Characteristics And Control Of Flow-Induced Vibrations Of Two Tandem Bluff Bodies

Flow-induced vibration(FIV)is an inevitable phenomenon for bluff bodies in the current.Vortex-induced vibration,galloping,flutter,buffeting are four types of FIV.Conventionally,FIV is harmful.It fatigues structure,affect the normal servies,and even directly lead to the collapse of the structure.Due to the complexity and particularity of fluid-structure interaction,people do not have a comprehensive understanding of FIV,especially some special structures and multi-body structures.The FIV characteristics need to be further explored.The development of effective FIV control measures thus has engineering significance.At the same time,everything has two sides.As a controllable and sustainable power source,flow-induced vibration can also be used to collect kinetic energy for power generation.Compared with the traditional hydropower facilities,the main advantage of the power harnessing technology based on flow-induced vibration is that it can maintain higher energy conversion efficiency at low flow speed,which makes it has a wider range of application.At present,the energy collecting device based on flow-induced vibration is in the early stage of research and development,and has a good research prospect.In this paper,taking the flow-induced vibration characteristics of bluff body as the main line,the vortex-induced vibration,galloping mode and energy harnessing performance of tow tandem-placed cylinders are analyzed,and an optimization strategy based on stiffness configuration is proposed.A control measure for the vortex-induced vibration of cylinder is proposed,by using porous material coatings.The flutter response of bluff body rigid plate is studied,which lays a foundation for the development of flutterbased energy harnessing devices in the future.The main research contents are as follows:The vibration modes of two tandem cylinders with equal stiffness are explored and classified.According to the vibration characteristics such as amplitude,frequency and phase difference.From more than 2000 cases of water channel tests,the vibration modes of double cylinders are classified into three major patterns: synchronization,partial coupled,decoupled,and nine sub-patterns.The characteristics and inducing mechanism of each model are explained in detail.The effects of the spacing,stiffness and damping on the vibration mode are studied.The energy harnessing optimization strategy of two tandem cylinders is explored,based on stiffness configuration.The vibration characteristics of the cylinder can be changed by changing the stiffness configuration of the upstream and downstream cylinders,so as to increase the vibrational amplitude,change the vortex vibration branches,and induce galloping at lower speeds.Such measures can improve the energy harnessing performance of the energy converter.The envelope of power and efficiency in the testing range and the optimal configuration at each flow speed are given.A control method of the vortex-induced vibration of circular cylinders,using porous material coatings,is proposed and verified by wind tunnel tests.Firstly,measurements are carried out on a static cylinder with a series of thickness and pores-per-inch.It is found that the coating can greatly reduce lift fluctuation.The influence of the coating on the wake vortex shedding is found by the flow field visualization technique.The reason for the decrease in lift fluctuation is due to further vortex formation.Then,through the dynamic test,it is found that the coating with different parameters can suppress or enhance the vortex-induced vibration,and the amplitude of vortex vibration can be reduced to 4%or increased to 220%.This measure is suitable for general engineering applications to suppress the vibration,or,the energy converters to enhance the vibration.The vortex-and-wake-induced vibration of a separated-box bridge girder is investigated.The mechanism of the wind-induced vibration of a separated-box girder at low speeds is revealed by dynamical tests and flow-field analysis.The effects of the vortex shedding in the central gap and the wake region are demonstrated.Fifth,the flutter characteristics of a rigid bluff plate are studied.Four stages in the flutter process of a two-degree-of-freedom plate are found.The amplitude of the unstable boundary in the subcritical Hopf bifurcation is obtained by external excitation load.The complete bifurcation diagram and the corresponding bifurcation equation are given.The flow field evolution and modal changes in the process of the flutter are shown by flow field visualizations.