Jong-Woo Ahn

Cavitation Observation and Visualization of the Gap Flows on a Rudder Influenced by Propeller Slipstream and Hull Wakes

In the present study, the influences on the gap cavitaiton of the semi-spade rudder are investigated experimentally in the condition with propeller and hull wakes. To reduce the scale effect in the given experimental conditions, 1/28.5-scale-down models of propeller and rudder are manufactured. We have the propeller rotate ahead of the rudder, inducing the three dimensional effects originated from the propeller action. Experimental methods are composed of the cavitation observation using high speed camera, PIV (particle image velocimetry) measurements to visualize the cavitaition and flows around the gap. The propeller slipstream affects both of the gap flows and cavitation of the rudder.

Measurements of High Velocity Gradient Flow Using Bubble Tracers in a Cavitation Tunnel

The objective of the present study is to investigate propeller wake using particle image velocimetry (PIV) technique with bubble type of tracers, naturally generated by the decrease in the static pressure in a cavitation tunnel. The bubble can be grown from the nuclei melted in the water tunnel and the size of bubbles is changed by varying the tunnel pressure. A series of experiments are conducted in the conditions of the uniform and high velocity gradient flows to find out the characteristics of bubble tracers and compared the measurement results using bubbles with those using solid particles. Bubbles showed good trace ability in the region of 15 <Re s <75; however, some discrepancies showed at high velocity gradient region of Re S ≈ 1000. The fitted vorticity reduction rate would give reference for the prediction in a real flow when bubble tracers are utilized in PIV measurements of a vortical flow In addition, the characteristics of bubble slip velocity can provide information on the vortex core center and the reduction in the Reynolds shear stress caused by bubbles deformability.

Visualization of Unstable Vortical Structure in a Propeller Wake Affected by Simulated Hull Wake

The characteristics of complicated propeller wake influenced by hull wake are investigated by using a two-frame PIV (Particle Image Velocimetry) technique. As the propeller is significantly affected by the hull wake in a real marine vessel, the measurements of propeller wake under the hull wake would be certainly necessary for more reliable validation and the prediction of numerical simulation with wake modeling. Velocity field measurements have been conducted in a medium-size cavitation tunnel with a hull wake. Generally, the hull wake generated by the boundary layer of ship`s hull produces the different loading distribution on the propeller blade in both upper and lower propeller planes. The difference of the propeller wake behaviors caused by the hull wake is discussed in terms of axial velocity, vorticity and turbulence kinetic energy distribution in the present study.