Computational Analysis Of Flow-Induced Vibration And Energy Harvesting Analysis Of A Cylinder With Different Cross-sections

Tidal current energy as a green clean energy,with the characteristics of renewable,predictability and substantial reserves,has gradually become the focus of world research and development because of the fossil resources are not renewable and gradually depleted.The flow kinetic energy conversion technique based on FIV is a relatively new concept,by using this technique,the kinetic energy of water flow can be extracted from the surrounding flow field by using flow induced vibration in low velocity environment,and converted to other forms of available energy,such as electric energy.At present,the study on FIV generating device mostly around the cylindrical VIV expansion,and mainly focuses on the influence of the parameters such as the mass ratio of the cylinder,damping ratio and spring stiffness.The geometric characteristics of the cylinder is an important factor in the study of the FIV,and the flow induced vibration characteristics of the cylinder vary greatly due to the change of the cross-section shape.In this paper,the numerical simulation of five kinds of cylinders with different cross sections is carried out.The vibration response,force,vortex patterns,energy harvesting characteristics of the cylinders with different sections under water flow are calculated,it provides reference for the design of FIV water generating device and the in-depth analysis of the interaction mechanism of "Column-Flow" under high damping and high Re.According to the change of the boundary layer separation point,this paper selects five cross section forms,such as circular circle,square,quasi-trapezoid cylinder,trapezoid cylinder and T shaped cylinder.In order to better compare the FIV's characteristics and energy transfer characteristics of the cylinders with different cross-section shapes,the parameters such as mass ratio,damping ratio,Re and Ur of the cylinder with different section shapes are carefully designed,so that the important dimensionless parameters of the cylinder with different shapes are consistent.The FIV of each column was numerically simulated by the finite element software ADINA,and the flow velocity range was 0.2-1.1m/s.Calculation results show that,frequency locking occurs in all cylinders.There is a "self-limiting"vibration phenomenon of circular cylinder,and the amplitude curve shows an obvious branch,VIV of circular cylinder completely,there exists a phenomenon of VIV and galloping switching of other cylinders,a sudden change in the amplitude ratio is accompanied by the mode switching,the maximum amplitude ratio of quasi-trapezoid cylinder and T shaped cylinder is 2.26 and 2 respectively.Vibration of circular cylinder is stable and the square cylinder is unstable.When frequency locking occurs,the frequency ratio is not strictly 1.The vibration branches can be observed obviously in frequency curve.Influence of cross-section geometry on the cylinder vibration frequency,it depends mainly on the cycle length of the vortex shedding cylinder.Due to the different geometric characteristics of each cylinder,the separation and evolution process of the boundary layer are different,so the vortex shedding period is also different.The vibration frequency of a smooth cylinder is much larger than that of a cylinder with an angular angle,and the ratio of the maximum frequency is 2.44.The vortex formation and shedding patterns of each cylinder are analyzed in detail,with the increase of velocity,the new attachment point of the boundary layer moved to the rear of the cylinder;when the cylinders are in different vibrational branches,the vortex patterns are different,too.The patterns of "2S","S+P","2P","2S+2P" and "2S+4P" are switched,the trailing vortex width changes as well,that is,the larger the vorticity,the wider the vortex channel,and the formation of the vortex will be accompanied by the emergence of the pressure cavity;the results show that the pressure cavities are easier to be produced by the cross-section cylinder with corners and section step,and the T-shaped cylinder has the best effect.The difference of vortex patterns caused by the geometrical characteristics of the cylinder has a great influence on the lift resistance of the cylinder,for example,the vortex falls off at the corners,and the formation of pressure cavities,all of these make great changes in the forces acting on the cylinders.The length of the trailing edge of the cylinder has a greater influence on the resistance,the longer the trailing edge(the rear corners is more prominent),the greater the resistance.The lift coefficient and lift drag ratio of T-shaped cylinder are generally greater than that of other cylinders,which shows that the hydrodynamic performance of T-shaped cylinder is better than others.Finally,the power and efficiency of each cylinder are calculated,in which the power and efficiency of the T-shaped cylinder is better than that of others,the maximum values were 10.57w and 42.5%respectively.The comprehensive energy conversion efficiency is T shaped cylinder>circular cylinder>quasi-trapezoid cylinder>trapezoidal cylinder>square cylinder.For each cylinder,the conversion efficiency varies with the velocity of flow,conversion device with circular cylinder is suitable for the low flow rates,while the quasi-trapezoid cylinder is suitable for the high flow rates.