Instabilities and Vortex Breakdown in Swirling Jets and Wakes

Abstract

The convective/absolute nature of the instability is determined for two distinct families of swirling wake/jet flows: the Batchelor vortex with external axial flow and the Rankine vortex with plug-flow velocity profile. It is demonstrated that, even in the absence of external axial counterflow, wakes and jets may exhibit a transition to absolute instability for sufficiently large swirl. Distinct transitional helical modes are found for wakes and jets, and the transitional swirl value for the Rankine vortex model is consistent with observed values for vortex breakdown onset. An experimental study of vortex breakdown in swirling jets reveals the existence of four distinct breakdown states (bubble, cone, spiral bubble, spiral cone) above a critical swirl value of 1.3-1.4. This threshold can be predicted from a simple inviscid criterion based on the existence of a stagnation point within the flow. Extensive hysteretic behavior prevails close to the critical swirl value for breakdown onset.