Response And Influencing Factor Of Flow-induced Motion Of A Circular Cylinder With Roughness Strips

When a bluff body is immersed in moving fluid such as ocean or river currents,vortex shedding behind the body and the resulting Flow-induced Motion(FIM)are commonly observed.With the rapid development of wind and ocean engineering in the last several decades,extensive research efforts have been conducted.Security problems in many engineering fields are related to FIM,because FIM can make structural fatigue damage to chimneys,cables,bridges,and offshore structure,etc.On the other hand,the application of flow-induced motion of elastically mounted rigid cylinder to extract hydrokinetic energy from water currents has been developed recently.In order to improve the output power density,it's crucial to find out the driving mechanism of FIM and enhance FIM of the cylinder intentionally.Passive turbulence control(PTC)in the form of two selectively distributed roughness strips alters the flow properties around the body and makes the transition from VIV to galloping thus enhance FIM of the cylinder.There is not a unified conclusion on the response of the FIM of a PTC cylinder and the driving mechanisms of a PTC cylinder in VIV-to-Galloping Transition still remains unknown.What is more,in engineering applications,the direction of incident flow is changeable.As the angle of attack changes,the FIM response and energy harvesting of the PTC-cylinder could present a different characteristic consequently.However,the effect of angle of attack still remains unclear.In this thesis,FIM of an elastically mounted PTC cylinder in the transition between VIV and galloping have been investigated by 2.D URANS simulations based on OpenFOAM.Amplitude response,frequency response,lift coefficient,near-wake structures and phase difference between lift and displacement of the cylinder are discussed.Furthermore,FIM of an elastically mounted PTC cylinder at different angles of attack has been investigated.The main conclusions are summarized as follows.FIM response of an elastically mounted PTC cylinder in the transition between VIV and galloping(5.50?U*water?9.63)have been investigated numerically and the driving mechanisms of a PTC cylinder in VIV-to-Galloping transition is revealed.The excitation of the cylinder galloping is captured successfully in simulation.The amplitude of the cylinder increases continuously after the VIV upper branch and the VIV lower branch is replaced by galloping with the maximum amplitude of 3.88 D.In VIV-to-Galloping transition,the frequency of the cylinder decreases continuously to the low-frequency and high-amplitude galloping oscillation.The amplitude and frequency in numerical results are very close to the experimental data.Numerical results are in good agreement with the experimental results.The driving mechanisms of a PTC cylinder in VIV-to-Galloping transition: The phase angle between lift and oscillation displacement is nearly 0° throughout the whole range and the lift is always in phase with the displacement.This explains the continuously increasing of the amplitude after VIV upper branch.The magnitude and direction of the lift determine the change of the amplitude response.Due to the surface roughness strips,the number of vortices and vortex intensity difference in one oscillation cycle increases with the inflow velocity and the vortex pattern becoming more and more complex.As a result,the vortex shedding and the lift characteristics of the cylinder are enhanced and leads to galloping.Finally,the effect of angle of attack on FIM of a PTC cylinder has been simulated: Within the test ?attack range,both suppression and enhancement of FIM are observed for the PTC-cylinder when compared with the case at 0° of angle of attack.The maximum amplitude in VIV upper branch observed and galloping is 1.30 D and 2.80 D with ?attack at 20° and 10°,respectively.When ?attack ? 65°,no obvious FIM take place in this range.The angle of attack has a significant influence on vortex mode and near-wake structures at the same Reynolds number.For ?attack? 55°,vortex shedding is suppressed and the near-wake region becomes quite slender,so that the FIM of the cylinder is greatly inhibited.A complex vortex reattachment in the near-wake intensifies the fluid-structure interactions and causes obviously higher amplitude.Power can be harnessed effectively for the VIVACE converter in the VIV upper branch and Galloping and the power output efficiency is achivevd when ?attack =10°or 20°.