LAUSR

Abstract We compute two-dimensional high Schmidt number mass transfer downstream of a backward-facing step in a laminar flow, in which a nonreactive solute is initially confined to a region adjacent to the step, with square cross-section and dimensions equal to the height of the step. For Reynolds numbers (Re; based on step height and far-field velocity) of 10 and 100, and for Schmidt numbers (Sc) of 7, 500, and 2650, we present concentration distributions in the fluid and on the surface, in addition to time histories for the maximum concentration in the domain and the amount of solute remaining upstream of various points in the domain. We show that the rate of removal of solute decreases strongly as the Schmidt number increases from 7 to 2650. For R e = 100 and S c = 500 and 2650, there is a time in the washout process when the location of the maximum concentration shifts from the relatively inaccessible bottom corner behind the step, to a point in the interior of the recirculation zone downstream of the step. When the washout process is approximated by a single exponential in each of three temporal ranges, that shift is seen to be accompanied by a dramatic reduction in the rate at which the maximum concentration decays. The joint effects of the Reynolds and Schmidt numbers on the washout process are discussed.