The Numerical Study Of Fluid-Structure Interaction In Cavitating Flow

The fluid-structure interaction(FSI)is normally referred to the interplay between the development of fluid and the deformation or vibration of structure,which widely occurs in many fields of engineering and nature science,such as naval architecture and ocean engineering,aeronautics and astronautics,civil engineering,biological medicine and military project.Because the engineering facilities are being designed with light weight and thin profile to increase the efficiency recently,the FSI effects are especially significant.Cavitation is a particular phase transformation phenomena of liquid with high velocities.The structure of the cavity is usually unstable,and the unsteady cavitating flow would induce significant fluid-structure coupling motion.In the present dissertation,an applicable and reliable numerical computing platform of FSI is established,and is used to study the FSI in the cavitating flow.The response of the elastic structure caused by the flow field loads and its influence on the cavitating flow is analyzed,and the phenomenon the mechanism in the coupling motion is also discussed.The primary researching contents and results are as follows:1.In the frame of closely coupled approach,the computing method of FSI which is adaptable to model the 3D unsteady cavitation-structure interaction in time domain is established.The fluid and structure field is solved separately.For the fluid field,the finite volume method is chosen to calculate the RANS equations in the arbitrary Lagrangian-Eulerian(ALE)coordinate,turbulence equations and cavitation model equation.The interface between various phases is captured by the VOF method,and several dynamic mesh technologies are combined to help the body-fitted grid of fluid field adapting the vibration of structure.The structure described by the equations in the Lagrangian representation is simulated by the finite element method(FEM).The material of structure is assumed linear elastic and the damping is neglected.The data transfer occurs at the interface of fluid and structure field,and the Smart Bucket interpolation method and GGI interpolation method are used to deal with the mismatching of interface grid.2.The oscillations of a plate in resting fluid and the vortex induced vibration(VIV)of a plate appended behind the square cylinder are simulated.The features of time and frequency domain agree well with previous numerical results,which verify the applicability and reliable of the FSI computing platform built in present thesis.The experienced equations are introduced to estimate the damping effects and added mass caused by the viscosity and density of fluid.In the case of VIV,the interaction between the vortex shedding and the motion of the plate is analyzed in detail.The effect on the vortex trajectory and the inhibition to vortex shedding caused by the plate vibration are discussed.And the resonance is also reproduced by the numerical modelling when the frequency of the vortex shedding is close to the natural frequency of the plate.Compared with the results of rigid plate,the amplitude of fluid loads on the flexible plate is growth,while the frequency is decline.3.The FSI motion of a three-dimensional cantilevered hydrofoil in the water tunnel,especially the coupled motion of the elastic hydrofoil in cavitating flow,is numerically investigated.The reliability and the accuracy of the computing platform based on partitioned algorithm are further verified by comparing experimental measurements of fluid and structure in the literature.The typical computing results show that the unsteady characteristic of the cavitaing flow is the major excitation mechanism to the vibration of flexible hydrofoil.In some specific cavitating flow regime,the cavity shedding frequency on the foil is dominated by the features of structure,but not the flow itself.The bending and twisting deformation of elastic foil would increase with the growth of cavity.However,the normalized results indicate that the cavitation does not change the deformation mode of the structure.4.The progress of the elastic cylinder shell exiting water vertically with certain attitude angle is simulated.The interaction mechanism between the structure and the cavitating flow is studied,and the influence of the cavitation number and Froude number are analyzed.Also the numerical simulation strategy on the water exiting of a unrestraint elastic vehicle is presented.The numerical results reveal that starting time of the cavity collapse for the rigid and elastic cylinder shell is almost the same,as well as the evolution and propagation of the pressure.When the shoulder cavity collapses,the elastic cylinder shell would vibrate violently in first order modal patterns.It is found that the effect of fluid-structure interaction could change the process of cavity collapse and shorten the duration.In the course of cavity collapse,the collapse pressure would decrease when the structure oscillation and the hydrodynamic load have the same direction,while the collapse pressure would rise when their directions are opposite.