Study On Vortex-Induced Vibration Response Of Four Cylinders In An Array Configuration

The deep-sea field is an important area for the sustainable development of the petroleum industry.With the implementation of the maritime power strategy,the deep sea oil development progresses steadily.As a ubiquitous form of deep-sea oil exploitation,the multi-cylinder array structure is widely used in offshore platforms and riser groups.Due to the comprehensive effect of multiple factors such as wind,wave,flow,traction and pull of the floating platform as well as the interaction among the column groups,the vortex-induced vibration response of riser becomes complicated,the collision between the cylinders easily happens,and even the cylinders are destroyed.Many scholars at home and abroad have carried out a large number of model tests and numerical simulation studies on serial or parallel double-column problems in recent years.Until now,the studies on four-column or even multi-column problems mostly focus on the flow past body.Therefore,based on the verification of numerical simulation,this paper studies the flow past body and vortex-induced vibration of four-cylinder array under different constraints by numerical simulation.The main study contents are as follows.(1)The numerical simulation study on the two-dimensional smooth cylindrical fluid-solid coupling with double degrees of freedom is carried out by means of dynamic grid in combination with cylinder control equation for UDF discretization.Then the results are compared with those of classic three-branch experiment of Williamson,and thus verifying the reliability of the numerical simulation method.The cylindrical vortex-induced vibration based on the similar scale is simulated,and then it is determined that prototypes based on Reynolds-structure dynamics and scale models have extremely high coincidence under the low Reynolds number(90≤Re≤240)by the comprehensive contrast on the dimensionless amplitude response,resistance coefficient,trajectory and space position,as well as the wake leakage vortex mode.Furthermore,the scale model can be regarded as the quantitative analysis to reflect the related characteristics of prototype.Under high Reynolds number,the model can’t restore some real characteristics of the prototype at partial reduced velocities due to the lack of three-dimensional turbulence details in two dimension.The effective values of stress and dimensionless amplitude of prototypes and models with high Reynolds number have a high similarity in the overall trend with the change of reduced velocity,which can be used as a qualitative reference.(2)The correctness of four-column grid strategy is determined by comparing and verifying the numerical simulation of four-column flow past body studied by Zhao(2016).Subsequently,the numerical simulations are carried out on the four-column flow past body,integral vortex-induced vibration of rigidly connected four-column,variable reduced velocity of individual vortex-induced vibration of four-column with the same spacing ratios(L=2.5D,G=3D).It is found that the sensibility of wake vortex removal model of fixed four-column flow to the Reynolds number is poor.Except that Ur is 7,the wake vortex leakage modes of the remaining reduced velocities are all pair of 2S mode.When Ur is 7,the unique in-phase leakage vortex appears,and the tail vortex fuses into a 2S mode.Moreover,as Ur is between 7 and 8,the flow state near the upper and lower cylinders gradually changes from shear layer reattachment to shielding flow.At this time,the effective values of the rising resistance of the upper and lower cylinders tend to be the same.(3)The dimensionless amplitude of integral four columns with a rigid connection changes with the reduced velocity is similar to that of single column.The vibration trajectory displays linear and classic 8,1 fonts.When Ur is 3,the rising phase on the upper side is opposite to that on the lower side,and they exactly offset.Hence,the horizontal amplitude is almost zero,then the wake vortex mode is similar to flow past body with the same velocity.As Ur between 4 and 6,since the vortex of the upstream column initially falls off and does not fuse with the rear,the vibrating column makes the leakage vortex of the upstream column scattered by impact,the part of the vortex smashed and scattered fuses with the downstream leakage vortex,thus resulting in the chaotic wake mode and the influence of multi-frequency on the flow direction vibration.Starting from Ur between 7 and 8,the separation point gradually moves backward with a flow spacing ratio of L=2.5D,and the upstream and downstream vortexes uniformly merge and fall off behind the downstream cylinder,so the wake vortex street presents a 2S mode.For each of four cylinder vibration system,the maximum amplitude of single column appears as Ur is 8,and the vibration trajectory forms of the cylinders of the upstream and downstream are similar.Similar to flow past body,the upstream vortex can fuse with the downstream vortex,and they are released behind the downstream column under the specific spacing ratio and specific flow rate,forming a vortex street with a reverse locking among four columns.