Characterization of Wake Vortices in a Transverse Jet in Crossflow

Jets in crossflow are a canonical combustor flow configuration characterized by a variety of vortical structures that enable effective control of mixture fraction. Among these vortical structures, wake vortices are present in the far downstream region and play a crucial role in controlling the far-field mixing, post-flame emission profiles, and flame lifting behavior for high-momentum jets. Studies by Fric et.al.[1] and Smith et.al.[2] have shown that these structures emerge as explosions from the crossflow boundary layer, triggered by flow separation caused by the adverse pressure gradient induced by the jet’s blockage.

Although the formation mechanism for these wake vortices is generally agreed upon, relatively limited work has been done to characterize the strength, timescale, and growth rate of these structures. This work aims to address this using time-resolved stereo-PIV (Particle Image Velocimetry), in the wake of non-reacting jets in crossflow, with the laser sheet oriented parallel to the jet-issuing wall at distinct locations from the wall. These measurements were obtained by systematically varying the jet-to-crossflow momentum flux ratio, the crossflow boundary layer, and the jet-to-crossflow density ratio.

The stereo-PIV data shows that the strength of the wake vortices are extremely sensitive to the jet-to-crossflow momentum flux ratio and the crossflow boundary layer thickness. The analysis also correlates wake vortex strength and wake separation velocity and characterized the shedding frequency of the wake vortices.



References:

[1] T. F. Fric and A. Roshko, "Vortical structure in the wake of a transverse jet," Journal of Fluid Mechanics, vol. 279, pp. 1-47, 1994, doi: 10.1017/S0022112094003800.

[2] S. H. Smith and M. G. Mungal, "Mixing, structure and scaling of the jet in crossflow," Journal of Fluid Mechanics, vol. 357, pp. 83-122, 1998, doi: 10.1017/S0022112097007891.