A vortex pair model for plume downwash into stack wakes

A model is proposed that uses a streamwise vortex pair embedded in the plume to produce a downwash velocity that varies with downwind distance and competes with the trajectory centerline plume rise velocity. This streamwise vortex pair is an idealization of the connection back to the stack tip of the vertical von Karman vortices shed from the stack. The model is compared with the measurements of trajectories from non-buoyant momentum jets injected from a circular tube with subcritical external Reynolds number into a uniform cross flow in a water channel. Fluorescent dye and laser sheet-lighting were used to define the trajectories over a range of effluent to crosswind velocity ratios from 0.25 to 8.0. With an empirical function to account for the efficiency with which the jet entrains vorticity from the stack-tip vortex pair, the model accurately predicts the plume downwash effects, and accounts for the continuous downward deflection of the jet trajectory.