Numerical Study On Free End Effect Of Flow Around A Cylinder

In marine engineering and other fluid environment projects,cylindrical structures are widely used,such as support columns for offshore platforms,columns for offshore drilling platforms and the like.These cylinder structures are immersed in the fliud environment,and the force of the fluid will form a periodically falling vortex at the rear of the cylinder,which increases the resistance of the cylinder and induces vortex-induced vibration,which is not conductive to the working state of the construction machinery.And even cause structural fatigue damage and other hazards.At present,research on the flow around a cylinder has formed a systematic problem and many branches,and the flow around a finite-length cylinder is one of them.Due to the free end effects,the stress of finite-length cylinder is greatly different from the state of the general flow around a cylinder,and it is also widely used in engineering practice.In the paper,the flow around a finite-length cylinder and square column is numerically simulated under the condition of Re=2×104,and the change of the free-end effect when the shape of the cylinder changes is explored.Further numerical simulations are carried out under the condition of the free end approaching the wall surface to draw some conclusions to guide the engineering practice.In order to balance efficiency and accuracy,the numerical simulation method used in the paper is Reynolds time-averaged turbulence model.Firstly,the numerical simulation of the two-dimensional and three-dimensional circular cylinders is carried out in order to vertify accuracy.The SST k-? turbulence model can be used to obtain the cylinder force results accurately.The defect correction model for the standard k-? turbulence model by predecessor is used for the verification of the three-dimension aquare cylinder.Numerical calculation of finite-length circular cylinder and square cylinder is based on accurate verification results.Through the numerical simulation method,this paper explores some effects of the free end effect on the flow around the circular cylinder and the aquare cylinder,including the drag coefficient,the lift coefficient,the St,the length of recirculation zone behind cylinder and the vortex shedding mode in the wake field,and compare some of the results with the existing experiments.It is found that the downwash flow near the free end has a greater inhibitory effect on the vortices on both sides of the column,which results in a large reduction in the drag coefficient and lift coefficient,as well as St.The length of the recirculation zone is lengthened by the compression of the downwash flow,and the mode of vortex shedding is also changed.Also explored the influence of the wall existing above the free end on these parameters.By adjusting the gap between the free end and the wall surface,different working conditions are simulated.As a result,under the working condition that the gap between the free-end and the wall is 3D,the drag coefficient of the finite-length cylinder is still obviously different from that of the non-wall surface condition.The working condition that the gap is 1D is completely different from the non-wall working condition,and the impact on two kinds of cylinder is fundamentally different.