Study On Turbulence Coherent Structure Of Non-smooth Plate Based On Large Eddy Simulation

The high friction resistance caused by turbulence greatly reduces the efficiency of energy utilization,and the coherent structure is closely related to the frictional resistance of turbulence.In this paper,the flow field information on the surface and groove surface is obtained by large eddy simulation.The flow characteristics and the evolution process of the coherent structure on the groove surface are studied,and the influence of the groove structure on the turbulent energy dissipation is analyzed.The numerical simulation results of turbulent boundary layer on plate surface and groove surface show that there is a swirling vortex inside the transverse groove structure,which changes the sliding friction of the wall into rolling friction.The thickness of the boundary layer of the groove surface significantly thicker,the velocity gradient in the boundary layer becomes smaller;the average speed of logarithm distribution comparison found that the groove surface velocity partition overall shift buffer layer thinning,logarithmic layer becomes thicker,shows that the drag reduction groove structure mainly affects the buffer layer and logarithm law zone(quasi the flow of vortex and Hairpin Vortex);the bottom of the groove surface viscous flow turbulence and turbulence component comprehensive are greater than the flat surface,and in the buffer layer,and outer layer of logarithm law were less than the flat surface,but normal turbulence components throughout the boundary layer are less than the flat surface.The results show that the groove structure significantly reduces the turbulent velocity in turbulent boundary layer,especially the normal pulsating speed,and reduces the turbulent characteristics of turbulent boundary layer.The Reynolds stress distribution of the two surfaces is compared,and the total Reynolds stress of the groove surface is less than that of the flat surface,which indicates that the groove structure reduces the momentum exchange and reduces the intensity of the burst event.The Q4 event detection center,were detected in the streamwise fluctuating velocity,fluctuating velocity and vorticity distribution to the surface,groove structure mainly inhibits the normal fluid velocity fluctuation,and reduce the vorticity makes the fluid flow induced effects on the bottom of the weakened viscous fluid with low velocity,reduce the probability of burst events.The Q criterion shows that the evolution of coherent structures in the flow field is found:The structure of the strip and the structure of the flow vortex are the important part of the self maintenance of the near wall turbulence.In the viscous sublayer low velocity zones under unstable disturbances and spanwise induction will develop streamwise vortex formation or hairpin vortex structure,the vortex structure to continue to develop to a higher level of flow velocity,due to vortex and vortex head legs are not the same,the streamwise vortex or Hairpin Vortex stretch,uplift.The vortex head of the high speed flow layer is shattered by a strong shear action,which forms a sudden occurrence.In the course of the flow vortex and the development of the Hairpin Vortex,the self maintaining process of the coherent structure in the turbulent boundary layer is formed by the instability,uplift and breakage of the low-speed strip.On the surface of the groove,the velocity of the fluid has a certain speed,which makes the velocity difference between the bottom fluid and the surrounding fluid less,and increases the stability of the ground floor.The smaller normal pulsation speed makes the fluid parallel to the wall flow and inhibits the further evolution of the vortex structure,thus forming a coherent structure dominated by stable streamlines and streamwise vortices.