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Numerical Study of the Unsteady Cavitating Flow Generated by Underwater High-speed Body

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Tutor: ZhangJiaZhong
School: Harbin Institute of Technology
Course: General and Fundamental Mechanics
Keywords: Bubble dynamics,Cavitation,Secondary cavity,Cavitation model,Model coefficients
CLC: U661.1
Type: PhD thesis
Year:  2013
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Abstract:
Cavitation often occurs around the high-speed submerged body and in thehigh-speed turbine system and other high-speed motion under water. Cavitation mayoften cause cavitation damage in the turbine system however may show therevolutionary drag reduction effect during the motions of the underwater vehicleenveloped in a supercavity. To strengthen the research of the cavitation mechanismhas a wide range of practical significance. The research began with the analysis ofthe bubble dynamics, the theoretical correction was implemented onRayleigh-Plesset equation and the Rayleigh-W equation was acquired. Then thesimplified Rayleigh-W equation was introduced to establish the cavitation modelwith cavitation mass rate just similar to the process of the derivation of the FullCavitation Model. The main research works are as follows.The paper first provides an overview of the basic control equations ofcavitation flow, and then points out the defects of the existing cavitation models.Secondly, based on the Rayleigh-W equation, a new bubble dynamics model wasestablished taking into acount the deformation characteristics of the single sphericalbubble in the incompressible flow. Taking ideal gas as an example, the changedirection of the cavitation phase-change during the bubble deformation was studiedand then the result was introduced into the Rayleigh-W model to rebuild the bubbledynamics model.Based on the derivative of the Rayleigh-W equation, the expansion andcontraction of a single bubble were studied, and then the pressure difference insideand outside of a bubble and the velocity of the bubble wall were estimated. Throughthe assumption that the gas inside a cavitation bubble experienced polytrophicprocess, the bubble collapse frequency was further studied. Through the variationalanalysis of the Rayleigh-W equation, the relationship between the bubble wallvelocity and the bubble diameter during the bubble collapse was studied, and theachievable contraction limit radius during the bubble collapse was also studied atnormal temperature.Based on the two-phase fluid continuity equation, the cavitation phase-changerate during the deformation of the spherical bubble was derived and then proved,and then the conclusions of which were generalized to an arbitrary cavity shape.Combined the Rayleigh-W equation, the cavitation phase change rate and phasechange direction during the bubble deformation process, a new cavitation modelbased on the simplified Rayleigh-W equations was established for the numerical study of cavitation flow. Taking the water tunnel experiment and literature data forreference, the validation and verification (V&V) were carried out on the newlycreated cavitation model.Numerical simulations were carried on the cavitation flow generated from theunderwater high-speed hemispherical head cylinder model using the new createdcavitation model. During the study, the calculations to verify the model coefficientswere firstly numerically carried out using enumeration method, and then thehydrokinetic layout and the variations of related parameters over time of thehemispherical head cylinder high-speed sailing underwater were analyzed. Thecalculation errors of the drag coefficients caused by the improper model coefficientswere studied. The relationship between the model coefficients and the cavity shapewas analyzed. The types of the cavities in the cavitation flow field and therelationship with the cavitation number has been summed up. The relationshipsbetween the cavitation number and the cavitation model coefficients in some typesof cavitation flow fields have been acquired.Indirect analysis was carried out to the relationship between the main cavityshape and the model drag coefficients in the supercavitation flow field using therelationship between the cavitation model coefficients and the shape of the maincavity.The thesis has studied the bubble dynamics model and numerical cavitationmodel, successfully capture the main unsteady details of the cavitation flow,provides a theoretical basis and foundation for the study of cavitation mechanismin-depth.
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