Study On The Mechansim And Force Reduction Method For 3-d Flow Around A Circular Cylinder Of Finite Length

Fluid flow around a circular cylinder is classical problem in Hydromechanics field.There are alternating vortices shedding in the rear of the cylinder when fluid flows past a cylindrical structure.As a result,the structure will oscillate all the time.The cylindrical structure is widely used in many offshore structure such as the spud leg for ocean platform,the suspension wire for Skye bridge,the marine riser for offore oil drilling,the mooring rope for submerged buoy and so on.The structure would get fatigue when keep suffering periodic force by alternating shedding vortices,which finally can decrease its service life.The cylindrical structure is not usually infinite in length,namely,research for finite cylinder is more practical.The turbulence structure is such sophisticated that it can barely be predicted completely.Therefore,based on previous research,further study on the mechanism and measurement to reduct resistance and vibration for the circular cylinder of finite length would be significant.In this paper,analysis on force and flow field for infinite cylinder in subcritical regime is conducted to understand its flow mechanism.Under the same condition,similar study is done.Finally,on the basis of previous research,circular cylinder with sinusoidal variation of cross-section along the spanwise direction,named wavy cylinder is involved,and then,flow patterns between infinite cylinders,finite cylinders and the wavy cylinders is compared to seek some measure to decrease vibration or resistance,and comprehend its theory.Based on Large-Eddy-Simulation method,the mesh refinement and sub-grid scale stress model is critically considered under subcritical regime for infinite cylinder computation.With the aid of the Courant equation,the suitable time-step is provided.All these work contribute to the accuracy of the simulation.Significant parameters in the process like vortex formation length,flow separation angles and lift or resistance curve is analysed.Similar analysis is conducted for finite cylinder,and it is found that there are Tip vortex on the free end of the cylinder,besides,in the rear of the free end,there is arch vortex,finally,horseshoe vortex is discovered above the bottom.The downwash phenomenon exist on the free end surface of the cylinder,which accounts for the reduction of the resistance force.Differences along the spanwise of the finite cylinder is enormous,that is to say that flow field for circular cylinder of finite length has a stronger 3-D effects.With former research,study for wavy cylinder is done.The wavy cylinder with different combinations of wavelength and amplitudes,is found to be able to decrease the resistance and the vibration under certain circumstances.Compared with circular cylinder,wavy cylinders various in separate angles,vortex formation length,flow patterns as well as the base suction coefficient,which means a more complex 3-D flow field structure.This paper is meaningful for the comprehensive of the flow characteristic for finite length cylinder,wavy cylinder,the measurement for the reduction of resistance or vibration as well as its mechanism.