Experimental investigation on cavitating and self-sustained oscillating flow in a horizontal axisymmetric cavity

An experimental study is performed to reveal the flow phenomena and the oscillation characteristics of water jet flow through a horizontal axisymmetric cavity. The test conditions include different Reynolds numbers and various cavity lengths. Two distinct states are identified inside the axisymmetric cavity for a cavity length range of 4.03 ≤ Lc/d1 ≤ 10.82, namely, the submerged state and the unsubmerged state. It is observed that low-frequency and large-amplitude (LFLA) self-sustained oscillations occur only in the unsubmerged flow state. Three factors lead to LFLA semiperiodic oscillations, among which the periodic connection of the inside cavity with the atmosphere is dominant, while the stochastic variation of spread angle and the jet deviation in the radial direction are minor causes. The flow pattern in the unsubmerged state is divided into four stages inside the cavity according to the variation of the number of bubbles. In the submerged state, the jet closes the cavity and cavitation occurs inside the cavity. The pattern of cavitation inside the cavity shows four distinct periods. As the cavitation number increases, cavitation first occurs in the shear layer and generates deformed bubbles, following which it develops into a full-cavitation pattern. During this process, the shape of the cavitating bubbles changes from densely packed deformed bubbles to large spherical bubbles. The cavitation number and the flow pattern inside the cavity remain almost steady in the full-cavitation regime. The critical Reynolds number for the occurrence of full cavitation increases linearly with increasing cavity length. The oscillation characteristics of the LFLA oscillations are semiperiodic, whereas the oscillations in the submerged flow state are chaotic. Nonetheless, the oscillation amplitude in the submerged state is much smaller than that in the LFLA oscillation state.