Experimental And Numerical Study On Turbulent Structure Characteristics Of Near A Wall Counterflowing Jet Flow Field

The near-wall counterflow jet is a complex flow in which the direction of the jet flow is opposite to that of the main flow near the wall.It is widely used in machinery,chemical and microelectronics and other fields.It has strong theoretical and engineering significance to study the turbulent and structural characteristics in different regions of the flow field.In this paper,an experimental model of the counterflowing wall jet is established,and the particle image velocimetry(PIV)and large eddy simulation(LES)were used to obtain near a wall counterflowing jet flow field data.In Eulerian and Lagrangian systems,the turbulence characteristics and structural characteristics of the flow field are studied.The main results are as follows:1.Experimental study on the statistical characteristics of the flow field structure along the streamwise direction in the near-wall area of the counterflowing wall jet.The flow field is partitioned according to the probability density distribution characteristics of the flow pulsation velocity,and the dominant events in each region are analyzed statistically.The average spatial scale of the turbulent structure in the jet shear layer is analyzed by the two-point correlation function.It is found that the turbulent structure changes along the flow direction in three stages:increase,maintain and decrease.The average scale is further divided into upstream scale and downstream scale.It is found that the upstream scale and downstream scale change along the flow direction in order of scale approximation,downstream scale is larger than the upstream scale,downstream scale is smaller than the upstream scale three stages.2.The main turbulent structures in the flow field are analyzed by dynamic mode decomposition(DMD)and spectral proper orthogonal decomposition(SPOD).The distribution characteristics of the main structure of the flow field were explored.The wall effectively inhibits the normal pulsation so that the free counterflowing jet whose main characteristic is radial pulsation is transformed into the counterflowing wall jet whose main characteristic is streamwise pulsation.The jet deflects when it interacts with the main flow,and a large number of vortexes are generated in the shearing process.The morphology of high-frequency structures is similar.These high-frequency structures are located in the jet shear layer,and the closer they are to the jet exit,the higher the frequency and the smaller the scale.The dynamic process of vortex ring formation and separation from the wall was observed by the virtual dye method,and the feedback mechanism in the counterflowing wall jet caused the vortex ring detachment and affected the material transport mechanism was revealed.3.The influence of different modes of jet flow and wall on flow field structure characteristics was studied by numerical simulation.There are obvious differences in flow field structure between jet deflection to wall and jet attachment to wall.When the jet deflects towards the wall,the inner structure of the jet is helical ring structure.The jet induces boundary layer flow separation.When the jet is attached to the wall,the inner structure of the jet is hairpin like structure,and there is horseshoe vortex near the stagnation point.