| As a common fluid-conveying structure in engineering,fluid-conveying pipe systems are widely used in many engineering fields,such as marine ships,aerospace engineering and petrochemical industries,etc.The dynamical system of fluid-conveying pipes also displays rich dynamical behaviors,thus it is of great academic significance.In this work,the impacts of internal axial flow and external cross flow(or external axial flow)on nonplanar dynamics of simply supported fluid-conveying pipes are investigated.Particular attention is paid on the bifurcation behavior and nonplanar dynamics of the fluid-conveying pipes when the internal or(and)external flow velocity is taken as the variable parameter.Based on theoretical and numerical results,the influences of the internal fluid flow or the combined effects of internal and external fluid flows on the nonplanar vibration characteristics of the fluid-conveying pipe system are revealed.The main work of this dissertation includes:(1)From the perspective of nonplanar dynamics,the effects of a steady internal fluid flow on the nonplanar postbuckling behavior of a pinned-pinned pipe are investigated.Based on the nonplanar governing equations,the natural frequencies of the buckled pipe are analyzed.The results show that the first-mode frequencies for in-plane and out-of-plane motions of the system are quite different,while their second-and third-mode frequencies are almost equal.By studying the effects of initial conditions on the postbuckling configuration of the system,it is found that the orientation angle of the postbuckling configuration plane changes with the initial conditions.On this basis,the nonplanar postbuckling behavior of a pinned-pinned fluid-conveying pipe with an axially sliding downstream end is further explored.The effects of initial conditions,mass ratio and gravity parameter are studied.It is found that initial conditions can affect the buckling displacement components in the two lateral directions but cannot influence the total(resultant)buckling amplitude.(2)A nonplanar dynamical model of two fluid-conveying pipes connected with linear springs is established.The nonlinear vibration behavior and synchronization phenomena of the two-pipe system are analyzed.Based on linear and nonlinear analyses,the effects of the internal fluid velocity of the two pipes on the coupled frequency,modal shape,bifurcation and synchronization behaviors are investigated.It is found that when the two pipes convey steady fluids,the internal fluid velocities of the two pipes have the same influences on the natural frequency and modal shape of the coupled pipe system.When the two pipes convey steady fluid and pulsatile fluid respectively,the oscillations of two pipes are qualitatively identical.In addition,when the two pipes undergo periodic or quasi-periodic motions,they are usually coupled with phase(or phase lag)synchronization patterns.(3)A nonplanar theoretical model for vortex-induced vibrations of a simply supported fluid-conveying pipe subjected to uniform or shear cross flow is established.The complex mechanism of vortex-induced vibrations of the pipe system is revealed.The frequency characteristics and bifurcation behavior of the pipe system in uniform or shear cross flow are investigated.It is found that when the internal fluid velocity is higher than the critical value,due to the external hydrodynamic forces,the orientation angle of the postbuckling configuration plane is time-varying.The pipe tends to vibrate periodically in a uniform cross flow.In addition,the dynamic responses of the pipe mainly consist of one single-mode and exhibit a standing wave pattern.However,the pipe subjected to a shear cross flow tends to undergo quasi-periodic motions.The dynamic responses usually contain the contributions of multiple modes and a mixture of traveling and standing wave patterns may be observed.(4)A nonplanar dynamical model of a fluid-conveying pipe subjected to external axial flow is developed.The effects of internal and external axial fluid flows on the inherent characteristics and bifurcation behavior of the pipe system are investigated.The results show that when the internal fluid velocity is steady,the internal and external fluid flows have similar effects on the natural frequency and postbuckling behavior of the system,since both of them generate centrifugal and Coriolis forces exerted on the pipe.When the internal fluid velocity is pulsatile,within the given pulsating frequency range of the internal fluid,the bifurcation diagram for displacement amplitudes of the pipe may be divided into five regions according to the oscillation feature of the pipe.It is observed that the pipe may undergo periodic,quasi-periodic or chaotic motions in different regions. |
