Fluid exchange between a recirculation region and the perturbed external flow

We present numerical simulations of incompressible, two-dimensional, unstea dy flow over a backward facing step. The velocity field is made time-depend ent through a vortical perturbation of given amplitude and frequency impose d upstream of the step. On being convected downstream by the flow, the pert urbation interacts with the detached shear layer and causes the recirculati ng flow to periodically shed large-scale vortices. By means of Lagrangian t racers 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 recircula ting structure and its release, which may be found in the lee side of a sha rp crest, such as a row of hills laid across the wind. (C) 2001 Elsevier Sc ience Ltd. All rights reserved.