Symmetry breaking and vortex precession in low-swirling annular jets

Abstract

In this paper, the flow dynamics in the wake of a turbulent annular jet is studied using Time-Resolved Stereoscopic Particle Image Velocimetry and Proper Orthogonal Decomposition (POD). In this wake, a central recirculation zone is present which, under certain conditions, shows a low-frequency precessing motion. POD analysis of the measured velocity data shows that at zero swirl, an asymmetry is present in the wake, which motion is random in time. This asymmetry originates from a bifurcation of the flow once a threshold Reynolds number is exceeded. For low-swirl numbers, ranging from 0 < S < 0.12, the asymmetry is still present and its motion becomes structured into a well defined precession. For S > 0.12, the precession is gone and the motion of the asymmetric wake is again random in time, similar like the non-swirling jet. In this paper, a model is developed to describe the influence of swirl on the wake dynamics. The model assumes that perturbations in the inner shear layer near the bluff body wall are convected towards the stagnation point. These perturbations cause a shift in the stagnation points position. This shift is convected back to the inner shear layer through convection in the recirculating flow. The dynamics of this feedback mechanism can be modeled by the nonlinear delayed saturation model. In this paper, the model is adapted for swirling flow and simulations show that good agreement is found with the experiments.