Flow and mixing characteristics of swirling wakes in blockage-effect regime

Abstract

The flow structures and turbulence properties of swirling bluff-body wakes are studied via the velocity fields measured by a two-component laser Doppler velocimeter. The velocity vectors, streamline patterns, vorticities, velocity distributions, and turbulence properties are presented and discussed. The streamline patterns show two characteristic flow modes, single bubble and dual rings, in the bluff-body effective regime. The single bubble is a closed recirculation bubble with a stagnation point on the central axis. Unsteady large-structure flow motions in the bubble result in high turbulence intensities. The dual-ring flow structure is an open-top recirculation zone, in which a pair of large-vorticity-containing counter-rotating vortex rings exists. The dual rings induce a central swirling jet to stabilize on the circular disc and expel the flows in the dual rings to the downstream area. In order to obtain and compare the mixing capabilities of the single-bubble and dual-ring flow structures, the tracer gas is released into different regions of the wake, then the concentration distributions are measured. Both the flow characteristics and tracer-gas concentration distributions are analyzed and logically argued to elucidate the influences of the flow patterns and turbulence properties on the mixing capability.