wall-Modelled Large-Eddy Simulations (WMLES) and hybrid RANS/LES

simulations of wall-bouned 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).