Massive separation of turbulent Couette flow in a one-sided expansion channel

Direct numerical simulation has been performed to study wall-driven flow over a backward-facing step at Reynolds number Re=5200Re=5200 based on the step height h   and the upper-wall velocity UwUw. The flow configuration consisted of a step with height equal to that of the upstream channel yielding an expansion ratio 2:1. Instantaneous enstrophy contours revealed the formation of Kelvin–Helmholtz instabilities downstream of the step. Intense velocity and vorticity fluctuations were generated in the shear-layer formed between the bulk flow and the massive recirculation zone in the lee of the step. Extraordinarily high turbulence levels persisted in the center region even 7.5h7.5h downstream of the step, i.e. where the separated shear-layer reattached to the wall. A fully redeveloped Couette flow cannot be reached in the downstream part of the channel due to the principle of mass conservation. The local wall pressure coefficient gave evidence of an adverse pressure gradient in the recovery region, where a Couette–Poiseuille flow type prevailed.