Experimental investigation on the breakdown and shedding mechanisms of unsteady attached cavitating flows

The breakdown and shedding mechanisms of unsteady attached cavitating flows are investigated by experiments in a convergent-divergent channel. Experiments are carried out in a rectangular test section of a cavitation tunnel using simultaneous sampling technique to synchronize the cavitation images with wall-pressure signals. The results show that there are two different kinds of breakdown and shedding mechanisms of unsteady attached cavitating flows, namely re-entrant jet and discontinuity propagation mechanisms. The re-entrant jet generates at the rear of the cavity and keeps moving forward along the wall. The thickness of the re-entrant is far thinner than that of the cavity. During the propagation of a re-entrant jet, the wall pressure fluctuation is relatively smooth. When the re-entrant arrives at the throat, it will break up the attached cavity, resulting in the cavity cloud shedding. Discontinuity propagation is another breakdown and shedding mechanism of unsteady attached cavitating flows. The thickness of the discontinuity is the same as that of the local attached cavity. Moreover, the vapor fraction in front of and behind the discontinuity has large differences. The pre-discontinuity is almost pure vapor with high vapor fraction and the post-discontinuity is water/vapor mixture with relatively low vapor fraction. During the discontinuity propagation, the pressure peak will exist at the location of the discontinuity. When the discontinuity arrives at the throat, a large area of low void fraction will exist, reducing the vapor generation rate. Under the convection of the main flow, the attached cavity will be separated from the newly generated vapor, resulting in the breaking up of the attached cavity and the cavity cloud shedding.