| To improve the cavitation performance and the operation stability of the hydraulic machinery,researches on hydrodynamic characteristics and theory of cavitation flows around hydrofoils have great significance for engineering application.In this paper,flows with and without cavitation around a three-dimensional(3D)hydrofoil are investigated by experimental and numerical approaches.First of all,inception cavitation,attached sheet cavitation,cloud cavitation and super cavitation in flows around a hydrofoil are measured by means of water tunnel experiments,and the periodic variation of cavitation shapes and structure at different cavitation stages are successfully observed.The inception cavitation bubble is generated in low-pressure area near the leading edge and the generation-expanding-shedding-collapse evolution of the inception cavitation bubble is observed.The attached sheet cavitation generates only at small angle of attack,expands with time until downstream-side part of sheet cavity detaches from hydrofoil surface and forms cloud cavitation.As the cavitation number decreases further,the characteristic of cloud cavitation stage is observed obviously in cavitation flows around hydrofoil.Under conditions of small angles of attack,cloud cavitation occurs in low-pressure area near the leading edge of the hydrofoil,and the size of cavities gradually increase with decrease in cavitation number.Under conditions of large angles of attack,the periodic shedding of cloud cavitation happens simultaneously in the leading edge and the trailing edge of the hydrofoil.In particular,cavitation vortex sheets form at hydrofoil trailing edge region.Keeping decrease the cavitation number,super cavitation regimes can be observed.The upper surface of the hydrofoil is completely covered by the cavitation region which extends to the downstream of the flow.An obvious vapor-liquid interface exists between the cavitation region and the main flow region.Based on the Chapman-Enskog expansion of LBM and the fundamental of LES,LBM-LES coupling model is proposed by combining LBM Single-Relaxation-Time model and LES Sub-Grid Scale Smagorinsky model,and the equivalent relaxation time is proposed to reflect turbulence characteristics.Numerical investigations of turbulent flows around the hydrofoil at Reynolds number Re = 2.5×104 are carried out by using LBM-LES coupling model.The number,position and scale of vortex,and the hydrodynamic characteristics at different angles of attack are in good agreement with those obtained from the hydrofoil experiments.Besides,some small-scale vortexes are only captured by numerical approach.In addition,the pressure,the lift and the drag coefficients are analyzed quantitatively and qualitatively in detail.All the results and analyses verify the feasibility and accuracy of the LBM-LES coupling model for predicting turbulent flows around hydrofoil.Based on the high precision of Carnahan-Starling(C-S)real-gas equation of state(EOS)and its proper computational method of intermolecular potential,a 3D cavitation flow SC-CS model is established by combining Shan-Chen model and C-S EOS for simulating gas-liquid two-phase flows with large density ratio.3D phase separation processes under different temperature are predicted successfully by means of the SC-CS model,and the inter-phase large density ratio over 2×104 are captured and the applicability of the presented model in solving cavitation problems are verified by the theoretical curves of the Maxwell equal-area construction.The evolution of creation,collapse and repetition of cavitation bubble is successfully reproduced by 3D cavitation flow SC-CS model.The effectiveness and reliability of present method are verified by energy barrier theory.The influences of temperature and pressure difference on cavitation bubbles are analyzed systematically.During the cavitation occurrence and development processes,under higher temperature and larger pressure difference,cavitation is more likely to occur and the expanding velocity of cavitation bubbles are faster.During the processes of cavitation contraction,the change rules of bubble radius are similar and smaller bubble can cause bubble collapse faster.The presented cavitation flow SC-CS model is applied further to cavitation flows around 3D hydrofoil.Heterogeneous cavitation simulations are carried out to investigate the evolution of pre-existing bubble in hydrofoil flow fields and homogeneous cavitation simulations are applied to study the phase transition at low-pressure area of the hydrofoil upper surface.In heterogeneous cavitation simulations,cavitation occurs when the pressure difference between the bubble and the liquid phase is larger enough to overcome the surface tension.The variation of bubble shape is predicted in the evolution of bubble first increasing and then collapsing.Homogeneous cavitation simulations provide the evolution of generation,development,shedding and collapse of inception cavitation due to phase transition at low-pressure area near the leading edge of hydrofoil.The simulations results related to homogeneous cavitation are compared with those of the cavitation experiments,the evolution of cavitation,the location of cavitation generation and collapse are in good agreement with the experimental.The successes in predicting 3D cavitation of bubble seeds and hydrofoil flow fields verify the effectiveness of present 3D cavitation flow SC-CS model for complicated cavitation flows with complex boundary conditions and extend the application areas of LBM.Based on 3D SC-CS model,parallel algorithm is developed by adopting MPI(Message Passing Interface)in C++ language program to improve the computational efficiency of cavitation flows.Parallel performances are analyzed systematically including execution time,acceleration rate and communication efficiency.The optimum parallel performance of minimum execution time,maximum acceleration ratio and high communication efficiency are investigated by using 5 processes for parallel calculations. |
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