Theoretical And Numerical Research On Ventilated Supercavitating Flows Based On Logvinovich’s Principle

Supercavitation that envelops its vehicle fully is the extreme state of cavitation.On the one hand, it provides the evidence for developing the high-speed underwatervehicle to get great potential of drag reduction because of very limited wetted area.In this case only the head and tail of the vehicle contact water. A betterunderstanding of it requires a deeper research of the academic circles as a physicalphenomenon in the field of fluid mechanics on the other hand, which invloves somechallenges such as phase change between gas and liquid, turbulence, re-entrant jets,gas and liquid compressibility and so on. As much supercavitating flows haveattracted widespread attention from researchers and engineers around the world.Although research on supercavitating flows has made great advances so far, somemechanism and technical questions still restrict its understanding and application,such as the gas-leakage mechanism, the stability of the unsteady ventilatedsupercavity, the blockage effect of the ventialted supercavity in water tunnel, themaneuvering motion and shape control of the ventilated supercavity and its mobilevehicle in unbounded flows, etc.From this view, this thesis is devoted to ventilated supercavitating flows fromexperiment, theory and numerical simulation. The concrete research contents are asfollows:(1) Research on the ventilated supercavity is done by experiment based on theforward facing model. The effects of blockage ratio on the dimensions of theventilated supercavity and its cavitation number in water tunnel are discussed; Thedimension differences of the ventilated supercavity in between water tunnel andunbounded flows are analyzed comparatively at the same cavitation number; Someexperiments are carried out to research the effects of ventilation on supercavitydimensions, its closure mechanism and the flows and its delayed effects; Gravityeffects on the dimensions of the ventilated supercavity and its cavitation number arealso discussed.(2) The gas-leakage rate of the unsteady ventilated supercavity is modeled, andis formulated based on the equation of cavity section from Logvinovich’s principleusing the numerical-analytical method. The obtained formula is validated byexperiment; The closure model of the two vortex tubes of the ventilated supercavity is derived in water tunnel based on the potential theory, and effect models of gravityand angle of attack (AOA) on it are established respectively in uniform flows; Onthis basis, the mathematical model of the unsteady ventilated supercavity isestablished, which consists of the established effect models, the closure model of itstail, and its gas-leakage model combined with the cavity section model for the givenventilation rate; For the choking flows in water tunnel, the mathematical model ofthe minimum cavitation number is also proposed based on the potential theory.(3) The cavitation number embedded coefficient algorithm is developed tosimulate the ventilated supercavity in water tunnel and analyze its characteristicsbased on the established unsteady ventilated supercavity model; The model of theminimum cavitation number of the ventilated supercavity in water tunnel ispresented, and its formula is obtained using the numerical-analytical method andvalidated by experiments; Ventilated supercavity is simulated to analyze the effectsof walls and gravity on its shape, drag and cavitation number in water tunnel, andthen the relation between the minimum cavitation number and the blockagecavitation number is discussed.(4) Effect models of AOA, gravity and inertial force on the ventilatedsupercavity in the curvilinear motion of the vertical and horizontal plane areestablished respectively based on the potential theory; The ventilated supercavity issimulated in the arbitrary curvilinear motion of the plane using the cavitationnumber embedded coefficient algorithm based on the established unsteadysupercavity model, and the effects of inertial force on the shape and its deformationand shedding caused by the change of control variables of supercavity are alsoanalyzed.(5) The effects of the disturbances of various control variables of ventilatedsupercavity from velocity, deflection angle of cavitator, ventilation rate and jets onthe supercavity stability are analyzed numerically using the cavitation numberembedded coefficient algorithm based on the established unsteady supercavitymodel; The stability of the unsteady ventilated supercavity with AOA is researchednumerically, and its pulsation characteristics are analyzed based on the numericalresults using FFT (Fast Fourier Transform Algorithm). Then the stability criterion ofthe supercavity with AOA is obtained using the numerical-analytical method; Theshedding of the ventilated supercavity with velocity disturbance is simulated tomake an analysis of the relation between its stability and the dynamics parameter ofthe gas in supercavity, AOA. (6) The shape control algorithm is developed based on the established unsteadyventilated supercavity model for the ventilated supercavity in the arbitrary motion;Another control algorithm combining the ventilation and the motion velocity is alsopresented to control supercavity dimensions in the vertical plane, and thesupercavity control with constant shape in the vertical motion is simulated.(7) Supercavitating vehicle is modeled, and based on the model in combinationwith the established supercavity model, the algorithm is developed to simulate thevehicle in the curvilinear motion of its longitudinal plane; Uing this algorithm, thevehicle making a diving movement is simulated and its planing forces are alsocomputed. The model is validated by comparing the results with the empericalmodel’s results to some extent.