Research On Hydrodynamic Characteristics Of Marine Risers Under Non-uniform Periodic Pulsation Disturbance

As an important link between the offshore platform and the subsea production system,the marine risers are indispensable and important components in the marine development process and its dynamic flow characteristics have been highly concerned by the academic community.At present,most researches focus on the flow-induced vibration of marine risers under uniform flow,while ignoring the interference effect of non-uniform periodic pulsating fluid.However,more fluid flow in the ocean occurs in the form of complex nonlinear flow,such as shear flow,oscillating flow,and non-uniform flow of ocean internal waves and vortices.So far,there have been few studies on these issues,but they are also important subject that ocean engineering have to face and solve.Therefore,this paper takes ocean risers as the research background and uses high-fidelity numerical simulation methods to study the dynamic flow characteristics of risers(cylinders)under the disturbance of non-uniform periodic pulsations.The main research results include:(1)Based on Incompact3 d high-performance three-dimensional parallel computing architecture,the high-fidelity fluid-solid coupling numerical calculation method based on immersed boundary is studied.The spatial discretization adopts the sixth-order compact difference format,and the time advancement adopts the second-order Adams-Bashforth format.In order to satisfy the incompressible condition,the pressure poisson equation is solved by the step projection method,and the fast Fourier transform is used to achieve a fast solution in the spectral space.For the fluid-solid coupling boundary,the direct forcing immersion boundary method(IBM)is used for effective processing.And through the classic numerical example of the flow around the cylinder,the reliability of the calculation method is verified.(2)The three-dimensional high-fidelity flow simulation of the flow around a single cylinder,the flows over two tandem circular cylinder,and the flow around a complex wing structure under high Reynolds numbers is studied,and the characteristics of the flow around the flow and the lift and drag are analyzed in detail.At the same time,the flow characteristics and stress changes of the flows over two tandem circular cylinder at different spacing were studied,and the existence of the critical interval was confirmed.(3)The sinusoidal periodic oscillation of the wing structure composed of the upstream circular leading edge and the sharp trailing edge produces non-uniform periodic pulsating flow field interference,and analyzes the characteristics of the wake disturbance flow field formed by it.Furthermore,the effect of different oscillation frequencies and amplitudes of the wing structure on the force characteristics of the cylinder in the downstream wake field was also investigated.At the same time,the influence of the different distance between the wing structure and the cylinder and the position of the cylinder in the Y direction on the force were studied.It is found that as the spacing increases from S/D = 5 to 8,the average drag coefficient of the downstream cylinder changes significantly from-0.25 to 0.5;The swing frequency of the wing structure has a greate influence on the amplitude of the downstream cylinder’s lift.With the increase of the frequency,the amplitude will suddenly drop to about 1/2 of the previous;The effect on the placement of the downstream cylinder in the Y direction is mainly reflected in the symmetrical position on both sides.The average resistance of the cylinder is similar and the direction is the same,but the average lift is similar in size and the direction is just the opposite.