Numerical Study On Cavitation Characteristics Of Central Body Nozzle Underwater Jet

The traditional high-pressure water jet cleaning method relies on the supply pressure of the pump,and uses water as a carrier to produce a high impact force to achieve a good cleaning effect.Although it has the advantages of high efficiency and environmental protection,the corresponding cleaning ability relies too much on pressurized equipment.The cavitating water jet is a more efficient cleaning technology that combines high-pressure jets with hydrodynamic cavitation.It has low requirements on supply pressure and a stronger jet impact cleaning effect.The main principle is to use the strong shear effect of high-speed jets in water to produce many bubbles.With the impact effect of the jet collapsing on the clean surface,the surface of the equipment is cleaned.Therefore,it is of great significance to develop green and efficient water jet cleaning technology by mastering the relevant theory of cavitation water jet flow,optimizing the structural parameters of cavitation nozzle and studying the pulsation characteristics of jet cavitation morphology.Firstly,the numerical simulation of cavitation water jet is studied.The turbulent viscosity of Renormalization-Group（RNG）k-εmodel is modified by introducing Filter-Based Density Correction Model（FBDCM）,and the cavitation model is modified by considering the influence of shear stress on cavitation formation.The numerical results are compared with the previous experimental results,and it is found that the dimensionless velocity of cavitation water jet,the shape of cavitation cloud and the fluctuation law of flow field are in good agreement.Secondly,the optimization design of the central cavitation nozzle with different structural parameters is studied.According to the simulation results,a cavitation nozzle structure with a flared outlet shape and a conical end of the central body is selected.The influence factors of various structural parameters of the cavitation nozzle on the cavitation performance were obtained by orthogonal test.When the diameter of the cavitation nozzle was 2 mm,the contraction section L₁=4 mm,the throat section L₂=4 mm,the expansion section L₃=5 mm,the expansion angleβ=60°and the center body indentation distance S=1 mm,the cavitation performance was the best.The factors with great influence were selected for in-depth study.It was found that the change of jet velocity caused by structural parameters led to the change of jet shear layer thickness,which affected the distribution of low pressure area in the flow field.The cavitation clouds of the jet flow field are different in size.Then,the pulsation characteristics of the central cavitation jet flow field are studied.The results show that during the initial stage of cavitation,with the development of the jet inside the nozzle,the low pressure area gradually increases,and the cavitation bubble grows larger.Subsequently,when the jet enters the unstable development stage of the cavitation cloud,the high-speed jet enters the static flow field to cause a strong pulsating shear effect,which causes a large number of vortex structures to merge and break.As the jet continues to develop in the flow field,the cavitation cloud morphology gradually appears stable pulsation periodic behavior of growth,shedding and collapse.With the increase of inlet pressure,the jet velocity increases,resulting in the increase of the structure and number of vortices in the shear layer,which further reduces the local pressure of the fluid and enhances the cavitation performance.Finally,the dynamic mode decomposition（DMD）is used to study the periodic flow behavior of cavitation clouds.It is found that the first-order mode is the average flow field,which is the main structure of the flow field.The second-order mode is related to the periodic shedding and collapse dynamic behavior of cavitation clouds.With the increase of inlet pressure,the evolution of cavitation shedding and collapse is more and more severe,and the cavity size and cavity length are gradually increased.With the increase of nozzle diameter,the evolution of cavitation shedding and collapse is first increased and then decreased.