Experimental investigation into influence of clearance size on cavitating vortical flows in hydrofoil tip

Cavitating vortical flows in a hydrofoil tip with different tip clearance sizes are studied experimentally. A series of experiments are conducted in a closed-loop cavitation tunnel. The cavities are observed with a high speed camera and are analyzed by using image processing. The cavitating flow developing characters in the different size clearances (0.5 and 2.0 mm) are compared. Based on the results, three typical cavitation stages can be defined with decreasing cavitation number, namely inci-pient cavitation (I), attached cavitation (II) and shear layer cavitation (III). In stage I, the streamwise length of cavity in the big gap is larger than that in the narrow gap. In stage II, the attached cavity inside the large gap develops faster than that in the small gap in both the cavitation inception and the developing phases with decreasing cavitation number. In stage III, the critical cavitation number for the formation of triangular shear layer cavitation in the large gap is higher than that in the small gap; meanwhile, the cavity extends further downstream in the large gap than that in the small gap as well.