Experimental Simulation of a Vertical Round Jet Issuing into an Unsteady Cross-Flow

This paper investigates the discharge of a vertical round jet into an unsteady cross-flow that consists of a mean flow and a sinusoidally oscillating component. An experimental technique is devised to simulate the unsteady cross-flow situation in the laboratory. A vertical jet-pipenozzle assembly is physically oscillated backward and forward in the steady flow stream of a laboratory flume, and the flow patterns are viewed by an observer moving with the jet. Dispersion patterns of the dye-marked jet fluid are studied with a phase-locked analysis of digitized flow images. Oscillations in cross-flow velocity are found to organize the jet fluid into regular large-scale fluid patches, which, after time averaging, lead to a widened jet width and enhanced dilution. Experiments are also carried out on a vertical jet discharging into a current with surface waves, a situation that approximates to a genuine oscillating cross-flow with a nonzero mean velocity. The two sets of experimental observations are found to match with each other. The validity of the experimental simulation technique is further supported by a computational fluid dynamics study of the flow problem, in which it is possible to produce an idealized oscillating cross-flow situation. The numerical results agree well with the experimental observations.