Study On Hydrodynamic Characteristics And Fluid-solid Coupling Mechanism Of Slender Structures

Slender structures are widely used in civil engineering,ocean engineering and other fields.This type of structures is subjected to the the hydrodynamic of wind and sea current during its service.When the flow through the it at a certain velocity and angle,it will cause the vibration of the structure and eventually lead to the fatigue damage of the structure.Furthermore,strong nonlinear fluid-structure interaction(FSI)is also involved in the flexible slender structures.It is of great academic value and important application significance to study and accurately predict the fluid-induced response characteristics and mechanism of slender structures,especially at turbulent flow.Based on the spectral/hp element numerical method,from rigid structure to flexible structure,from 2.5D to 3D,from low Reynolds number to high Reynolds number and from no FSI effect to consider FSI,the typical slender structures are analyzed systematically.Flow over stationary catenary risers with different span ratios and incoming flow directions have been studied systematically by using spectral/hp element numerical method.Firstly,three-dimensional flow past stationary catenary risers with the free stream aligned with the plane of curvature are investigated at Re=100.It is found that four wake topologies,namely of single body,bi-stable,vortex impingement and steady wakes,are successively appeared with the increase of the span ratio.Besides,to investigate the wake characteristics at transition flow state,the Reynolds number is increased to 500 at this part.Furthermore,for the maximum span ratio case(A=10,‘A' is a catenary coefficient used to scale the span ratio),the wake characteristics of stationary catenary risers with different incoming flow directions are also investigated in the current work.Based on the numerical simulation results,the relationship between the range of the vortex shedding along the span of the riser and the angle of the incoming flow is proposed.By using fully coupled model approach(Parallel Fourier spectral/hp element method based on body-fitted coordinates),flow-induced vibrations of infinite long flexible cable allowed to oscillate in the cross-flow direction are deeply studied,including the circular cable and the triangular cable at Re=100,200 and 3900.The influence of angle of attack and section form on the dynamic response and the flow field characteristics of the flexible cable is deeply revealed.Finally,the galloping response of asymmetric flexible cable is systematically explained.Based on the ‘thick' strip method,vortex-induced vibration of super slender flexible cable subject to uniform currents is investigated in the present thesis.The vortex-induced vibration model of long length-to-diameter ratio is considered and ‘thick' strip technique based on high-order spectral/hp element method is employed for the computational simulation.By comparing the calculated results with the existing numerical and experimental results,the accuracy and validity of the results by this method is verified.Eventually,the mechanism of vortex-induced vibration response of super-slender flexible riser is revealed.The structural response characteristics of the super-slender flexible catenary riser under sinusoidal excitation are studied and analyzed by using the Geometrically-exact composite beam model.Based on the engineering practice,reasonable drag coefficient,excitation frequency and structural parameters are fixed to study the dynamic response of the superslender flexible catenary riser under sinusoidal excitation by using SHARPy.In this work,the mechanism of dynamic response of flexible catenary riser is systematically revealed,which provides a reference for the design of the strongly geometrically nonlinear catenary riser,and provides a research basis for the subsequent numerical simulation analysis of the fluid-structure interaction of the strongly nonlinear riser.The academic achievements of this thesis have important values to improve the understanding of slender structures,flow field characteristics and flow-induced vibrations and then provide a theoretical basis and practical guideline for engineering practice,including ocean engineering and civil engineering.