Experimental Study On Wake Characteristics And Vortex Evolution Of Side-by-Side Circular Cylinders Placed Near Wall

In this paper,the multi-body flow around the wall is simplified into a near-wall double-sided cylindrical flow model,and the flow characteristics of wakes and the evolution process of vortex shedding were studied,so as to clarify the interaction between boundary layer flows near the wall and side-by-side circular-cylinder wakes,which has important theoretical and engineering significance.Firstly,the experiment of near-wall side-by-side circular cylinder flow around a water channel is carried out,and the flow field data are obtained by particle image velocimetry（PIV）.Then,the POD method was used to reconstruct the flow field,the vortex identification method was used to capture the vortex structure in the flow field,and the virtual dye method was used to trace the transient vortex structure.The effects of different spacing ratios and different Reynolds numbers on the flow around the near-wall cylinder are studied.The main results are as follows:The turbulence characteristics and vortex structure evolution of the parallel double cylinder flow around the wall with spacing ratio were studied.Based on the cylinder diameter D,the Reynolds number is 1.5×10³ and the clearance ratio is 1（/=1）.The spacing ratios T/D=3,1.5 and 1.1 were selected for comparison.On the near wall,the vortex shedding frequency of the lower cylinder increases due to the deflection of the gap flow between the lower cylinder and the wall.The power spectral density shows that the lower cylinder induces a secondary vortex structure on the wall periodically,and the vortex shedding frequency of the lower cylinder is equal to that of the secondary vortex,and the boundary layer flows near the wall can affect the Reynolds stress distribution at the wake of the double cylinder.The deflection of wall gap flow leads to the increase of vortex shedding frequency in the wake of the lower cylinder.The effect of the spacing ratio on the flow characteristics of the gap flow around the double cylinder near the wall and on the structure of wake turbulence is studied.The instantaneous flow field was reconstructed by POD analysis method and₍(8(8(8（8） vortex identification method,and the merging and evolution of vortex structures were captured by virtual dye visualization method.When the spacing ratio is medium（/=1.5）,the deflection direction of the clearance flow between the cylinders changes,and the deflection of the wall gap flow leads to the increase of the vortex shedding frequency at the wake of the lower cylinder,and the peak value of the vortex shedding frequency appears.As the spacing ratio decreases,the vortex interaction between the two cylinders increases.The vortex merging occurs in the lower wake vortex of two cylinders with the same rotation direction,while the upper cylinder wake vortex near the wall dissipates faster.In addition,the downward deflection of the clearance flow will reduce the induced effect of the double wakes on the wall surface so as to restrain the uplift of the secondary vortices.The effects of Reynolds number on turbulence characteristics and vortical structure evolution characteristics of near-wall cylindrical flow were studied.PIV experiments with Reynolds number Re=1500～4500 at?=3,1.5,1.1 were carried out.When/=1,?=3,a separation bubble is generated near the wall of Re=1500.With the increase of Reynolds number,the induced effect of the cylindrical wake vortex on the wall is weakened,and the separation bubble gradually disappears.With the increase of Reynolds number,the probability of large-scale structure becomes higher.The Reynolds number has an obvious effect on the evolution of vortex structure in side-by-side circular cylinder flow near the wall.At the same time,the size of the side-by-side circular cylinder vortex increases and the gap flow increases,which leads to the decrease of deflection and the suppression of the uplift effect of the secondary vortex structure formed by the wall boundary layer curling.