Abstract
In this paper a study of the flow dynamics in the wake of a turbulent annular jet is made using Time Resolved Stereoscopic Particle Image Velocimetry (TR-SPIV) and Proper Orthogonal Decomposition (POD). Behind the bluff body of the jet, a transient asymmetric recirculation zone is formed. This asymmetric wake originates from a bifurcation of the flow once a threshold Reynolds number is exceeded. For the non-swirling jet, the large scale motion of the asymmetry can be reconstructed using the first two POD spatial modes and the associated time-coefficients are random in time. Hence there is no periodic motion of the wake. When swirl is imparted (swirl numbers ranging from 0 < |S| < 0.12), the large scale motion of the wake can also be reconstructed by the first two POD spatial modes as for the nonswirling jet and those modes are very similar for both jets. Looking at the associated time coefficients shows that the motion of the asymmetric wake becomes structured into a precession. For |S| > 0.12, no precession can be found anymore and the motion of the asymmetric wake is again random in time, similar to the non-swirling jet. In this paper a hypothesis for this precession is proposed, which states that the precession itself is not an instability as such but is the result of a physical mechanism which involves feedback in the recirculating flow. For the non-swirling jet, there is symmetry breaking of the recirculation zone as disturbances in the inner shear layer are convected downstream towards the stagnation point, changing its location. Then the disturbances near the stagnation point are convected back to the inner shear layer of the jet and the process repeats itself. When swirl is added to the flow, the feedback mechanism is correlated with the swirl and the motion of the wake becomes correlated with the swirl number into a coherent precession. For higher swirl numbers, this feedback mechanism is disturbed by the appearance of an inverted triangle zone and the motion of the wake is again decoupled from the swirl and hence no precession is present in the flow