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 $15<ReS<75$⁠; however, some discrepancies showed at high velocity gradient region of $ReS≈1000$⁠. 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 bubble’s deformability.