Research On Simulation Analysis For Stability Problem Of Pressure-Penstock With Imperfection

The construction of large-scale hydropower stations and large-capacity pumped-storage power stations make penstocks construction be large-scale and huge-scale. The use of high quality steel makes penstocks become more and more flexible and thin. The penstock stability has become the key factor in penstock design. But now ,the theory and method of penstock design is short of relevant research. The conventional analysis-resolution and half-experience formulas are used in home and abroad. The calculation models for these formulas have its great limit, therefore, in home and abroad, penstocks of some hydropower stations often occur buckling accidents. Finite element method is an excellent numerical method,but according to general shell theory, more parameters are needed to describe the geometry shape of shell and its shape function. It is difficult to construct the elements, that not only satisfy the harmony of elements and the rigid displacement but also satisfy the constant-strain items,for solving the stability problem of the thin shell by FEM.According to above condition, the simulating technology of Element-Free numerical Method and experiment is firstly applied to the research on the stability analysis of penstock bearing external pressure.At first, a systematic analysis about the latest development of EFM(element-free method) and the theories and methods about stability analysis of pressure penstock is made. A new model bearing external pressure is established for calculation of stability analysis of geometric nonlinear ribbed shell with initial imperfections . At the same time ,the seams between penstock and concrete were creatively turued into initial imperfections, and this translation makes the research of this kind of imperfections become more scientific.Secondly, some improvement for the new rising Element Free Method (EFM) is accomplished . A new method for constructing the field function of EFM is put forward. This method inherits the reasonable inner kernel of constructing field function in EFM and FEM, retains the "local approximant" , "the moving" , and "high times completeness" of EFM field functions, and retains "directness"and"interpolating" of constructing FEM field functions. It also eliminates the two " bottle-neck" problems ,which hinder the development of EFM, that are the heavy inverse calculating in determinting implicitly EFM field function ,and the noninterpolating at nodal of field function, which cann't treat the essential boundary conditions precisely. Calculating the high-gradient field in unconvex boundary areas such as the crack tipsand acuity nooks, we often encounter the instances that the influence field is cut off by the discontinuity boundary. But the geometry quantity in the formulas of EFM to deal with the problems are difficult to calculate. Because the calculation in these areas is very important ,a new calculating criterion —arc—string criterion is presented. The new criterion is not only convenient but also efficient. Besides above works , high precise intergral skill for irregular boundary field in Gauss intergral grid is given.Thirdly, the thick curved-beam geometry equation is advanced by means of the tensor analysis technology and the nonlinearity geometry equation of thin shell with any initial geometric imperfections is put forward. This geometry equation can be conveniently turned into the perfect shell, linearity shell and so on, such as the columniation and sphere shell. The model of ribbed shell is regarded as the combination of thick curved-beam and thin shell,and it has changed the analysis model of the traditional computitional method. The new model can reflect the real stress and strain state of penstock. Then the shape function of thick curved-beam is constructed. This shape function can eliminate the "locking" phenomenon when the thick cured-beam degenerate to the thin curved-beam.Finally, as one of the key works in this paper, the experimental scheme for the stability analysis of penstock under negative-pressure is creatively designed. In order to watch the changing digit on the meter and observe the process of buckling of penstock, the buffering-pots are used to buffer the added pressure. With the help of experimental skills,the buckling mechanism of perfect penstock and imperfect penstock are researched. The reliability of new model ,new theory and new method are tested and verified. Their correctness is proved. By the emulation skills of the numerical calculation and experiment measures , the sensitivity of critical load to imperfection and geometric size is analyzed, and by which the limitation of traditional method is pointed out..In a word , by the aid of three measures, which are theoretical research, computer simulating and experiment analysis, the new theory and new method for the stability analysis of penstocks bearing external pressure are successfully presented . As well as the relative software has been expolited .All these achivements create a new electronic road for penstocks design and its stability analysis.