Fluid exchange between a recirculation region and the perturbed external flow

We present numerical simulations of incompressible, two-dimensional, unsteady flow over a backward facing step. The velocity field is made time-dependent through a vortical perturbation of given amplitude and frequency imposed upstream of the step. On being convected downstream by the flow, the perturbation interacts with the detached shear layer and causes the recirculating flow to periodically shed large-scale vortices. By means of Lagrangian tracers we study the fluid exchange between the recirculation region and the incoming flow. The numerical experiments have application to processes of enviromental relevance, i.e. the trapping of a pollutant within a recirculating structure and its release, which may be found in the lee side of a sharp crest, such as a row of hills laid across the wind.