Formation of side jets from V-notched nozzles under strong forcing

Abstract

V-notched round jets at a Reynolds number of 2200 were subjected to strong forcing and were experimentally investigated at forcing frequencies corresponding to Strouhal numbers of 0, 0.25, 0.5, 0.75, and 1. Time-resolved particle image velocimetry results are presented, and the fundamental differences in the near-field vortex dynamics of the jet flow at the peak and trough planes of the V-notched nozzles are discussed. The flow phenomena, side jets, are of particular interest as they drastically increase the jet spread rate and mixing, and were observed to form in different nozzle planes depending on the forcing frequency. Forcing at the lowest frequency resulted in strong amplification of mechanical disturbances for the V-notched jets. This led to the suppression of the breakdown of the primary vortex ring and the formation of large-scale, symmetrical coherent flow structures near the jet centerline. Additional smaller-scale flow structures and side jets are also observed along the nozzle peak plane leading to significant enhancements of the jet spread rate. Flow bifurcation and forking of the vorticity branches were attributed to the formation of streamwise vortices that formed because of the heightened azimuthal instabilities incurred by the primary vortex ring. As the forcing frequency increases, the flow structures are observed to be increasingly destabilized with reductions in their length scales. Flow bifurcation in the peak plane would cease to exist and side jets appear along the trough planes instead. The formation of these side jets results from the jet fluid seeking to achieve pressure equilibrium with the ambient fluid due to the significant perturbations induced by the strong forcing. Based on the two V-notched aspect ratio nozzles studied, the sharper V-notched nozzle is more destabilizing to the primary vortex ring, which led to earlier breakdown of large-scale coherent structures.