| Cavitation is a complicated and unique multiphase phenomenon in the liquid.It often occurs on the surface with a high relative motion,such as ship propeller and rudder,hydraulic machinery and projects.As one of the most typical cavitation phenomena,the cloud cavitation is characterized by the distinct unsteady dynamics during the whole evolution process,including growth,development and collapse.It can be the source of noise,vibration,erosion and poor hydrodynamic performance that are harmful to ship system and hydraulic system.Therefore,the research on unsteady cloud cavitation has become a hot topic.Moreover,the evolution and the unsteady dynamics of the cloud cavitation over the hydrofoil are significant for the avoidance and prevention of cavitation damage and the utilization of cavitation benefit in engineering.The cloud cavitation around a NACA0015 hydrofoil has been investigated by Large Eddy Simulation(LES)method embedded in FLUENT.It focuses on the unsteady dynamics of the cloud cavitation and the characteristics of lift and drag coefficients and cloud shedding frequency.Subsequently,the most probable locations of cavitation erosion on the hydrofoil surface are predicted.First,the numerical simulation of non-cavitating flow around a two-dimensional NACA0015 hydrofoil is investigated by LES method.The grid size sensitivity has been analyzed by using the lift and drag coefficients as the benchmark.Then the numerical simulation of unsteady cavitating flow around the two-dimensional NACA0015 hydrofoil is performed by the LES method.It thoroughly carries out investigations of the influence of different sub-grid scale models(SGS),computational grids and Courant numbers on the calculation results,such as the lift and drag coefficients,and the cloud shedding frequency.Analysis of these changes lead to different results of the calculation.Comparing with experimental data and URANS result,the lift and drag coefficients,the shedding frequency and the typical cloud cavitation shape of the simulation results under different conditions are analyzed.By comparison with the experimental results and the URANS results,the further detailed investigation is performed by the Smagorinsky-Lilly sub-grid scale model,the medium grid G2,and the Courant number of 1.0.It focuses on the capability of LES method to predict the unsteady behavior of the cavitating flow over the hydrofoil,such as the cavity shed and regeneration,the development of the re-entrant jet and the influence of the re-entrant jet on the cavitation shedding process,and the quantitative prediction of the shedding location and frequency.Based on the two-dimensional simulations,this paper explores the capability of the LES method to predict the evolution process of the cloud cavitation around a three-dimensional NACA0015 hydrofoil and its unsteady dynamics.It also analyzes the correlation between the unsteady transient behaviors and the lift and drag on the hydrofoil.The locations of the high cavitation erosion risk area are finally predicted by the analysis of the pressure field.Compared with the URANS method,the unsteady evolution process of the cloud cavitation captured by LES can display more vortex sub-structures.The predicted lift and drag fluctuates more obviously than the URANS results.And the shedding frequencies of the cloudy structures is much closer to the experimental results.It is proved that the flow field information predicted by the high-resolution turbulence model(e.g.LES)has important significance on the quantitative prediction of cavitation induced phenomena.It can provide numerical foundation for further investigation of pressure fluctuation analysis,quantitative assessment of cavitation erosion,and cavitation noise and vibration,etc.However,there are deviations of the locations of some structures from the experimental observations.It is supposed to be reduced by improving the quality of the mesh and selecting the appropriate parameters,which will pave the way for more accurate quantitative prediction. |
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