Fluid-structure Interactions Of Three Tandem Circular Cylinders

The growing energy demand caused by the rapid development of economy make how to make better use of the energy generated by waves and tides and exploit the rich mineral resources on the sea be done of the hotspots in current research.It involves a problem that needs to be discussed in depth: flow-induced vibration.In recent years,the Vortex-induced Vibration Aquatic Clean Energy(VIVACE),which can convert ocean energy into electrical energy by stimulating strong flow-induced vibration,has been widely concerned.However,the vibration of the important structural components in the deep-sea mining system,such as offshore platforms and submarine pipelines under the influence of the complex incoming flow is obviously unfavorable,which will influence the safety and durability of the structure to a certain extent.Based on the above background,the flow-induced vibration of three cylinders in tandem in planar shear flow is simulated numerically in this paper.The main research work is as follows: 1)The research on the single-degree-freedom flow-induced vibration responses of three circularcylinders in tandem in planar shear flow is carried out,and the influences of three keyparameters of the Reynolds number,shear ratio and reduced velocity on the dynamic response,fluid force characteristic,phase change,spectrum characteristic and wake mode of threecylinders in transverse direction are carefully analyzed.It is found that as the Reynolds numberincreases,the irregularity of the non-dimensional maximum amplitudes of three cylinders versusshear ratio and reduced velocity is significantly enhanced,and there are two peaks in theamplitude curve of the downstream cylinder when Re=160 and k=0.1.At different Reynoldsnumber and shear ratio cases,the influence of the incoming flow on the cylinders is graduallyweakened,which resulting in the mean value of the drag coefficient is reduced,additionally,therms value of the lift coefficient shows great volatility in the resonance region.As the Reynoldsnumber increases,the sag phenomenon after the “phase switch” of the phase difference betweenthe transverse displacement and lift coefficient of the midstream and downstream cylinderssubjected to the shear flow is more and more obvious.The increase of the Reynolds number andshear ratio will significantly increase the order of vibration frequency and vortex sheddingfrequency in the resonance region.In addition,except that the flow field exhibits regular 2S,P+S and 2P for the case of Re=100 and k=0.0,the distribution of the wake vortex of threecylinders is chaotic under other conditions.2)The research on the two-degrees-freedom flow-induced vibration responses of three circularcylinders in tandem in planar shear flow is carried out,and the influences of three keyparameters of the Reynolds number,shear ratio and reduced velocity on the dynamic response,frequency characteristic and motion orbit are carefully analyzed,and the dynamic mechanism ofhow the change of shear ratio of the incoming flow influencs the structural vibration response isrevealed.It is found that the dynamic response of three cylinders can be classified into twodifferent stages with the increasing of the Reynolds number.The in-line amplitude is moresensitive to the change of Reynolds number and shear ratio compared to the transverse one.Meanwhile,the increasing of the shear ratio causes the extension of the resonance region and theincrease of the maximum amplitude,and the dual resonance is observed.As the Reynoldsnumber and shear ratio increase,there is an obvious transform in the frequency valuecorresponding to the resonance phenomenon and in the vibration response mechanism in in-linedirection.The increase of the shear ratio makes the motion trajectory diverse,and thefigure-eight and irregular shapes transform into the raindrop,dual-raindrop,dual-eight andirregular shapes.Especially,the “2T” wake pattern is observed by analyzing the flow field.3)The comparative research of the flow-induced vibration of one and two degrees of freedom threecylinders in tandem in planar shear flow is carried out,focusing on the analysis of the effect ofthe vibration in the in-line direction on the transverse response of three cylinders for the case ofRe=160.It is found that for the case of different degrees of freedom,the trend of the amplitudeof three cylinders versus the shear ratio and reduced velocity is basically consisitent,and thenumerical difference is small,which attributed to the flow field characteristics under laminarflow are more stable compared to turbulent one.