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Effect of Cavitation and Turbulence Model on Numerical Study of Unsteady Cavitation

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Tutor: ZhangXiaoBin
School: Zhejiang University
Course: Refrigeration and Cryogenic Engineering
Keywords: Unsteady cavitation,Dynamical cavitation model,NACA66,Twist-11N,Turbulence visco
CLC: TK121
Type: Master's thesis
Year:  2014
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Abstract:
Cavitation is the formation of vapor bubbles within a liquid where the flow dynamics causes the local static pressure to drop below the vapor pressure. And it causes so many damages, such as vibration, fluctuation of lift and drag coefficient, erosion and decrease of performance of hydraulic machine. Unsteady cavitation brings even more serious and complicated effect. The main purpose of this paper is to study the effect of cavitation model and turbulence model, which are the two most important key factors, on the transient character of cavitation.The Full Cavitation Model (FCM) has become an internal model of the commercial CFD software Fluent as a result of its strong stability and accuracy. However, the assumption itadopts that the bubble radius is a constant is not appropriate for unsteady cavitation. Based on the assumption of thermal and chemical balance, we recalculated the pressure-dependent radius and purposed the Dynamical Cavitation Model (DCM), which had considered not only the first-order pressure effects but also zero-order effect and could provide greater insight into the physical process of bubble producing, developing and collapsing compared to FCM. Numerical results of unsteady2d cavitating flow around NACA66and3d cavitating flow around Twist-11N of DCM is in accordance with that of experiments. Compared with FCM, the error of the cycle of DCM is smaller and DCM is able to depict the shedding character of cavity, which is beneficial to the analysis of the flowfields and performance of hydraulic machine.The popular used cavitation model needs to be verified in the modelling of the cryogenic cavitation, as the property of the cryogenic fluids shows much difference from the normal room-temperature fluids and also the experimental coefficient of these models are obtained in the normal fluids. The Sauer model and Zwart model have been tested for cryogenic cavitating flow and the results show that the Sauer model is more accurate in the calculating of the length of the cavity and the position of the lowest temperature is more close to the experiments.On the other hand, the effect of turbulence model on unsteady cavitation is conducted. Three kinds of correction functions are adopted and tested in the unsteady cryogenic cavitating flow. After comparing the results of these modified k-¦Å models with that of the Large Eddy Simulation (LES), it is suggested that density correction model is more appropriate in simulating unsteady cavitating flow while the results of others are still steady or almost steady.
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