Dynamics of coherent vortex rings in a successively generated turbulent pulsed jet

Abstract

This work focuses on the dynamics of coherent vortex rings in a successively generated turbulent pulsed jet at a Reynolds number (Re) of 1.0×104 using planar particle image velocimetry (PIV). In the experiments, the vortex rings are issued into the free jet mainstream by the reciprocation of a stepper-motor driven piston. The stroke-to-nozzle diameter ratio L/d is 1.56, which produces compact vortex rings without a trailing jet in single pulsed jet flow without a free-jet mainstream. The flow fields generated by four pulsing frequencies are investigated, and the free jet at the same Re is also measured for comparison; the maximum dimensionless pulsing frequency (St) is 0.32. The effect of enhancement of turbulent fluctuation intensity (turbulence enhancement) is found to be closely related to the generation and structure of the vortex rings in a flow field. The compactness of the vortex rings is positively related to St, as more compact vortex rings exhibit stronger entrainment and turbulence enhancement. The generated vortex rings convect at a speed of 5St∙U0 in the near field (x/d<7), and then convect at a slightly decreased speed downstream. In addition, the distance between two adjacent vortex rings decreases with increased St. Finally, the spatial dynamic mode decomposition (DMD) results show that the decay rate and the propagation time for an entire flow field subjected to vortex ring generation decrease almost linearly with increased St. This indicates that vortex ring generation increases the propagation time and decay rate of these types of flow fields.