Study On Flow Field Inside Cavitation Region And Unsteady Characteristics Of Cavitating Flow Around An Axisymmetric Body

Underwater vehicle,hydraulic machinery and ship propellers will be affected by cavitating flows,in terms of hydrodynamic performance,cavitation erosion and noise.Deep understanding the unsteady characteristics of cavitating flow and internal flow structure are useful,which will provide scientific basis for numerical simulation,and prediction of hydrodynamic loads and control techniques for hight-speed underwater vehicles.In this thesis,we study both the internal cavitating flow field and the unsteady cavitating flow characteristics through water tunnel experiments and numerical simulations.In order to solve the difficulties of optical measurement inside a cavity,dual-tip electrical impedance probe(dtEIP)system is developed to measure the void fraction,statistical chord length distribution(CLD),and bubble size distribution(BSD)inside the cavitation region of different cavitating flows.Using the homemade dtEIP system,statistical information on the void fraction and CLD are obtained for supercavitation and sheet/cloud cavitation of different cavitation numbers at laboratory scales.For supercavitating flows,the maximum void fraction of vapor? can reach 1.0 inside the cavity region,the water border of cavity region is found to coincide with the edge of the pure water turbulent boundary layer,the values of mean chord length of the micro bubble in the supercavitating flows are around 2.9–4.8mm.For the sheet/cloud cavitating flows,the maximum void fraction of vapor is less than 1.0 in the cavity regions,which decreases with increasing the cavitation number,mean chord length of the micro bubble are aroud 1.9-4.4mm.Measurement results imply the main flow structures inside a cavity region depend on the cavitation number,while Reynolds number has little effect among the experimental parameters of the water tunnel tests.Numerical simulations are conduct for cavitating flows over a flat plate.Four turbulence model and three cavitation model are used to validate and evaluate numerical convergence,stability and computational accuracy.Numerical results indicate that the k-? turbulence model incorporate with the Singhal cavitation model is suitable for simulations of a cavitating flow.The numerical results are accurate for cavity length,cavity thickness and pressure distribution.Comparing with the experimental results,less accurate distribution of the void fraction can be obtained from the numerical simulations.At this point,numerical simulating flow structure inside a hydrodynamic cavitation region is still a challenging work.For unsteady cavity shedding under steady flow conditions,RANS equations are used to model the three dimensional unsteady behavior of cavitating flow for an axisymmetric body with four headforms at different angle of attack.By modification of the eddy viscosity coefficient in the transition area,the shedding process of cavitating flow can be successfully captured.At a given cavitation number,head shape has a significant influence on the cavitation development and shedding process.For the hemisphere headform,the cavity length is higher,and shedding frequency is bigger.For the blunt headform,the cavity length is longer,shedding frequency is lower.The effects of the attach angle on the shedding frequency are also investigated.Increasing the attach angle will lead to a slight lower shedding frequency,and the high angle of attack will destroy the periodicity of the cavity shedding.The hydrodynamic characteristics of a maneuvering ventilated supercavitating underwater vehicle are studied by using dynamic mesh technique.The cavity shapes and flow field are investigated for an underwater vehicle at circular motion.The influences of centrifugal effects are compared with the cases of the pure gravity effects.The results show that the gravity effect and circular motion generate different gas leakage type respectively.The simulation results show that centripetal force cannot be directly considered as a body force for multiphase flows,especially for the hydrodynamic cavitation.To understand the generation mechanism of the unsteady characteristics of hydrodynamic loads and cavity,the cavitator with continuous deflection motion is investigated.For the activator deflection problem,a sudden change of angel of attack will cause a strong impact,the duration time equal to the ratio of vehicle length and flow velocity.If the cavitator deflects continuouly,the cavity deformation appears unsteady characteristic,and induces the cavity breakdown when the deflection angle is aroud 8 degree.For conclusion of this thesis,the method of measuring flow field inside the cavity have been developed based on the dtEIP probe,and the understanding of characteristics of flow field in the cavity region has been improved;The numerical simulations of the typical unsteady cavitation characteristics are achieved successfully;The frequency characteristics of fracture and evolution of cavitating axisymmetric body with angle of attack are obtained;The unsteady characteristics of the unsteady motion of ventilated supercavitating vehicles and the simulation method are summarized.