Numerical Simulation Of The Unsteady Cavitating Flow And Radiated Noise Of Three-Dimensional Hydrofoil

Cavitation flow involves complex problems such as turbulence,phase transition,vis-cous and compressible effect.Cavitation phenomena can be found in hydraulic valves,fuel pumps,marine propellers,etc.Cavitating flow is often unsteady,and unsteady cavi-tating flow is the main source of cavitation erosion,flow-induced vibration and noise ra-diation.Research on the cavitating flow of marine propeller shows that the cavitation will not only reduce the propulsion efficiency but also increase the flow noise level.Therefore,it is necessary to study the unsteady cavitating flow field and its flow noise.In this paper,the unsteady cavitating-flow and its noise radiation are simulated using the unsteady RANS equations and FW-H_pds?FW-H equation for penetrable data surface?.Through the analysis of the non-linear relationship between Reynolds shear stress and the mean strain rate in the tip vortex core region,the applicability of the non-linear turbulence model to the swirling flow is found.It is applied to the simulation and prediction of the three-dimensional unsteady cavitating flow around a twisted hydrofoil and the unsteady cavitating tip clearance flow around a truncated three-dimensional hydrofoil.Finally,the corresponding flow noise is simulated and predicted with the self-developed noise module.In terms of unsteady cavitation shedding characteristics of the three-dimensional twisted hydrofoil,both the main and secondary cavitation shedding phenomena were ob-served.And after the collapse of secondary cavitation,a high-pressure concentrate on the end of the sheet cavitation,which will lead to the formation of counter-pressure gradient and provide energy for the re-entrant jets.That is,the secondary cavitation shedding is the cause of the formation of the re-entrant jets.After the main shedding,the side-entrant jets still exists,and still have enough energy to cut a small piece of cavity structure from each side end of the sheet cavitation and form secondary cavitation,so the side re-entrant jets are the cause of secondary shedding of cavitation.Finally,the ratio range between dimension-less cavitation shedding frequency based on maximum cavitation length St_Lc=?fL_c?/U and cavitation number expression?1+??^1/2is analyzed.And the linear relationship between the St_Lcand?1+??^1/2proposed by the previous researcher for two-dimensional cavitating flow is extended to the three-dimensional problem by combining the existing experimental results and the numerical results in this paper.In the aspect of noise simulation,a self-developed noise module is added into OpenFOAM,and the flow noise at the designated receiving point is calculates using the Farassat 1A integral formulation of the FW-H_pds.The reliability of the self-developed noise module is demonstrated by comparing with the NASA aerodynamic noise experiment and the China Ship Science Research Center?CSSRC?hydrodynamic noise experiment.The analysis of the characteristics of three-dimensional cavitation noise shows that,the cavitation noise of the unsteady cavitating flow around the twisted hydrofoil is mainly concentrated in the low-frequency region,and the intensity of the cavitation noise is not only related to the length of the cavity,but also the cavitation shedding modes and the existence of secondary cavitation shedding will lead to a significant increase in the cavitation noise.Finally,the unsteady cavitating tip leakage flow around a truncated hydrofoil and its cavitation noise are simulated.The effects of clearance size on tip vortex trajectory,hy-drodynamic performance,unsteady characteristics and noise rediation are systematically analyzed at different angles of attack.The results show that with the decrease of clearance,the flow around the clearance will gradually change into a jet like flow,and the tip vortex trajectory will gradually move away from the wing suction surface under the action of clearance jet.For cavitating-free flow,the lift coefficients of the foil will increases with the decrease of gap size,while the tendency of drag coefficient is opposite,and the lift-drag ratio will reach its maximum when the gap size is 0.The hydrodynamic performance of the foil will decrease after cavitation,and the harm of cavitation will be magnified under the condition of a large angle of attack and small clearance.As for the unsteady charac-teristics of the flow field,the analysis shows that with the decrease of the angle of attack,the source of the flow disturbance will gradually change from the vortex shedding at the suction surface to the wake vortex shedding at the trailing edge.For a better analysis of the influence of clearance size,we propose the concept of the key influence area of tip vortices.It is found that the normalized local lift coefficient C_lL/C_lL?z=1.5c??0.97 is a reasonable definition for the key influence area of tip vortex.It is also considered that the strong suppression of the tip vortex on the unsteady of the flow field for large gap size is due to a large key influence area,while the weak suppression effect of the tip vortex for s-mall gap size is due to a small key influence area.Finally,the analysis of flow noise shows that both the angle of attack and gap size have significant effects on the noise radiation of the three-dimensional tip leakage cavitating flow.The noise sources of the flow field are different at different angles of attack,and the smaller the angle of attack,the weaker the flow noise.The change of gap size will not change the distribution of noise sources significantly,but it will have a significant impact on the intensity of flow noise,and the smaller the clearance,the stronger the noise radiation at the same angle of attack.And we also find that the flow noise is sensitive to the unsteadiness of the flow field:even a weak flow fluctuation will lead to strong noise radiation.To sum up,a complete set of numerical prediction methods for flow field and flow noise based on the non-linear turbulence model and FW-H_pdsare established in this paper.Through detailed simulation,verification and analysis of the unsteady cavitating flow and tip leakage cavitating flow around a large number of three-dimensional hydrofoils,the unsteady flow and noise characteristics of the turbulent cavitating flow dominated by eddy separation are obtained,which can provide a technical support for rapid prediction of the flow field and noise radiation in engineering practice.