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.

Effect of Jet Inclination Angle and Hole Exit Shape on Vortical Flow Structures in Low-Reynolds Number Jet in Cross-Flow

“…Based on Figure 5 (right), the jet "lifted-off " is more important in the elliptic jet hole case in which the upperdeck vortex pair, promoting the entrainment of the crossflow toward the wall surface, is relatively weak compared to those identified in the other two hole geometry cases. The results reported here are in good qualitative agreement with the experimental studies of Haven and Kurosaka [16], New et al [18], and DNS of Sau et al [11].…”

“…Similar to Sau et al [11], results presented here were taken at an instantaneous dimensionless time unit of t = 18, as the main objective of the study is to visualize the vortex flow structures. Discussions will focus on understanding the underlying flow physics in JICF at different scenarios.…”

“…Figure 2(a) depicts contours of spanwise vorticity (ω z ) at a midspan plane (z = 0) and it reveals the presence of the shear layer vortex that emerges at the upstream side of the initial portion of the jet. The roll-up process of the shear layer upstream of the jet has been attributed to the Kelvin-Helmholtz (K-H) instabilities [2,11,13]. Figure 2(b) reveals the evolution of the three-dimensional streamlines participating in the formation of a K-H vortex system in the near-wake region that emerges at the upstream (wind side) of the initial portion of the jet flow.…”

“…In case of square jet, it is located in 4D and 5D from the cross-flow inlet plane in the streamwise direction and 2.5D and 3.5D in the spanwise direction, respectively (see Figure 1). Following a previous numerical study of Sau et al [11], a computational grid of 241 points in the streamwise (with 11 points in the jet domain), 81 points in the wall normal, and 61 points in the spanwise direction (with 11 points in the jet domain) is adopted. Both domain influence and grid refinement studies have been carried out [12].…”

Numerical visualization of vortex flow behavior in square jets in cross-flow

“…1). The lateral spacing of 6D was primarily based on an existing numerical of this kind (Sau et al, 2004) and further confirmed by our numerical study on domain influence study.…”