Effects of circumferential microjets on the near-field behaviour of a round jet

Abstract

An experimental study on the effects of introducing two secondary cross-stream microjets opposite each other into a laminar round primary jet has been carried out using laser-induced fluorescence technique. The primary jet Reynolds number ranged from 1000 to 2400 and the microjet injection velocity ratio between 0.5 and up to a maximum of 4.5, depending on the exact configuration. The results showed that at low velocity ratios, the microjets did not penetrate the primary jet significantly and mainly perturbed the primary jet shear layer into producing shear layer vortices earlier than the unperturbed case. At higher velocity ratios when the microjets are able to penetrate into the primary jet core but did not impinge against each other, an additional vortex roll-up region was formed near to the jet center and adjacent to vortex roll-up region along the primary jet shear layer. The qualitatively smaller vortices emanating from the two regions were found to interact intensely with one another and hence led to an abrupt and early transition to turbulence for the primary jet. When the microjets did impinge against each other at the highest velocity ratios used in the present study, the intense near-field interaction between the two vortex roll-up regions described previously was observed to be largely reduced. The primary jet began to produce large-scale structures closely resembling those found at low velocity ratios, albeit in a more turbulent state. Results suggested that the greater distance of the microjets from the primary jet shear layer at high velocity ratios, as well as the opposite circulation of the microjet counter-rotating vortex-pairs caused this to occur.