| Cavitation is the whole process of the nucleation in liquid flow growing into cavity and the cavity growing,shedding,flowing and collapsing when the local static pressure drops to critical pressure.Cavitation occurs in a variety of fluidic machinery and brings negative effects like vibration,noise and erosion.Cavitating flow is a complex unsteady two-phase flow with high turbulence and vortex motion.The flow is even more complicated in cryogenic fluids concerning thermal effect and energy exchange in addition to the above factors.There is amount of numerical simulation and experimental research of cavitation in water at room temperature.However,simulation of cavitation in cryogenic fluids is limited to two-dimensional structure and experiments are rarely published.Numerical simulation based on the LES for unsteady cavitating flow in liquid nitrogen(LN2)is carried out in this thesis.The simulation results agree well with experimental data.The structure used in simulation is a three-dimensional twisted hydrofoil Twist-11N.The simulation results indicate that the mechanism of cavitation in LN2 and water is significantly different:cavity shedding in water is caused by re-entrant flow and that in LN2 is mainly caused by side-entrant flow.In addition,the interaction between vortex motion and cavity evolution in flowing field is illustrated in this research.A visualization experiment setup of cavitation in LN2 is designed and the preliminary experiments are carried out,and the dynamic cavity shedding process is captured.Some categories of cavitation phenomena such as partial cavitation,sheet cavitation and cloud cavitation are observed in the experiment.The periodicity of cavity shedding is studied by measuring the cavity length in different instants.The cavity evolution in the period is partitioned into three stages according to different shedding morphology. |
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