Analysis Of Vortex Characteristics Of Mixing Layer In Two-side Sudden Expansion Channel Flow

Separating flow is a common phenomenon occurred in natural and industrial production.The mixing layer formed between the main flow zone and the recirculation zone contains a large number of vortices,which dominates the mass and momentum exchange process between the main flow zone and the recirculation zone,which has an important impact on the environment of the recirculation zone and its boundary.In this thesis,the typical model of separating flow"two-side sudden expansion channel flow"is selected as the research object.Starting from the length of recirculation zone,the law of recirculation zone length is obtained through particle image velocimetry(PIV)experiment and numerical simulation,and the characteristics of macroscopic parameters of recirculation zone are explored.At the same time,this thesis analyzes the turbulence and vortex structure properties of the mixing layer based on Proper Orthogonal Decomposition(POD),then further explores the evolution law of the vortex structure in the mixing layer.The results show that the flow region can be divided into non-friction region,transition region and friction region according to the influence of bed friction.In non-friction region,the influence of turbulence is dominant,the flow presents turbulent state and both sides are not equal in the length of recirculation zone on.In the transition region,the turbulence and the viscosity offer comparable effect,leading to the rapid change and the similar size of the length of the recirculation zone on both sides.In the friction region,the viscous effect is dominant,and the flow presents laminar state.Both sides is equal in the length of the recirculation zone,and decreases with the increase of the bed friction effect.Three factors influencing the length of the dimensionless recirculation zone L/d are obtained by dimensional analysis:bed friction number S,dimensionless depth h/d and ratio R_b.These three parameters represent the effects of the bed friction,water depth and channel structure on the length of the recirculation zone respectively.The numerical simulation results show that L/d decreases with the increase of S,h/d and R_b in the friction zone respectively,which reflects the increase of the bed friction,water depth and channel structure accelerates the dissipation and collapse process of vortex structure,macroscopically.The development of the mixing layer is similar among different S.The thickness of the mixing layer in the short recirculation zone firstly increases,and then decreases.Finally,a wake is formed at the end of the recirculation zone.Correspondingly,the vortex in the mixing layer undergoes the processes of initiation,development,dissipation,and finally collapses in the wake.The mixing layer in the long recirculation zone is closer to the sudden expansion and develops toward the main flow zone and the recirculation zone simultaneously,resulting in the rapid increase of its thickness.With the increase of the bed friction number S,the flow is more restricted by the free surface,which cause the vortex motion frequency decay more slowly.Meanwhile,the vortex energy dissipation is slower,which reflects that the length of the short recirculation zone in the non-friction region increases with the increase of S.The low-order mode shows the evolution process of the increasing size of the vortex in the upstream of the recirculation zone,which shows the development stage of the vortex.The vortex evolution law of mixing layer in two-side sudden expansion channel flow further reveals the exchange mechanism of mass and momentum in the separating flow.It provides a theoretical basis for the construction of waterway facilities and the improvement of the efficiency of industrial heat and mass transfer equipment.