Turbulent flow over a backward-facing step. Part 1. Effects of anti-cyclonic system rotation

摘要

The effects of rotation on turbulent flow with separation and reattachment are investigated by means of direct numerical simulations. The backward-facing step configuration is rotated about a spanwise axis such that the sudden expansion of the channel is on the pressure side. The upstream flow is a fully developed plane Poiseuille flow subjected to orthogonal-mode rotation, which subsequently detaches from the step corner and eventually reattaches further downstream. The size of the resulting separation bubble with recirculating flow diminishes monotonically with increasing rotation rates and the reattachment distance is reduced from about 7 to 3 step heights. This is ascribed to the augmentation of the cross-stream turbulence intensity in the anti-cyclonic shear layer formed between the bulk flow and the recirculating eddy due to the destabilizing influence of the Coriolis force. The spanwise-oriented vortex cells or roller eddies found in non-rotating shear layers were disrupted by the enhanced turbulence. The flow along the planar wall is subjected to an adverse pressure gradient induced by the sudden expansion. The stabilizing influence of the system rotation in this cyclonic shear layer tends to damp the turbulence, the flow becomes susceptible to flow separation, and a substantial cyclonic recirculation bubble is observed at the highest rotation rates. The resulting meandering of the bulk flow is associated with interactions between the anti-cyclonic shear layer at the stepped side and the cyclonic shear flow along the planar surface. These give rise to enhanced turbulence levels at the cyclonic side in spite of the otherwise stabilizing influence of the Coriolis force. Exceptionally high velocity fluctuations in the spanwise direction are observed in the vicinity of flow reattachment behind the step and ascribed to longitudinal Taylor-Grtler-like roll cells which extend into the backflow region. These roll cells arise from a centrifugal instability mechanism associated with the convex streamline curvature in the reattachment zone.