Experimental Research On The Flow Field Characteristics And The Vortex Evolution Behind The Circular Cylinder Placed Near A Wall

The flow around the circular cylinder placed near a wall is often encountered in natural world and engineering applications,and the researches of the flow characteristics and the vortex evolution have extensive engineering application background and strong theoretical significance.In this paper,the particle image velocity technology（PIV）is used to measure the flow flied around the circular cylinder placed near a wall,and the proper orthogonal decomposition（POD）and the vortex identification method λ_ci are applied to study flow field characteristics and the vortex evolution behind the circular cylinder placed near a wall.The following conclusions have been drawn.（1）The effects of the gap ratios on the cylinder wake vortex of flow around the circular cylinder near a wall on a laminar boundary layer are conducted by experimental method.In PIV measurement of flow around the cylinder near a wall,the gap ratioG?Dvaries from 0 to 2.5and the Reynolds number is Re=1500.As the gap ratios increases,the upper wake vortex and lower wake vortex are shed from stretching and curling of the upper shear layer and the lower shear layer of the cylinder,respectively.At the same time,the cylinder wake vortex shedding become more and more symmetric about the centerline of the cylinder.The gap ratios can affect the cylinder wake vortex shedding frequency.As the gap ratio decreases fromGG?DD=2.5 toG?D=0.25,the corresponding Strouhal number（St）increases form St=0.192to St=0.369,and there is nearly a 92%increase of the wake vortex shedding frequency,which is caused by the deflection of the gap flow.（2）The effects of the gap ratios on the vortex structures evolution of flow around the circular cylinder near a wall on a laminar boundary layer are conducted by experimental method.ForG?D≤0.25,the formation of the lower wake vortex is suppressed,and the wake vortex is shed from stretching and curling of the upper shear layer of the cylinder.For 0.25<G?D≤0.5,there is coupling process between the lower shear layer and the wall boundary layer.The wall boundary layer separation is induced by the lower wake vortex,and the unstability and curling of the separated wall boundary layer promote the formation of the secondary vortex structures with clockwise rotation.At G/D=0.5,there is a vortex merging process among the upper wake vortex and the secondary vortex during the convection of latter.For 0.5<G?D≤2,the lower wake vortex can make interaction with the secondary vortex.The secondary vortex can be affected due to the upward induction exerted by the lower wake vortex,and the secondary vortex moves downstream together with the lower wake vortex.ForG?D>2,the effect of the cylinder wake on the wall boundary layer is reduced,and the flow is similar to that of an isolated cylinder.（3）The interaction between the flow around the circular cylinder near a wall and the wall turbulent boundary layer are conducted by experimental method.Firstly,the flow field characteristics of the plate turbulent boundary layer at Re_τ=327 without the effect of the circular cylinder are revealed.The POD reveals that there are sweep events Q4（high-velocity downward motion of flow）and ejection events Q2（low-velocity downward motion of flow）in the wall turbulent boundary layer.Finally,the PIV measurements of flow around the circular cylinder placed inside the turbulent boundary layer are conducted,and the flow field characteristics and the vortex evolution are considered at G/D=0.5,G/D=1 and G/D=2.As the gap ratio increases,the Reynolds stress and the turbulent kinetic energy behind the cylinder are increased.At the large gap ratio,the effect of the wall on the cylinder wake vortex is reduced,and the cylinder wake vortex shedding gradually become symmetric about the centerline of the cylinder.The lower wake vortex can induce the wall shear layer,thus promoting the formation of the secondary vortex,and the secondary vortex moves downstream together with the lower wake vortex.