| The phenomenon of Vortex-induced vibration(VIV)of blunt body structures can not only induce serious structure damage to the important components of aviation and ocean engineering,but also provide new clean energy thro ugh vibration energy harvester.Therefore,VIV is always one of the hot issues in scientific research whether from the perspective of vibration suppression or efficient use of vibration energy.Due to the complexity of fluid-structure interaction(FSI),the mechanisms of fluid-structure coupling is not mature so far.The purpose of this paper is to develop a simple numerical simulation method for solving FSI problems.In order to verify the feasibility and accuracy of method,numerical validations are carried out.The effects of inflow conditions,structural characteristics and flow interferences on VIV responses are comprehensively analyzed,and the related internal physical mechanisms are clearly elucidated.Firstly,a multiple relaxation time lattice Boltzmann flux solver is proposed for viscous flow.Based on the idea of coupled macroscopic equation and mesoscopic model,according to the expansion of multi-scale Chapman Enskog,the relationship between the variables and fluxes in incompressible Navier-Stokes equations and density distribution functions in lattice Boltzmann equation is established,which effectively realizes the unified solution of non-viscous flux and viscous flux and avoids solving the higher-order term of macroscopic flux.The accuracy of the method is verified by using the driven cavity flow at different Reynolds Numbers.Secondly,a newly developed loose coupling method,which combined boundary condition-enforced immersed boundary method with MRT-LBFS,is proposed based on the idea of non-conforming boundary for FSI problems.Due to bringing in the lattice model and immersed boundary method,fluid-structure coupling calculation can be implemented in a Cartesian grid,without generating the body-fitted mesh and using moving mesh technique.Therefore,the computational processis considerably simplified.In order to verify the validity and feasibility of this proposal method to solve FSI problems,flow past a stationary bluff body,rotationary cylinder and one-degree freedom VIV of a circular cylinder are simulated.The purpose of above research is to provide an effective and simple method for the further study of VIV of elliptical cylinders.Then,numerical simulations of flow past stationary elliptical cylinders at low Reynolds number are carried out.For single elliptical cylinder(0.7?A_R?1.5),numerical results show the force coefficients and Strouhal number decrease with the axes ratio increasing.And the wakes represent periodic shedding vortex structures.For two elliptical cylinders in tandem arrangement(0.7?A_R?1.5 and 2?L/D?11),the critical gap spacing increases with the increase of the axial ratio.The variation of the force coefficient of the upstream elliptical cylinder is the same as that of the single bluff body,which decreases with the increase of the axial ratio.When the gap spac-ing does not reach to critical value,the force of downstream conforms to the rule.When the critical spacing is reached,the rule is opposite.The flow pattern of the tandem system is divided into single blunt body mode,alternating reattachment mode and double shedding vortex mode.Next,numerical simulations of two-degree of freedom VIV of an elliptical cylinder are carried out.The axis ratio has a significant influence on the starting and stopping point of the lock-in region,response amplitude and force.The range of lock-in firstly increases and then decreases with the increase of axis ratio.The main frequency of vibration is the main frequency of lift coefficient,resonance occurs in the transverse direction,and the motion trajectory presents‘8'shape.The transverse amplitudes are 10~2 times as much as axial amplitudes.For low Reynolds number.Based on the evolution of transverse amplitude with time,different response branches are divided in detail,including desynchronization regime I,quasi-periodic initial branch,periodic initial branch,periodic lower branch,quasi-periodic lower branch,desynchronization regime II.The transverse maximum value is linearly de-creasing with A_R.At last,numerical simulations of VIV of two elliptical cylinders in tandem ar-rangement are carried out.Results reveal that fluid-induced vibration accelerates the process of wake shedding vortices formed by the shear layer in comparison with two stationary elliptical cylinders.As gap spacing increasing,the vibration characteris-tics and flow patterns of upstream elliptical cylinder are close to the VIV character-istics of single elliptical cylinder which possess the same axis ratio.Because the downstream bluff body is greatly affected by the upstream wakes,including shear layer attachment,shedding vortex alternating adhesion or effective preventing in-coming flow,the vibration characteristics and flow patterns of downstream elliptical cylinder are comlex.When the shedding vortex formed by the upstream flows through one side of the downstream cylinder,the vibration response of the downstream cylinder is small.When the shedding vortex formed by the upstream alternately adheres to the surface of the downstream cylinder,the vibration o f the downstream cylinder is enhanced.When the gap spacing and axis ratio are larger,the vibration response of the downstream cylinder is stronger. |
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