High Schmidt number "washout" of a viscosifying solute downstream of a backward-facing step*

We consider high Schmidt number, two-dimensional, unsteady laminar convective mass transfer downstream of a backward-facing step, behind which a nonreactive viscosifying solute is initially confined to a layer whose thickness is equal to the height of the step. The solute is "washed away" by a flow of "clean" fluid over the step. The relationship between solute concentration, viscosity, and molecular diffusivity is approximated using the Stokes-Einstein relation. We focus on the distribution and removal of the solute for several combinations of Reynolds number (based on step height and infinite-dilution viscosity; primarily 10 and 100) and Schmidt number (the ratio of kinematic viscosity to the infinite-dilution molecular diffusivity; primarily 500 and 2650). Comparison is made to the case in which the viscosity and diffusivity are independent of solute concentration.