The Dynamic Characteristics Of Pipe Conveying Fluid With Local Weakness Or Reinforcement

As a widely used engineering structure,pipes conveying fluid may have local defects,cracks or weakness,etc.in engineering applications,which will reduce the stability of the pipe conveying fluid and make it prone to damage.Therefore,it is of great engineering value and academic significance to propose effective means to analyze the vibration characteristics of the pipe conveying fluid and to design pipes,as well as to explore the optimization methods to improve the stability of the pipe conveying fluid.In this dissertation,theoretical research and numerical analysis are carried out for the pipe conveying fluid with local weakness or reinforcement.The main contents are as follows:1.Based on the transfer matrix method,a mathematic model for dynamic analysis of the cracked pipe is established.First,the finite element method is used to discretize the vibration governing equation of the pipe conveying fluid;then,the equivalent dynamic stiffness matrix of the crack element is obtained according to the principle of the local strain energy release rate;finally,the effects of single crack and double cracks on the first four natural frequencies and the critical flow velocity of the pipe conveying fluid are analyzed by numerical examples.The research results show that cracks usually reduce the natural frequency and the critical flow velocity of the pipe conveying fluid.However,cracks at some special positions may also increase the critical flow velocity of the system,and the critical flow velocity will increase with the increase of crack depth.2.By changing the arrangement of pipeline materials,a dynamic model of the fluidconveying pipe with stiffness-weakened or stiffness-reinforced segment is constructed.On this basis,the influence of the position,length and other related parameters of the stiffnessweakened segment and the stiffness-reinforced segment on the stability of the pipe conveying fluid is discussed.The research results show that,for cantilever pipes,the stiffness reinforcement near the fixed end or the stiffness weakening at the free end is beneficial to improve the stability of the pipe conveying fluid,and vice versa.For simplesupported pipes,the introduction of stiffness-reinforced segment will improve the stability of the system and the degree of stability improvement is related to the length and position of the stiffness-reinforced segment.3.A dynamic model of a hybrid rigid-flexible pipe conveying fluid is established.Based on the derived governing equations,complex frequencies and critical flow velocity of the system are solved based on Galerkin method.The influence of the system parameters,such as the stiffness of the rotating spring,the mass ratio,and the length ratio of the rigid segment to the flexible segment,on the stability of the system is analyzed in detail.The research results show that by changing the key parameters of the hybrid rigid-flexible pipe,the instability mode order and the critical flow velocity of the system may be significantly changed.4.By analyzing the stability mechanism of pipes conveying fluid with local weakness or reinforcement,a local optimization method based on increasing the critical flow velocity of the pipe conveying fluid is proposed.Using this method,the local optimization results of two ways with material transformation and wall thickness increase and decrease are analyzed in detail,and the possible optimized configuration of the cantilever pipe is given.