Unsteady Effluent Dispersion in a Round Jet Interacting with an Oscillating Cross-Flow

Abstract

Dispersion of a vertical round jet issuing into an unsteady cross-flow is investigated by laboratory and numerical experiments. An experimental technique has previously been devised to simulate a sinusoidally oscillating cross-flow situation with a nonzero mean flow velocity. The parameters of the cross-flow can be selected with ease. With this experimental technique, 12 cross-flow situations with systematic varying flow parameters are produced. The dispersion pattern of a jet in each cross-flow situation is studied by phase-locked dye visualizations and the dilution level of jet effluent is estimated using image processing. It is found that in a cross-flow of a large unsteadiness parameter, the jet dispersion pattern is significantly different from that of the same jet in a steady cross-flow. The jet effluent is organized into successive large-scale effluent clouds which are connected on the inner side by a bent-over effluent fetch. Specially designed experiments using time-controlled dye ejection are performed to investigate the formation mechanism of the effluent structures. Computational fluid dynamics (CFD) studies are carried out to supplement the experimental concentration data. In addition, the CFD results help to support the formation mechanism of the effluent flow structures and to explore their dynamics. In the time-averaged sense, unsteadiness in the oscillating cross-flow leads to a two- to threefold increase in jet width. The reduction in time-averaged concentration level of jet effluent is not as dramatic because there still remains high concentration of effluent inside the effluent clouds.