Control Strategies Of Cross-flow Induced Vibration Of Cylinders

Cross-flow induced vibration of cylinders commonly occurs in our daily life and engineering application.These cylinder structures usually play a crucial roal in their respective fields.The cross-flow induced vibrations(CFIV)of cylinder are usually considered to be undesirable side effect as they are of large amplitudes and high frequencies,which have the great potential to cause damage in a short term.Thus,it has great significance to predict and suppress the cross-flow induced vibration to enhance the safety and lifetime of structures.On the basis of project background above,the control strategy of cross-flow induced vibrations of cylinders is researched in this thesis.A variety of approaches,such as theoretical modeling,analytical method and numerical simulation,are applied to investigate the effect of the parameters of the strategy on the control of crossflow induced vibrations.A series of new nonlinear dynamic responses are observed in this study and the predecessor's research has been expanded.The main features of the present paper are organized as follows:1.The suppression of oscillations of an elastically mounted prism under galloping by a dynamic vibration absorber(DVA)with linear damping and stiffness is systematically investigated.A model considering the dynamic coupling of the prism and the DVA is constructed,with the aerodynamic loads acting on the system represented by a quasi-steady approximation.Based on the coupled nonlinear governing equations of motion,a linear analysis is first conducted to explore the coupled frequency and damping,and the onset speed of galloping in the presence of the DVA.Subsequently,the normal form of the Hopf bifurcation for the coupled system near the onset of galloping is derived to characterize the type of instability(supercritical or subcritical),while evaluating the effects of the DVA parameters.The results show that with appropriate parametric values,the DVA has great impact on the onset speed of galloping and can significantly alleviate the oscillation amplitude of the prism.2.A new dynamic vibration absorber composed of both linear and nonlinear springs in parallel(Lin-non DVA)is proposed to improve the stability properties of the prism galloping,and compared with other dynamic vibration absorbers which have been investigated in the literatures.The results show that dynamic vibration absorbers with linear spring have a better performance on the control of prism galloping.Rich nonlinear dynamic responses such as limit circle oscillation,multiple stable responses and even chaotic motions can be observed when the nonlinear spring exists.It is found that the occurrence of multiple stable responses can enhance the nonlinear energy pumping effect,resulting in the increment of transferring energy from the flow via the cylinder to the absorber.There exists an effective limited cross-flow velocity range for the converntional nonlinear energy sink,which is found in this thesis for the first time.Morever,the Lin-non DVA has been proved to be more suitable for vibration control of galloping in a wide range of flow velocity.3.The effectiveness of linear and nonlinear time-delay feedback controls to suppress high amplitude oscillations of an elastically mounted square cylinder undergoing galloping oscillations has been investigated.A representative model that couples the transverse displacement and the aerodynamic force is used.The quasi-steady approximation is used to model the galloping force.A linear analysis is performed to investigate the effect of linear time-delay controls on the onset speed of galloping and coupled frequencies.It is demonstrated that a linear time-delay control can be used to delay or minish the onset speed of galloping.The normal form of the Hopf bifurcation is then derived to characterize the type of the instability(supercritical or subcritical)and to determine the effects of the linear and nonlinear time-delay parameters on their outputs near the bifurcation.The results show that the linear time-delay can either reduce or amplify the amplitude of galloping responses and the nonlinear time-delay control can be efficiently implemented to significantly reduce the galloping amplitude and suppress any dangerous behavior by converting any subcritical Hopf bifurcation into a supercritical one.4.The possibility of using a time-delay feedback control to suppress the vortex-induced vibrations of an elastically mounted circular cylinder has been investigated.An appropriate reduced-order wake-oscillator model is employed to determine the cylinder's displacement and lift fluctuating coefficient in a coupled manner.A parametric study is performed to investigate the effects of the time-delay feedback control on the cross-flow oscillations of the circular cylinder.To study the effects of this controller on the coupled frequency and damping of the aeroelastic system,a linear analysis is performed.It is demonstrated that the presence of time-delay feedback control can result in an increase or decrease in the coupled damping of the aeroelastic system,varying from negative to positive values periodically.Then,the effects of this time-delay feedback controller on the nonlinear responses of the circular cylinder are determined.The results show that a good choice of time-delayed controller parameters can be efficiently implemented to significantly decrease or amplify the vortex-induced vibrations amplitudes in the lock-in or synchronization region.Morever,for high values of the delayed time and gain,complex variations in the coupled damping and frequency,and hence complicated dynamic response are obtained.This may be resulted from the presence of two different excitations in the synchronization region: vortexinduced vibrations and self-excited oscillations.In conclusion,based on theory analysis and some numerical calculations,two control strategies are deeply investigated and a detailed analysis of the control parameters is performed.The control strategies presented in this thesis features simple modeling,easy to utilize,good performance and strong controllability.The research results is of great engineering practical value and theory guiding sense.