Three Dimensional Numerical Simulation On Flow Around And Vortex-induced Vibrations Of A Circular Cylinder Near A Wall

Submarine pipeline plays an essential role in the modern offshore oil production.Due to the existence of the pipeline,seabed scouring happens and forms a suspended span.The suspended pipeline may suffer from the vortex-induced vibration which causes the fatigue of the structure.Once the pipeline leaks,it will not only cause the oil production loss but also present severe damages to the environment.Considering the harsh surroundings pipeline locateing in,high expense,complex repairing technology and time wasted are not beyond expectations when the oil transporting system is repaired.Though there is always enough safe redundant in the construction and design process of pipeline,it is just a short-term palliative,not a long-time solution.Flow around pipelines and vortex induced vibrations near a wall has been studied extensively for a long time.But three-dimensional numerical simulation is still relatively rare.For the better understanding of the essence of flow-solid interaction,the pipeline is regarded as a smooth cylinder,and the seabed as a solid plane wall.Based on the immersed boundary method,a parallel computing program called CgLES_IBM was used,and a fixed solid cylinder along with a flexible cylinder model was generated separately to simulate the wall-cylinder-flow interaction problem.In this paper,the mass ratio of the cylinder is 6,the aspect ratio is set to 25 and Reynolds number is 350.The boundary layer height is set to 0,0.7D,1.6D.The gap ratio is set to 0.2,0.6 and 1.0 to represent the small gap ratio,medium gap ratio and large gap ratio condition.The main research conclusion contains the following:1.In the case of flow around a cylinder,the location of the front stagnation point represents the velocity difference between the upper side and down side layer of the cylinder.The difference between the peak velocity in the gap and the upper shear layer shows a strong correlation with the angle of the front stagnation point.The smaller the angle,the upper the front stagnation point locates in,the larger the velocity difference.2.Within a thicker boundary layer,there existed two conditions with negative lift force representing a downward direction,while the front stagnation point still located in the upper surface of the cylinder.Hence,the location of front stagnation point is not suitable for the benchmark of the lift force direction.The boundary layer and the wall can either push the cylinder away from them or absorb the structure coming closer in certain flow conditions.3.When the cylinder approached the wall,there formed a high velocity flow in the gap,a high pressure area in the upstream and a low pressure area in the downstream.The existence of the cylinder can dramatically change the flow structures.The high velocity flow in the gap can cause serious seabed scouring.The high pressure area in the upstream is generated by the cylinder's blocking effects while the low pressure area in the downstream is formed by the sheded vortex.The pressure gradient between the upstream and the downstream may lead to the piping and scouring beneath the pipeline and the growth of the suspended span.4.The vortex-induced vibration of cylinder near a wall is relatively simple.There just existed the first mode in both in-line and cross-flow directions,and the cylinder vibration trajectory is in oval-shape.5.When the cylinder locates beyond the boundary layer,the growth of the boundary layer has a similar effect as the decline of the gap ratio.