Slip effects in vertical structure behind cavitating propeller wake

Abstract

The cavitating propeller wake is investigated in detail using PIV and shadow graph techniques to figure out the trace ability of bubble type of tracers, naturally generated by the decrease of the static pressure in a cavitation tunnel. Experiments are conducted in the conditions of the cavitating propeller wake containing strong vortices. The flow behaviors of bubble tracers are compared with those of normal solid particles. The bubbles grown from the nuclei melted in the cavitation tunnel could not influence on the additional buoyancy, producing good trace ability in the uniform flow. The comparison between bubbles and solids shows some discrepancies in the area around the tip vortex core of cavitating propeller wake. The slip velocity, indicating the difference of moving velocities between bubble and solid, induces rather high difference in the vorticity values of it. The results of comparison in terms of vorticity values showed good agreement in the region of mild ReS (Reynolds number based on the slip velocity), however, disagreement at high ReS over 1000. The large slip velocity and high ReS provided a velocity difference, especially in the high velocity gradient region. The slipstream region gives a range of 15 <ReS < 75; however, ReS is about 1000 at a high velocity gradient region of a tip vortex. The fitted vorticity reduction rate would give a reference for the prediction in a real flow when bubble tracers are utilized in PIV measurements of a vortical flow. The vorticity reduction rates are obtained according to the bubble size which is considered to affect the slip effect in the high velocity gradient region.