Study On The Dynamic Characteristics And Physical Mechanism Of Attached Unsteady Cavitating Flows

Cavitation can be observed in a variety of fluid machinery like marine propellers, hydraulic turbines, pumps and underwater vehicles. Most of the cavitation phenomena in engineering are attached cavitating flows which form on the body surface. The unsteady process of attached cavitating flows involves growth, shedding and collapse of cavity and many other complex turbulent flow phenomena. Undesirable features of cavitation are vibration, noise, erosion and so on. Fundamental research on unsteadiness and dynamics of attached cavitating flows is of great importance in engineering, because it can help to enrich the knowledge of cavitating flows and propose measures to avoid the adverse effect of cavitating flows.Both experimental and numerical researches are carried out to explore the dynamics of cavitating flows. On the experimental side, a simultaneous sampling system for cavitating flows in cavitation tunnel is established, cavitation induced pressure fluctuation and pressure pulses in convergent-divergent channel are analyzed, and air-injection technique is applied to adjust the dynamic characteristics of attached unsteady cavitating flows. On the numerical side, the mechanism of pressure fluctuation associated cavity evolution and vortical motion characteristic are analyzed. The main contents and findings are as follows:(1) A simultaneous sampling system for cavitating flows in cavitation tunnel is established, it combines the high speed visualization system and pressure measurement system, based on the simultaneous sampling system, the measurement of cavity shape can be synchronized with and measurement of pressure signals. Then, the cavity shape and pressure fluctuation at the same instant in attached unsteady cavitating flow can be analyzed to illustrate their correlation.(2) Based on the simultaneous sampling system, the cavity shape and pressure fluctuation of attached unsteady cavitating flows in convergent-divergent channel are obtained. The time domain characteristic and frequency domain characteristic of pressure fluctuation under different cavitation numbers are investigated. The growth, shedding and collapse of quasi-periodic cloud cavity evolution and excited pressure fluctuation are obtained experimentally, the relationship between cloud cavity evolution and pressure fluctuation is analyzed. With the development of cloud cavity, large-amplitude pressure pulses with magnitudes of the order of several atmospheres and typical durations of the order of milliseconds are captured, the propagation speed of pressure pulse in different cavitation regions is found to be related with the bubble density in the flow field. According to the distribution of pressure impulse, it’s found that the pressure impulse in the region covered by attached cavity is much lower than that in the attached cavity closure area. Besides, particle image velocimetry technique is used to measure the distribution of velocity, vorticity, and turbulent fluctuation in cavitating flow field under different flow condition, the influence of cavity evolution on flow structure and turbulent field is analyzed.(3) The mechanism of pressure fluctuation excited by cloud cavitation development and the vortex motion characteristic of cloud cavitation are illustrated. Both experimental and numerical result show that the second derivative of cavity volume over time is proportional to the pressure amplitude, this conclusion is consistent with the result based on bubble dynamics analysis. Finally, the three dimensional vortex motion characteristics of cloud cavitation are illustrated by lagrangian method, isosurface of Q and three dimensional streamlines.(4) The basic principles of adjusting the dynamic characteristics of cavitating flows by air-injection technique are obtained. Simultaneous sampling technique is applied to acquire cavity shape and pressure fluctuation under different cavitation numbers and air flow-rates. It’s found that the pressure pules are surpressed significantly by air-injection technique, and with the increasement of air flow-rates, the shedding frequency of attached cavitating flow is reduced significantly, the development of attached cavitating flow becomes more stable, accordingly, the magnitude of pressure fluctuation is reduced a lot.