Numerical Simulation Of Flow Characteristic And Wall Effect With Oscillating Cylinder Based On Lattice Boltzmann Method With Immersed Boundary Method

Lattice Boltzmann Method is a kind of mesoscopic research approach with new fluid system modeling and analysis.Compared with finite volume method,it is easy to solve the problem with Lattice Boltzmann Method.The complicated mesh and several interpolations are not needed,and so the precision and efficiency can be improved.The shortcoming of this method is that it is blurred in boundary processing,and Immersed Boundary Method has advantage in the boundary processing.Therefore,the coupling method,IB-LBM,is used in this paper.And to increase the numerical stability and improve the accuracy of calculation when the Reynolds number increases,IB-LBM is optimized combined with LES.The flow near the wall is one of the important problems in naval architecture and marine engineering,many engineering structures involve this problem,such as the interaction between subsea pipeline and seabed,the study of propeller-ship interaction and so on,thereinto,subsea pipeline is the most typical problem.In the influence of waves and uneven seabed,pipeline always arises above the seabed.In the coupled influence of seabed wall and vortex shedding from the pipeline,pipeline bears complex unstable forces,vortex-induced vibrations may occur,which may easily cause fatigue damage and affect service life.The research of the flow near the wall can deeply understand the forces and flow field of pipeline and provide theoretical evidence and technical foundation for the laying of pipeline,fatigue damage and safe assessment,therefore,it has high engineering value and realistic meaning.Firstly,the effect of oscillation and rotating on flow field and force without the wall is studied.It is proved that the IB-LBM used in this paper has advantage over the solution of the moving boundary and the accuracy is guaranteed.The results show that change of amplitude and frequency can realize the mutual conversion of 2S,2P,and P+S modes.Oscillating amplitude can affect the three-dimensionality of the flow field and change the range of frequency lock.The effect of oscillation on forces is analyzed and the turning point is found.Rotating effect can change the characteristics of positive and negative vortex shedding from the cylinder and make the direction of vortex shedding have included angle with the horizontal direction.With the increase of rotating rate,the vortex shedding is suppressed,meanwhile,the stagnation point moves upward.Reynolds number can also change the direction of vortex shedding,with the increase of Reynolds number,the three-dimensional characteristic becomes more obvious,the flow field becomes complex,and the drag coefficient and lift coefficient have different change rulesThen,the numerical simulation about cylinder in the still and oscillating state near the wall is conducted.The effect of boundary layer thickness on the flow is analyzed and the thickness in which the flow can reach stable state is acquired.The effect of gap ratio on the flow field and forces is analyzed and the results show that vortex shedding suppression can be found when the gap ratio is about 0.2,and oscillation can promote the vortex shedding to a certain degree.Meanwhile,the wall affects the oscillation of the cylinder and change the range of frequency lock.Finally,the flow field and forces of two tandem cylinders near the wall is analyzed.The effect of gap ratio and pitch ratio is studied,compared with the two parts above,the oscillation effect,wall effect and coupling effect are analyzed.The results show that the upstream cylinder has similar flow field to the single cylinder,and the downstream cylinder has certain difference as the block of upstream cylinder.Gap ratio and pitch ratio limit each other in the effect of vortex shedding,and this will disappear when the gap ratio is up to 0.4 There is obvious difference of the forces between upstream and downstream cylinder and the drag of downstream cylinder is negative in some conditions.Oscillation promotes the vortex shedding and weakens the effect of pitch ratio.The oscillating upstream cylinder leads to the increase of pressure,drag and lift,and the effect of lift is much larger than that of drag.