Wash-away of contaminant downstream of a backward-facing step over a range of Schmidt number.

We report computations of two-dimensional unsteady convective mass transfer in flow over a backward-facing step, in which a contaminant initially present downstream of the step is "washed away". Results are presented for a range of Schmidt numbers, showing how the recirculation region downstream of the step not only serves to retain contaminant near the step, but also transports contaminant \textit{upstream} towards the step. The results for the highest Schmidt number considered (2650) are relevant to wash-away of low-molecular weight species in liquids, for which some implications are discussed.