Experimental Study On The Structural Characteristics Of Incompressible Pulsed Transverse Jet With Medium Velocity Ratio

Pulsed transverse jet is a complex multi-scale flow form,which is widely used in the fields of energy,chemical industry and aerospace.At present,the research on transverse jet mainly focuses on the characteristics of penetration,mixing and diffusion,but there is still a lack of adequate understanding of the macroscopic and microscopic multi-scale structure of the pulsed transverse jet and the controlling factors.In view of this,this thesis comprehensive use of Laser Induced Fluorescence(LIF)and Particle Image Velocimetry(PIV),in the selfdesign small tunnel,the experimental measurements on incompressible steady and pulsed transverse jet are carried out in detail than R = 3,4,5,and pulse frequency f = 0.5,0.75,1,duty ratio of ? = 35%,50%,65%,100% of the macro structure and micro turbulent coherent structure characteristics.Firstly,LIF method was used to continuously image the different sections and the transient structure of the steady and pulsed transverse jet,and the evolution and development characteristics of the multi-scale structure of the pulsed transverse jet were obtained.On this basis,the average turbulence statistics of steady and pulsed transverse jets,including average velocity and turbulence dissipation rate,were obtained through PIV measurement to characterize the penetration and mixing characteristics of transverse jets.Then,the instantaneous velocity field obtained by PIV is processed by the Proper Orthogonal Decomposition(POD),and the statistical distribution characteristics of the size and swirling strength of the vortex core in the transverse jet are obtained by combining the coherent vortex identification and extraction algorithm.Finally,Finite-Time Lyapunov Exponent(FTLE)analysis method was applied to extract the Lagrange Coherent Structure(LCS)in the transverse jet,and the potential fluid transport and mixing path in the transverse jet were analyzed.The results show that in the steady transverse jet,the macro mixing region is mainly distributed in the fully developed section of the jet and its backflow region,while the micro turbulent coherent structure is mainly distributed in the near-wall boundary layer and the shear layer of the jet downstream,and its potential transport path includes the backflow region of the transverse flow and the boundary of the leading edge of the jet.After joining pulse modulation,transverse jet penetrating and mixing are significantly enhanced,main mixing area transferred from the jet downstream region to the fully developed section of the jet shear layer near the nozzle exit,and the flow field of the turbulent coherent structure scale increases,the intensity is enhanced,meanwhile at the jet downstream near the wall,the small scale vortex structure is gradually disappear,fluid micro unit transport before and after mixing path also gradually focus on jet shear layer.Finally,the comparison results show that the duty cycle is the main control parameter of the pulsed transverse jet.