Numerical Study On Unsteady Cavitating Flow And Erosion Based On Homogeneous Mixture Assumption

In the present dissertation a new cavitation erosion model is proposed based on the released energy during cavities collapse,and homogeneous mixture flow assumption.The study provided important theoretical support for design and operation of hydraulic machinery.Previous studies were carried out using commercial or free software to describe the main mechanism of the cavitation as well as erosion with experimental validations.Numerical results showed that the unsteady cavitating flow can be reproduce fairly well.For erosion,there are results related to the pressure fluctuation and the distance between the bubble centre and wall without considering the vapor void fraction.Therefore,the prediction of erosion is qualitative other than quantitative with errors in the shape of the eroded region.For erosion prediction based on the mixture flow assumption,the first step is the optimization of the numerical simulation of unsteady partial cavitation,considering asymmetric conditions for condensation and vaporization.Therefore,the cavitation model of Zwart-Gerber-Belamri(Z-G-B)has been written in C++ language and implemented in OpenFOAM as a new solver.Secondly,a innovative turbulence model,i.e.,ILES has been used to avoid the explicit coupling between mass transfer and the subgrid model for high turbulence.Numerical simulations of partial turbulent cavitation around hydrofoils such as NACA0015,NACA66 and plane-convex have been carried out.Results show that the unsteady cavitation procedures,the attached leading edge cavity,the re-entrant jet,cavitation shedding,detached cavity and cavity collapse have been reasonably reproduced and match fairly well with the experimental results from previous studies with a Strouhal number close to 0.2,which indicates that the cavitation phenomena can be replicated.Furthermore,a variable time step controlled by CFL condition has been used to avoid floating points due to the refined mesh and the difference between transmission of the information and the predicted velocity.Finally,a new cavitation-erosion model has been proposed to assess the flow aggressiveness,the total affected region and the material plastically deformed.The model is based on the homogeneous mixture flow assumption and the transformation of potential and kinetic energy between the initial and the end of the collapse.In this context,the microjet speed has been calculated using the proposed model and compared with the magnitude of the deformation velocity to predict plastic deformation.Numerical simulation of partial cavitation around a plane-convex hydrofoil with semicircular obstacle and the flow in an axisymmetric nozzle have been carried out.Cavitation-erosion results of the plane-convex hydrofoil with semicircular obstacle and the axisymmetric nozzle have been compared to experimental results from the high speed cavitation tunnel of the école Polytechnique Fédérale de Lausanne(EPFL)and from the Laboratoire des écoulements Géophysiques et Industriels(LEGI),respectively.Results show similar regions affected by the flow aggressiveness and improvements over previous models.In summary,numerical simulations of cavitation erosion have been optimized using the implemented model of Z-G-B,ILES and a variable time step.Results of cavitation have been post-processed using the proposed model of cavitation-erosion,which is based on homogeneous mixture flow assumption and the released energy during bubble collapse.Furthermore,the studies have been carried out using free open source software as OpenFOAM,Paraview Salome,GMSH and programming language as C++ and python.