Research On Nonlinear Vibration Mechanism And Driving Utilization Of Cantilevered Pipe Conveying Fluid

Since fluid-conveying pipe is widely applied in major engineering like ocean engineering,aerospace and nuclear industry,the potential various vibrations of this system have become dynamical problems that researchers worldwide focus on.In this dissertation,a series of investigations are carried out on the nonlinear mechanical problems of the cantilevered fluid-conveying pipes,aiming to reveal the nonlinear statics and dynamics mechanism,propose the effective control strategies,and explore the possible driving utilization value,which can provide the research foundation for the structural design,control and utilization of the pipe in engineering.The main research work of this dissertation is as follows:(1)Through dynamic theoretical modeling and analysis,the planar and non-planar dynamic responses of a straight cantilevered pipe subjected to an axial base excitation are revealed.The linear analysis based on Floquet theory points out that,this pipe system could display the dynamic behaviors of subharmonic resonance and combination resonance.The nonlinear analysis indicates that,the axial base excitation can not only suppress the flutter instability of the pipe,but also force the non-planar oscillation of the pipe into a planar one.(2)A nonlinear theoretical model of a straight cantilevered fluid-conveying pipe subjected to an axial external flow is established,concerning on the influence of the geometric configuration of pipe’s free end on the linear stability and nonlinear dynamic behaviors of this considered pipe system.The linear results show that when the tip end of the pipe is tapered,the buckling,flutter and combined buckling-flutter instability may occur in this pipe system;while the pipe with a blunt free end only experiences flutter.These complex dynamic characteristics have been verified by the nonlinear numerical calculations.(3)Based on the absolute nodal coordinate formulation,a nonlinear theoretical model of the slightly curved cantilevered pipe conveying fluid is established.The static deformations,linear stability and nonlinear vibrations of this system are discussed,and some fresh mechanical behaviors are observed.For example,the slightly curved pipe may have large static deformations,and the corresponding static equilibrium configurations are strongly dependent on the initial curved shape of the pipe;the critical flow velocity of this system is closely related to the critical static equilibrium configuration of the pipe;the results of nonlinear dynamic analysis show that the pipe performs a limit-cycle motion after instability.(4)Based on the absolute nodal coordinate formulation,the nonlinear statics,linear stability,nonlinear self-excited vibrations and nonlinear forced vibrations of a L-shaped(straight-curved combination)cantilevered fluid-conveying pipe are studied in details.The numerical results indicate that when the base excitation is absent,the L-shaped cantilevered pipe suffers from static deformations under the action of subcritical internal flow.When the internal flow velocity exceeds the critical value,the pipe undergoes a period-1 motion.However,when the base excitation exists,complex nonlinear dynamic behaviors,such as multi-period,quasi-periodic and chaotic responses can be found in this pipe system.(5)A control method based on the local stiffening of the pipe is proposed to enhance the stability of flexible cantilevered straight and curved pipes,and suppress the deformation or the vibration amplitude of the curved pipe.The results indicate that the local stiffening may not only weaken the stability of the pipe system,but also greatly increase the critical flow velocity of this system,which mainly depends on the location of local stiffening.In addition,this control strategy can affect the static equilibrium configurations(only for curved pipes),vibration amplitude and the instantaneous vibration shapes of the fluid-conveying pipe.(6)A novel underwater thruster propelled by static deformations of cantilevered curved fluid-conveying pipe is designed.This thruster is composed of a head and three curved flexible fluid-conveying tentacles.The initial curved configuration of the tentacle is optimized with the aid of theoretical analysis.Simulation calculations and experimental studies indicate that,compared with the pure jet propulsion mode,the swing of the flexible tentacles due to the static deformation and recovery hebaviors can provide a higher propulsion speed,which offers a new idea for the research of underwater thruster.