Dynamical aspects of a backward-facing step flow at large Reynolds numbers

A contribution to the study of separating and reattaching flows over a backward-facing step configuration is reported. Simultaneous unsteady wall-pressure and velocity field measurements are made to investigate the dynamical aspects of the separated flow field at high Reynolds numbers, up to 182,600. An array of 25 unsteady pressure sensors in the streamwise direction coupled with PIV velocity fields is used to analyze the statistical properties as well as the streamwise time–space characteristics of the separated flow. A comparison with some previous studies is done to highlight the expansion ratio influence and dependency on the main separation flow. Emphasis is placed on the dynamical aspects of turbulent flows at a wide range of Reynolds numbers, specifically in the convective motion of the vortical flow structures. Unsteady pressure spectra show, even for high Reynolds numbers, the relative dominance of low-frequency flapping motion over high-frequency modes of the large-scale vortical structure. The latter shows a ‘waked mode’ behavior, where these large-scale coherent structures grow in place and further accelerate in the downstream direction. The ability to understand the flow field unsteadiness will lead future investigations to develop active and/or passive flow control techniques in separating and reattaching flows.