The structure and growth of the mixing layer region of a round jet

A submerged round jet was studied visually and quantitatively to determine the effects of time dependent perturbations in the initial jet velocity on the structure and growth of the mixing layer region of the jet. The experiments were performed on a water jet at a Reynolds number based on diameter of 5700. The shear layer at the lip of the nozzle was laminar, with a slope thickness of 0.034 diameters. Time dependent velocity perturbations were introduced uniformly over the exit of the nozzle producing the jet, thereby producing a jet with a nearly top hat instantaneous velocity profile and with a time dependent average velocity. The roll up of the laminar shear layer into ring vortices and the subsequent pairing interactions of these vortices were dramatically affected by forcing the jet. Furthermore, the growth of the jet in the mixing layer region was dependent on the merging of vortices. It was observed that without pairing there was virtually no growth.