Study On The Mechanism Of Transverse Vibration And Its Control Of Pipes Conveying Fluid

Now, pipes conveying fluid have been extensively applied in aeronautic and aerospace engineering, hydraulic engineering, petrochemical industry, agriculture and our daily life. Due to the effect of fluid-structure interaction (FSI), pipes always vibrate violently when the inner flow velocity is high or pulsatile, which may even result in pipe rupture in some serious cases and cause incalculable loss. So, in order to make the pipe work safely and stably in all industries, an intensive study of the vibration mechanism and its control of the pipe conveying fluid should be made.In this dissertation, the existing achievements in pipe FSI vibration are summarized first, then the stability, parametric resonance, forced vibration with internal resonance and vibration control of pipes are investigated by using theoretical and experimental methods. The conclusions in present work may provide a theoretical basis for the stability analysis and hazard assessment of the pipes in the engineering, and also establish the theoretical foundation for the feasible vibration control of the pipes. The main contents are as follows:(1) The mechanical systems of pipes conveying fluid are modeled, and the differential equations of transverse nonlinear vibration for the pipes are derived through Newton's method. After nondimensionalization and discretization, a differential equation of motion with simplest type is obtained.(2) Static and dynamic stabilities of supported pipes and those on the elastic foundation are analyzed. Applying the Galerkin and complex mode methods, the natural frequencies and critical flow velocity are calculated. For the case of pulsating inner flow, the regions of principal parametric resonances for first two modes and combination resonance are obtained using the averaging method. The contributions of the linear, sheer foundation rigidities and other parameters on the stabilities of the systems are discussed. The results reveal that the natural frequencies and the stability decrease with the flow velocity increasing; the effect of the sheer rigidity on the stabilities is great and can't be neglected; moreover, some parameters such as tension force, mass ratio of fluid to pipe, viscoelastic coefficient and mean flow velocity have effect on the stabilities of the systems.(3) Nonlinear equation of motion for the pipes is solved by incremental harmonic balance (IHB) method, and the results are verified by using numerical simulations. According to the solutions of the equation, dynamic response characteristics of the pipe systems in the case of pulsating inner flow are analyzed. The results demonstrate that the semi-analytical and semi-numerical IHB method is precise and effective for the nonlinear vibration problems; with the pulsation frequency increasing, the solutions of the response amplitude may bifurcate, which results in nonresponse, stable response or unstable response with them; when the parametric resonance of the nth mode occurs, the motion of this mode is dominant, and others may contribute weakly.(4) Forced vibrations with internal resonance of pinned-pinned and clamped-clamped pipes are researched through the multiple dimensions Lindstedt-Poincaré(MDLP) method, and the results obtained are compared with those of the IHB method. The resonance responses of the first two modes and combination resonance are investigated and the relationship between the internal resonance and excitation amplitude is discussed. The motions of all the modes are analyzed. The results show that the internal resonance may occur as the excitation frequency is near the first, second natural frequency or half the sum of them under the condition that the second natural frequency is three times the first one, when the first two modes are excited by each other. However some of the internal resonances are decided by the excitation amplitude.(5) With taking the ceramic piezoelectric gauge as the control actuator and modal transducer, parametric resonances of pipes conveying pulsating fluid on elastic foundation are controlled actively base on the optimal theory. The effect of the control parameters on the performance of the controller is analyzed. Numerical simulations demonstrate that all kinds of parametric resonances of the pipes can be controlled well with present optimal controller, and it has the ability to resist the perturbation of parameters.(6) An experiment is conducted to study the dynamic stability of pipes conveying pulsating fluid. The experimental apparatus is set up and effective methods to measure some important parameters are put forward. The first sub-harmonic parametric resonances of order 1/2 for the two pipes are observed and analyzed. The unstable regions are obtained experimentally and they are compared with the theoretical results. It can be seen that qualitative agreement is fairly good. Furthermore, the reasons inducing errors are discussed.