The effect of cube-roughened walls on the response of rough-to-smooth (RTS) turbulent channel flows

Direct numerical simulations of cube-roughened rough-to-smooth channel flows are performed with the objective of studying the response of turbulence statistics in the developing flow over smooth walls. Non-equilibrium effects persist and the global recovery is slow and incomplete by the streamwise exit of the computational domains, which is at about 10 channel half heights. The estimated recovery distance in the outer regions of the flow is on the order of 50 channel half heights, but different statistics have disparate relxataion rates. The turbulence structure swiftly relaxes to a 'near' equilibrium very close to the wall. Within this wall layer, due to a strong mean shear, turbulence statistics and instantaneous motions resemble their fully-smooth equivalents. However, the reversion is not complete because it is interrupted by large structures that persist from the upstream roughness. As the flow encounters the step change in roughness, it expands producing strong mean-advection effects, which prevent the canonical log-law region from being established. The expansion of the mean flow also results in an adverse pressure gradient across the channel. It recovers gradually, only becoming favourable near the exit of the computational domains.