Three-dimensional simulation of square jets in cross-flow.

Abstract

Direct numerical simulations are performed to predict the three-dimensional unsteady flow interactions in the near-field of a square jet issuing normal to a cross-flow. The simulated flow features reveal the formation of an upstream horseshoe vortex system, which is the result of an interaction between the oncoming channel floor shear layer and the transverse jet; the growth of a sequence of Kelvin-Helmholtz instability-induced vortical rollers in the mixing layer between the jet and the cross-flow, which wrap around the front side of the jet; and the inception process of the counter-rotating vortex pair (CVP), which is initiated through the folding of the lateral jet shear layers. It has been observed that for a square jet in cross-flow, the developed Kelvin-Helmholtz instability induced shear layer rollers do not form closed circumferential vortex rings. Along the downstream side of the jet, the extended tails of such rollers gradually join the locally evolving CVP. The very inception of the CVP is, however, observed to take place within the cross-flow-induced skewed lateral jet shear layers, and such inception was seen to occur slightly below the jet orifice. The simulated results also reveal the growth of the upright wake vortices from the topological singular points that developed on the cross-flow floor boundary layer. The accumulated floor vortices are seen to spiral around the critical points and subsequently leave the channel floor uprightly. During upward motion these vortices eventually get entrained into the CVP core. It has been made clear topologically that the unstable local surface excitations are the seeds from which upright vortices grow. Interestingly, such findings remain quite consistent with the existing experimental predictions for a round jet. Simulations were performed for two moderate values of the Reynolds number 225 and 300, based on the jet width and the average cross-flow inlet velocity, and for two different values of the jet to cross-stream velocity ratio, 2.5 and 3.5.