An experimental study of coherent swirling vortex rings in a turbulent pulsed jet

Abstract

This experimental study examines the coherent swirling vortex rings in a turbulent pulsed jet at a Reynolds number Re=1.0×104 using planar particle image velocimetry. The coherent vortex rings are produced by a stepper-motor-driven piston successively injecting fluid into a background jet flow. The swirl is generated by axial swirlers which are made by three-dimensional printing technology and mounted behind the nozzle. The swirl number S generated in the experiments ranges from 0 to 1.10, which covers the critical swirl number of a swirling jet. The stroke-to-nozzle diameter ratio L/d=1.56 produces compact swirling vortex rings in a single pulsed jet without a background jet flow. Two pulsing frequencies (St=0.16 and 0.32) that generate coherent vortex rings are investigated, with the free jet also measured at the same Re for comparison. The combination of vortex rings and swirling flows is found to enhance the turbulent fluctuation intensity; this effect is positively related to the compactness of the vortex rings (St) and central recirculation zone (S). At the early stage, the generated swirling vortex ring develops until it reaches its limit, which decreases as S increases but increases with St. The axial distance at which this limit is achieved decreases with both increasing S and St. Then, the ring decays and is eventually dissipated at a decay rate that grows larger in proportion to S and St. Moreover, the swirling vortex rings evolve at their corresponding constant axial and radial velocities in the near field. The axial velocity decreases as S increases and increases with St, while the radial velocity increases with S and is independent of St. In addition, the axial distance between two adjacent swirling vortex rings decreases with increased S and St. Furthermore, the temporal variation of the axial distance also decreases at higher S and St, indicating that the axial propagation velocity decreases downstream. Finally, detailed structures of the swirling vortex ring cores and the corresponding structures induced by the rolling-up process are presented.