Unsteady pressure fluctuation characteristics in the process of breakup and shedding of sheet/cloud cavitation

Abstract

The objective of this paper is to investigate the unsteady pressure fluctuation characteristics in the process of breakup and shedding of unsteady sheet/cloud cavitating flows via combined experimental and computational methods. Experiments are conducted in the divergent section of a convergent-divergent channel using a simultaneous sampling technique to synchronize the transient cavitation behaviors and wall-pressure signals. In the numerical simulations, the Zwart cavitation model and the modified RNG k-ε turbulence model are solved, with the compressibility effects of both water and vapor considered. In addition, one-dimensional bubbly shock wave relationship is applied to analyze the process of the vapor fraction discontinuity propagation. Two different types of cavity breakup and shedding existing in the unsteady sheet/cloud cavitating flows are observed, which is induced by re-entrant flow and vapor fraction discontinuity propagation mechanism, respectively. The re-entrant flow generates at the rear of the cavity, moving forward along the wall. When it arrives at the throat, it breaks up the attached cavity, resulting in the cloud cavity shedding. During the process, the wall pressure fluctuation is relatively small. The vapor fraction discontinuity propagation is resulted from the bubbly shock in water/vapor mixture of the sheet/cloud cavity. There is a significant difference of vapor fraction between the pre- and post of the discontinuity. The pre-discontinuity area is almost pure vapor, and the post-discontinuity area consists of water/vapor mixtures with relatively low vapor fraction. During the discontinuity propagation, the pressure peak exists at shock wave front. When the discontinuity arrives at the throat, the void fraction will suddenly decrease, which indicates the low vapor generation rate. Under the convection of the main flow, the attached cavity will be separated from the newly generated vapor, resulting in the attached cavity breaking up and the cavity cloud shedding.