Experimental Study Of Gap Ratio And Reynolds Number Effects On Turbulent Structures In Flow Past The Circular Cylinder Placed Near The Wall

The flow around the circular cylinder placed near a wall is often encountered in natural world and engineering applications.The study of the turbulent structures in flow past the circular cylinder placed near the wall is of reference value for the engineering applications.In this paper,the research subject is the flow around a near-wall circular cylinder placed on a laminar boundary layer,and the 2D-2C PIV is used to measure the flow flied.The flow field is reconstructed by the proper orthogonal decomposition（POD）,and the vortex identification method_（84）are applied.The main conclusions are as follows:（1）The effects of the gap ratio on turbulence characteristics and vortex evolution in flow past the circular cylinder placed near a wall are investigated.The PIV experiments are conducted at four gap ratios（?=0.5～1.5）,and the Reynolds number of ee11500 is considered.At?=0.5,the gap flow deviates from the wall,and two separate bubbles are formed near the wall.The wall boundary layer is gradually separated and rolling up due to induction of the lower shear layer,which promotes the formation of the secondary vortex.The secondary vortex moves upward at an angle.Finally,there is a vortex merging process occurred between the upper wake vortex and the secondary vortex.At?=1,the secondary vortex gradually lifts up due to the upward induction exerted by the lower wake vortex and interacts with the lower wake vortex.At?=1.5,the induction effect of the lower wake vortex is weakened,the upper wake vortex and lower wake vortex move nearly parallel with the wall with the secondary vortex.At?=1.5,the phenomenon of"shear sheltering"is found,which weakens the influence of the wake vortex on the secondary vortex.According to the distribution of probability density function,it is found that the probability of large scale vortex structure increases with the increase of gap ratio.（2）The effects of the Reynolds number on turbulence characteristics and vortex evolution in flow past the circular cylinder placed near a wall are investigated.The PIV experiments are conducted at three gap ratios（?=0.5,1 and 1.5）,and the Reynolds number of ee11500～5540 are considered.At?=0.5,two separation bubble are formed on the near wall of ee11500.With the increase of Reynolds number,the small separation bubble disappears firstly,and the size of the large separation bubble gradually decreases.And there is a critical Reynolds number region between 3000 and 3200.When the Reynolds number is smaller than the critical Reynolds number region,a small separation bubble will form on the front wall of the circular cylinder,which hinders the flow of upstream fluid through the gap and reduces the intensity of the gap flow,and then deviates from the wall.With the increase of Reynolds number,the probability of large scale structure becomes higher,and Reynolds stress and turbulent kinetic energy have a wider range of fluctuations.The effects of the Reynolds number on vortex evolution in flow past the circular cylinder placed near a wall are obvious.At?=0.5,as the Reynolds number is greater than the critical Reynolds number,the scale of the upper wake vortex and the lower wake vortex increases,deflection of the gap flow decreases,and the secondary vortex cannot merge with the upper wake vortex,the secondary vortex directly interact with the lower wake vortex.At?=1 and 1.5,the energy content of the secondary vortex decreases gradually with the increase of Reynolds number.