Observations and measurements on unsteady cavitating flows using a simultaneous sampling approach

Abstract

The main purpose of this study was to shed light on the unsteady cavitating flow and corresponding wall-pressure fluctuation characteristic. A simultaneous sampling technique is used to synchronize the observations of cavitation images and the measurements of wall-pressure signals in a convergent–divergent channel. The results show that, with the decreasing cavitation number, cavitating flows in a convergent–divergent channel display several types of cavitation behavior, such as cavitation inception, sheet cavitation, and sheet/cloud cavitation. The intensity of the pressure fluctuation increases with the decrease in cavitation number. However, with decreasing cavitation number, the dominant frequency of the unsteady pressure fluctuation decreases significantly, and for sheet/cloud cavitation, the dominant frequency of pressure fluctuation is consistent with that of global cavitation area fluctuation. A typical quasi-periodic sheet/cavitation development cycle is characterized by three stages such as: (1) the growth of attached cavity, (2) the shedding of attached cavity, and (3) the development and collapse of detached cavities. In the stage one, the magnitude of pressure fluctuations under the attached cavity is small; however, it is large in the closure region of attached cavity, especially when attached cavity obtains its maximum length. In the stage two, the attached cavity begins to shed and some small detached cavities are observed, and small local pressure fluctuations with higher frequency are detected. In the stage three, a large detached cavity is formed in the rear of attached cavity. When the detached cavity collapses rapidly in the downstream region, pressure pulses with the magnitude of the order of several atmospheres are detected. The propagation speeds of pressure pulses in different cavitation regions are found to be related with the bubble density in the flow field. It is also found that the pressure impulse in the region covered by attached cavity is much lower than that in the attached cavity closure area.