Three-Dimensional Diffusion of Matter in a Turbulent Recalculating Region Behind a Backward-Facing Step

Abstract

A water-channel experiment has been performed on the concentration in a recirculating region behind a backward-facing step. A plume of matter was issued from a circular hole on the surface located in the recirculating region. An optical-fiber probe was employed to measure fluctuating concentrations at a Reynolds number (based on the step height and the mainstream velocity) of 1250. The time-mean concentration in the recirculating region was found to be strongly dependent on the ratio of the effluent velocity and the mainstream velocity. When released matters are harmful or toxic, not only the time-mean concentration but the instantaneous concentration should be predicted. In order to meet this need, spatial distributions of the probability of finding a concentration higher than a particular value and the time when such concentration first appears at a given position are also presented and discussed.