Dynamics of compact vortex rings generated by axial swirlers at early stage

Abstract

This work concentrates on the study of flow dynamics of swirl vortex rings at the Reynolds number Re = 20 000 using a combination of the planar- and stereo-particle image velocimetry (PIV) measurements and dynamic delayed detached-eddy simulation. Particular attention is paid to the identification of the large-scale azimuthal modes in the vortex ring propagation process. In the experiments, vortex rings are issued from piston-driven axial swirlers with the swirl number ranges from S = 0 to 1.10. The stroke ratio L/D = 1.5 is used to produce a compact vortex ring without a trailing jet. PIV measurements are conducted in a water tank, while the in-plane component flow velocities on the longitudinal center plane and the three-component flow velocities on the cross section plane at several downstream locations according to the ring trajectories are obtained. In the simulation, the axial swirlers are also included, while the piston motion is realized by imposing a time-dependent inflow condition. Two types of dynamic effects in the vortex ring propagation process are captured by the planar-PIV measurement: the arriving time effect and the azimuthal effect, which induce parallel shift of the vortex ring core and the radial tilting of the vortex sheet, respectively. These modes are identified using the stereo-PIV results by applying the fast Fourier transform in the azimuthal direction, followed by the proper orthogonal decomposition on the radial and temporal directions. It shows that both m = 0 and 1 modes (m is the azimuthal wave number) coexist in the weakly swirled vortex rings, while the m = 2 mode arises and the m = 0 mode decays at high swirl numbers. The simulation also identifies the m = 1 and 2 modes, while the m = 2 mode has a large pitch with respect to the formation time.