Study On The Flow Characteristics Of Water Jet In Confined Space

Water jet is a new technology developed rapidly in recent decades,which is widely used in many fields such as mechanical processing,coal mining,power engineering and medicine.In the field of mechanical processing,water jet,called cold working technology,can be used to cut and break any metal and non-metal materials efficiently,Therefore,water jet is considered as the one of the promising cutting process.However,some defects,such as the slit width and slit roughness,have seriously affected the popularization and application in the fine processing field.At present,the previous researches about those problems mostly focus on material crushing mechanism.Whereas,there are few studies on the relative interaction between water jet and cutting section in the cutting slit.Therefore,this paper takes the interaction between the jet fluid and the cutting material in the process of hydraulic cutting as the breakthrough point,and studies the flow characteristics of the jet fluid in the cutting slit from theoretical perspective,and then analyzes the forming mechanism of the cutting slit.In this paper,large eddy simulation(LES)is used to simulate the flow law of water jet with the different widths and Reynolds number in the confined space.In terms of data analysis,the time-averaged characteristics,fluctuating characteristics and Reynolds stress distribution characteristics are analyzed in the turbulence statistical theory.For the purpose of studying the influence of confined width on the large-scale coherent structure in the jet flow field,proper orthogonal decomposition(POD)algorithm was used to reduce the dimensionality of the flow field,and then the distribution characteristics of large scale coherent structures in low order modes are analyzed,and the transient fluctuating field is reconstructed by using the first modes with 40% turbulent kinetic energy to analyze the distribution characteristics of the vortices.The results are as follows:(1)The influence of the confined width on fluctuating velocity and Reynolds stress distributions is mainly reflected in the core section and the transition section,while,which on the axial time-averaged velocity occurs in the transition section and the fully developed section.The influence of Reynolds number on the axial mean velocity is mainly reflected in the transition,which on the Reynolds stress occurs in the core section and the transition section,however,the influence on the fluctuating velocity occurs in the whole flow region.(2)The symmetric structure of double-wall has an inhibitory effect on the development of large-scale vortex structure in the jet flow field,and that inhibitory effect gradually decreases with the increase of the confined width.The phenomenon explains the law that the time-averaged velocity,fluctuating velocity and Reynolds stress cannot fully develop in longitudinal direction on the longitudinal section.(3)The low-order modes in the water jet field have large turbulent kinetic energy,and can accurately reflect the characteristics of large-scale coherent structure.The coherent structure with positive and negative rotating is generated in turn,and with the development of water jet,the size of coherent structure gradually increases,but the number decreases.(4)The geometrical morphology of cutting slit is mainly due to the uneven distribution of axial time averaged velocity,axial fluctuating velocity and Reynolds stress in the profile section,and the spatial delay phenomenon.