The unsteady flow structure of a backward-facing step

The unsteady now field behind a backward-facing step is studied experimentally in a water towing tank facility. The Reynolds number based on the step height and free-stream velocity is 1,000. Particle Image Velocimetry technique is used to measure the instantaneous now structure. Inside the shear layer, a localized instability, due to the emergence of a strong now reversal, appears to induce the accelerated formation of a large scale vortex. Near the reattachment zone, a low-frequency oscillatory movement of the separation region is found to be closely related to the convection and interaction of large scale vortices.