Fluid-Structure Interaction(FSI) Study Of Marine Flexible Structures

Fluid-Structure Interaction(FSI)is one of the most important and challenging problems of multi-physics applications ranging from aeronautics and astronautics to marine and ocean engineering.The coupling between the surrounding fluid flow and deformable elastic structure can produce the undesirable FSI phenomenon(such as flutter;Limit Cycle Oscillation(LCO);static deformation;and Vortex-Induced Vibration(VIV))that may compromise structural integrity.The efficient and promising solution for these FSI problems is of critical importance in the design and operation of many complex real world applications.With recent advances of computer technology and numerical methods,high-accuracy simulation approaches related to FSI have improved significantly,however,still computational expensively and time consuming.Therefore,it is meaningful to establish robustness modeling and simulation methods to meet the challenges to become a real industrial tool and suitable for the field of engineering as well as in the applied sciences.In this thesis,the different from the current FSI modeling is the present research work mostly carried out based on a high-performance dynamics method named Transfer Matrix Method for Multibody Systems(MSTMM).Subsequently,the methods for rapidly modeling and simulations of FSI problems based on MSTMM combined with Theodorsen,Van der Pol fluid theory and Computational Fluid Dynamics(CFD),Computational Structural Dynamics(CSD)including Finite Element Method(FEM)are proposed in details.FSI problems are widely existed in the naval architecture and ocean eng:ineering.FSI study of marine structure has an important scientific impact.In the present research work,two industry applications in the fields of naval architecture and ocean engineering are fully addressed namely;(?)hydroplanes system of an underwater vehicle which include free-play nonlinearity,and(?)marine flexible riser system which include fluid nonlinearity.Following the proposal FSI analysis methods,intensively study has been taken for both applications.The vibration characteristics,mathematical method of the flutter model and the influence of dynamic parameters on the flutter are considered in the first application.The mechanism of the vortex induced vibration and the suppression method of the cylinder structure,the vibration characteristics and the vortex induced vibration responses of the flexible composite riser are also considered for the second application.The main achievements of the thesis are as follows:1.Bending-torsion coupled Euler-Bernoulli beam model is introduced,and the three-dimensional(3D)hydroplanes system's dynamic model is established based on MSTMM.Moreover,the method for determining the dynamic parameters of the hydroplanes system is proposed and the vibration characteristics are calculated,and the results are compared with the commercial software ANSYS FEM simulation results.2.The frequency and time domain linear flutter models of the 3D hydroplanes system are established based on the Theodorsen unsteady fluid theory and MSTMM,and the accuracy of the models are verified by comparison with the experimental data and commercial software simulation results.The main idea of this part provides a reference for rapidly modeling and simulation of the multi-flexible bodies systems with similar FSI problems in the field of engineering.3.The idea for how to simplify the 3D hydroplanes system to two degrees of freedom(2-DOF)flutter model is introduced.The pure bending and torsion frequencies of the hydroplanes system are obtained using MSTMM methodology.Meanwhile,the linear and nonlinear flutter models of the hydroplanes system are established based on the Theodorsen unsteady fluid theory and MSTMM.The rationality and validity of 2-DOF linear and nonlinear flutter modeling methods are verified by CFD/FEM two-way coupling simulation results and the experimental data as well as reference simulation results.Then,the influence of the dynamic parameters of the hydroplanes system on the linear flutter and the limit cycle oscillation(LCO)are studied,which provides theoretical support for structural vibration reduction design of the hydroplanes system.4.Considering Van der Pol model and the CFD/CSD model(based on CFD software user define function and overset mesh technology),respectively,the FSI dynamic models of the cylinder structure are established.Compared with the experimental data,the reliabilities of the models are verified.The advantages and disadvantages of the two numerical models are compared and analyzed.Force coefficients,vibration amplitudes and the vortex shedding modes under different conditions are studied,and the VIV mechanisms of the cylinder structure are explored.On this basis,the effect of the nonlinear energy sink(NES)on the VIV response of the cylinder structure is further studied.5.3D composite riser dynamics model is established based on MSTMM and then the vibration characteristics of the composite riser are calculated.Subsequently,the FSI model of the 3D composite riser using MSTMM/Van der Pol is established,and the influence of the riser length,top tension and flow distribution on the FSI characteristics of the 3D composite flexible riser are simulated and discussed.The last but not the least,the 3D short riser with helical strakes' FSI model is established utilizing CFD/CSD two-way coupling method,and the VIV suppression effect of the 3D riser with different helical strakes structural parameters are calculated.All the models are compared with experimental data and simulation data from references,which verifies the accuracy of the models.It provides a reference method for rapid prediction of vibration characteristics,VIV characteristics and the VIV suppression devices'design of the risers in the field of engineering.