Flow-induced Vibration Characteristics Of A Cylinder Neighbored By Another Of A Smaller Diameter

Most engineering structures have some connections with flow-induced vibration(FIV).For examples,the graphite bar in nuclear reactor,the cooling tower,the tube in heat exchanger,etc.The forces that enhance the vibration may cause failure of the structures and bring enormous losses.This dissertation investigated FIV of a smaller cylinder with diameter d in the neighborhood of another cylinder(diameter D),with d/D = 0.4.The smaller cylinder is fixed,and the larger cylinder is allowed to vibrate in the transverse direction only.The FIV characteristics of different configurations can be obtained by changing ?(which is the angle between free-stream velocity and the line connecting the centers of two cylinders)and spacing ratio L/d(where L is the spacing between the centers of two cylinders minus the radius of the large cylinder).In this dissertation,the vibration of the larger cylinder was measured with the freestream velocity varied from 1.8 m/s to 20 m/s.And four vibration patterns are found: no vibration(0),vortex-excitation(VE),galloping(G),small vibration(SV).Based on the vibration response,the test area can be divided into different regimes.Then,some cases which represent the flow characteristics of each regime are selected to measure the pressure distribution and the flow structures.Thus,the mechanism can be elucidated clearly by connecting the vibration response,the pressure distribution around the cylinder and the flow structures.By changing ? and L/d,different vibration responses of the large cylinder can be observed.Based on the result,the test area can be divided into five regimes,i.e.VE+G(Regime I),0(Regime II),0(Regime III),VE+G(Regime IV),VE+SV(Regime V).The large cylinder vibrates at its natural frequency in every regime,and the lift force frequency is almost the same as the natural frequency in every regime except when the vibration response is SV in Regime V.The mechanisms of Regime II and Regime III are different.Because the position where the smaller cylinder is located has an impact on the gap and wake flow structures.So do Regime I and Regime IV.From above all,changing ? and L/d significantly affects the flow structures and the flow mechanism of each regime.