Research On The Discrete Vortex Method Based On Instantanous Vorticity Conservation And Its Application In Vortex Induced Vibration Of Marine Risers

With the shortage of oil and gas resources on the land, oil and gas exploitation has gradually stepped into ocean. The deep-ocean risers are among the major devices that connect the floating platforms and seafloor wellheads, and responsible for drilling, exploiting, and transporting oil and gas. Ocean current makes effects on risers, and incurs vortex-induced vibration that would lead to the wear and fatigue of the risers, and even cause severe broken accidents. Thus, the research on vortex-induced vibration has always been the focus of both academy and industry.The dissertation proposes a novel vortex method that bases on the instantaneous vorticity conserved boundary condition (IVCBC), and explores the flow around double two-dimensional circular cylinders, and the vortex-induced vibration on three-dimensional risers using this method within the framework of the viscous-flow theory. Being aimed at the issue of handling the vortices penetrating into the boundary of the cylinders for flow around two cylinders, another novel vortex scheme based on IVCBC is proposed which is called DIVCBC. Combined with the structural features of two circular cylinders, the numerical models of computing the flow and vortex-induced vibration around the two circular cylinders are established. A method of distinguishing the wide wake and narrow wake is proposed based on the fact that the direction of the biased flow is synchronized with the velocity direction at the mediumpoint of the distance between the two cylinders. With the introduction of Finite Volume Method (FVM) and dynamic stiffness matrix, the numerical methods of calculating the static balance and dynamic response of single and double risers are established. All the relevant numerical methods in this dissertation are programmed on Fortran, and post-processed on Tecplot.Firstly, being aimed at the issue that some of the boundary vortices penetrate into the circular cylinder, a pure-Lagrangian vortex scheme based on IVCBC is proposed. In order to make the vorticity zero inside the cylinder, for each vortex blob, a new vortex blob is brought in at the same location of the original vortex blob with vorticity of same size and opposite direction. Meanwhile based on the Circle Theorem, identical number of new vortex blobs outsidethe cylinder are introducedat the imaged position of the inside ones.The results show that the scheme keeps the cylinder's surface being streamline, and meets the requirement that the vorticity inside the cylinder should be zero. Furthermore, the scheme ensures not only the instantaneous vorticity conservation, but also the total vorticity conservation, which makes the calculation accuracy improved significantly and extends the calculation scope of discrete vortex method. This vortex method satisfies the instantaneous vorticity conservation, thus, it is called Instantaneous Vorticity Conserved Boundary Conditions based vortex method, i.e. IVCBC vortex method.Secondly, being aimed at the issue that some of the boundary vortices penetrate into the circular cylinder for double circular cylinders, the scheme of handing voetex inside cylinders is proposed based on the confomal mapping and Double Circular Theory. The results show that it is scarcely any influence on surface pressures for the vortex number outside cylinders in velocity formula of Double Circular Theory. Hence, in order to reduce the calculation amount, being aimed the features of the flow around circular cylinders under high Reynolds numbers, two numerical models of flow around two circular cylinders are established by IVCBC vortex method with the configurations of side-by-side arrangementand in tandem arrangement. The characteristics of the flow under these two models are investigated at Reynolds numbers Re= 6.0×104 and Re= 2.2×104 respectively. A method of distinguishing the wide wake and narrow wake is proposed based on the fact that the direction of the biased flow is synchronized with the velocity direction at the medium point of the distance between the two cylinders. The results show that it is capable to distinguish exactly the wide wake and narrow wake. Meanwhile, an intermediate frequency between frequencies of NW and WW was found and approached to that of a single cylinder in gap ratios 1.1?T/D?2.6. Five wake modals exist in the wake under the twoside-by-side arrangement, where at the symmetric regime 2.6<T/D?7 the anti-phase flow patterns are predominant and it can persist for a long time at certain pitch ratios. With two circular cylinders in tandem arrangement, the shear layer of uppercylinder connect with that of the downcylinder and enclose the downcylinder at the gap spacing 1.1< L/D<1.4. Small vortices can be observed clearly on upside and down side between two circular cylinders at the L/D=1.2. With increasing of the L/D, the shear lay connecting the uppercylinder and downcylinder gradually breaks at the gap spacing 1.4<L/D<3.25, forms reflux region between the two cylinders, but no complete vortex pairs exist. The L/D=3.25 is called critical spacing, where all the flow forces occur sudden changes. When the gap spacing is L/D>3.25, complete vortex pairs occur.Finally, being aimed at the feature of large slenderness ratio of risers, the numerical structural model is established based on strip theorywith the introduction of the Finite Volume Method and dynamic stiffness matrix. Combined with IVCBC vortex method, a three-dimensional numerical model of calculating the vortex-induced vibration is established. The results show that the model has much higher reliability and effectiveness. For the vortex-induced vibration at high Reynolds numbers, the shedding frequencyincreases obviously, and does not obey theStrouhal law any more, which leads to the phenomenon of multi-frequency shedding, and the locking phenomenon of vortex-induced vibration of risers. The response frequency of the riser includes not only the shedding frequency that is in accord with main frequency, but also other minor frequencies. There aredifferent frequency response phenomena at different points of the cross section of riser, which causes asymmetric complex flexural deformation withcoexistence of multiple high-order modal vibration. Under the action of the same currents, the main vibration order of both single riser and double risers will increase with the gap ratios increase. The amplitude of riser will decreases with gap ratios increase.