Study Of The Numerical Simulation Method For Complex Multiphase Cavitating Flow

In recent years, the research of cavitating flows has become an importanthydrodynamic topic due to the supercavity drag-reduction technology for theunderwater vehicle. As moving through the water within a supercavity, the underwatervehicle can achieve very high speed limit by virtue of significantly reduced drag.Technically, the supercavity is easy attained at the comparatively lower mainstreamvelocity by injecting gas into the cavity. In the practical application, ventilationtechnology can also be used to control the shape and hydrodynamic force characteristicsof cavity. Thus, a very complex multiphase cavitating turbulent flow over the vechicle isformed relating to phase transition and mixing, mutual influence between air bubbledevelopment and natural cavitation.The purpose of this paper is to develop the mathematical models and accurate andefficient numerical method employed and to exploit the in-house developed software,which are used to study the flow characteristics and cavitation mechanism of themultiphase cavitating flows, further to provide a useful theoretical basis and technicalmeans for experimental research and engineering application.The dissertation includes the main contents and innovative achievements as follows:(1) Based on the homogeneous equilibrium flow hypothesis, a mathematical modelfor simulating the gas-vapor-liquid multiphase cavitating flow was developed bysolving the mixture RANS equations in the whole flow field. Four kinds of cavitationmodels based on transportation equation of phase fraction and six types of eddyviscosity turbulence models were introduced to quantify the phase transition rate andturbulence influence. The numerical approach based on the FVM and SIMPLEalgorithm was built up. A number of high order and high resolution convection discreteschemes were introduced such that the calculation resolution was improved and thelarge gradient of quantity near the cavity interface was best captured. The stability andfalse diffusion of the different schemes were investigated.(2) A code for simulating the unsteady, multiphase turbulent cavitating flow wasdeveloped. The code has a good applicability and computational efficiency forcomplicated flow field and geometry. Multiple boundary conditions, cavitation models and turbulence models are available. Based on the given grid files, boundary set andcombination of the different program models, the code is applicable to simulate the2-D/3-D ventilated cavitating flows or multiphase cavitating turbulent flows. A specialprogram interface was designed to import the structured grid from GAMBIT, and thepretreatment process on a graphic interface was implemented.(3) The flow characteristic of base ventilated cavitating flows over a symmetricalwedge in a channel was studied. The simulated flow pattern of one-wave pulsation onthe wall of the ventilated cavity agreed well with the experimental phenomena. Theresults showed that a critical ventilation rate which causes the cavity to pulse existedunder a given flow condition. The cavity morphology, gas leakage type and the cavitypressure of the pulsation cavity evolved periodically, violent perturbation wave on thecavity wall resulted in the pinching-off and shedding of the rear of the cavity. At thesame ventilation rate, the increase of the model submergence depth led to the increaseof the average cavity length and the decrease of the pulsation frequency.(4) The gas-vapor-liquid multiphase cavitating flows over an underwater vehicle werestudied. The relevance of ventilated cavitation number, cavity dimension and dragcoefficient to ventilation rate and the natural cavitation number was investigated. Themechanism of mutual influence between the natural cavitation and the ventilatedcavitation was analyzed. The results showed that the ventilated gas suppressed the vaporcavitation or caused the vapor cavity to collapse, and the collapse of the vapor cavityresulted in the pressure oscillation in the local flow field and pressure wave propagation.At a relative small ventilation rate, gas was entrained into the boundary layer on thewall of the vapor cavity and moved towards to the downstream at the vaporization zone,or accumulated at the wall of the body at the non-vaporization zone, eventually rejectedfrom the rear of the cavity.