Numerical Simulation Method Research Of The Nonlinear Fluid-Solid Interaction Problems

Fluid-Solid Interaction (FSI) is an important problem in fluid mechanics research, one of which is fluid and solid mechanics by the formation of the mechanical cross-branch. Under the fluid action, the structure in the flow field will occur deformation and displacement. In turn, the deformation and displacement can impact the fluid, so fluid and structure form an interconnected, complex system. FSI problems exist in many engineering fields, such as aircraft, buildings and bridges anti-wind, ships and ocean engineering. Especially in the ocean engineering field, FSI analysis plays an important role. Such as offshore platforms and deep-water risers, which withstand the waves and currents, in their design processes are inevitable to carry out SFI analysis, especially the riser Vortex-Induced Vibration (VIV) is a typical strong nonlinear complexity FSI problems, is also one of a main reasons of riser fatigue damage, and an important factor in reducing its service life. Therefore, the problem of nonlinear FSI study has important scientific significance.Nonlinear FSI problems of have been the difficulty to deal in fluid mechanic field. The current analysis methods all require the grid-based information, such as the Finite Element Method (FEM), these methods in dealing with nonlinear problems, especially if deep-water risers, anchor cable and other non-linear large deformation of fluid-structure coupling, the unit will be a serious distortion, resulting in reduced accuracy or even not converge. And moving grid computing would entail a considerable amount of computation.Element-free method (EFM) does not rely on the information grid cell characteristics. But their research focused on solid mechanics, fluid mechanics research in the still relatively small, and the EFM has not been found to use in nonlinear FSI analysis reports such as VIV phenomenon.For these problems, This thesis use riser VIV as example, to introduction EFM into FSI analysis, try to explore a new method of nonlinear FSI numerical simulation without relying on grids or elements information, and achieved some initial achievements. The thesis provides theoretical basis and technical support for analysis of such issues. This thesis’ work can be summarized as follows:1. Overview the research in the fields of the FSI and deep water risers, explains the basic theory of VIV, such as important control parameters, the wake vortex shedding mechanism, the important phenomena and characteristics. This thesis also described the main experiments and numerical simulation methods of the VIV.2. Describes the characteristics of EFM and analysis process, compared the distinction with the traditional finite element method. Analyze its strengths and weaknesses. Described EFM classification, node support domain theory, shape function construct method based on interpolation and least mobile squares, and the choice of weighting functions. Describes the theory of the system equations discrete with element-free method, how to deal with the nature of boundary conditions and numerical integration theory of EFM method.3. Give the nonlinear FSI analysis method, the method joint use Element-Free Galerkin (EFG) method and third-order split-step finite element method, using the speed and pressure separation mode, respectively discrete the two-dimensional viscous incompressible flow equations in time and space, and analyzed the fixed cylinder VIV parameters. The thesis also given an automatically EFM nodes distribute algorithm for two-dimensional VIV. Using two different nodes distribute methods to analyze two-dimensional fixed cylinder VIV.4. Givens the algorithm of move nodes around the vibration cylinder boundary, and used the "virtual control unit" to solve the spatial derivative of EFM shape functions so that to deal with the high Reynolds number flow field. Using EFM to calculate the damping considered and elastic supported two-dimensional rigid cylinder VIV’s dynamic response, and simulation the2S,2P mode and initial, upper and lower three branches of two-dimensional VIV, described the VIV "Lock-in "phenomenon.5. To consider the application of EFM simulate the flow of vibration of two degrees of freedom VIV dynamic response of the elastic support cylinder, the cylinder near the anisotropy field of local support for a "virtual control unit" algorithm for solving the spatial derivatives of shape functions, the control units can use to background integral grid. Calculate the amplitude response in two directions, as well as its lift, drag coefficient changes with time, and the numerical results with the model experimental results were compared.