Study On Flow Around Cylinders In Equilateral Triangular Arrangements

With the development of architecture science and technology,the construction of high-rise buildings,sea-crossing bridges and other structures develops towards higher height and larger span.When natural disasters such as typhoons,giant waves and dust storms occur in the area where the above structures are located,the interaction between multiple column structures in the flow field will affect their safety and durability,which will cause damage to their main body and infrastructure,and cause human and economic losses.At the same time,the interaction between multiple column structures will also affect its safety and durability to a certain extent.Therefore,the in-depth study of the flow around the group-column structure has very important engineering significance.The lattice Boltzmann method,which is proposed as a meshless method,has distinctive advantage,e.g.,adaption and stability,in the simulation of the flow around a cylinder structure.In summary,based on the immersed boundary-lattice Boltzmann method(IB-LBM),this paper conducts a numerical analysis on the flow around three circular cylinders in equilateral triangular arrangements.The main research contents are as follows:(1)The basic equations,models and boundary conditions of IB-LBM are deduced.The multi-relaxation model and the force model are introduced.The basic theory of the IB-LBM is introduced based on the modified velocity method to solve the external force term.Based on the above numerical calculation method,the numerical verification of typical flow around a fixed single cylinder,a rotating single cylinder,two cylinders arrangement in tandem or side-by-side is carried out.By comparing the results of the cylinders force coefficient and the characteristics of the flow field with those of the existing literatures,accurate numerical results are obtained.It is proved that the IB-LBM used in this paper is suitable for solving the multi-cylinders flow problem.(2)The numerical simulation of flow around three fixed cylinders in equilateral triangle arrangements is carried out based on IB-LBM,the influences of Reynolds number(Re),attack angle of coming flow(?),spacing ratio(Kd)on the flow force coefficient,wake mode and spectrum characteristics of the cylinders were emphatically studied.The research results show that the wake morphology under different Reynolds numbers can be divided into single bluff body,double vortex shedding,the transition of double vortex to triple vortex shedding,and anti-phase triple vortex shedding,in-phase triple vortex shedding.With the increase of the spacing ratio,the wake mode changes from single bluff body mode to anti-phase or in-phase triple vortex shedding mode.The increase of Reynolds number accelerates the conversion process of the wake mode of the flow field from single bluff body mode to anti-phase or in-phase triple vortex shedding mode..When the wake mode of the flow field changes,the fluid force coefficient of the cylinders will have an abrupt change.The wake modes at different attack angles can be divided into single bluff body,double vortex shedding,the transition of double vortex to triple vortex shedding,and the triple vortex shedding.When the spacing ratio is relatively small,the wake modes of the flow field under each attack angle of the incoming flow present single bluff body and double vortex shedding respectively.With the increase of the spacing ratio,the wake flow of the flow field presents a transition mode from double vortex to triple vortex shedding,and the spacing ratio of the transition mode of the flow field will also change with different attack angles.When the spacing is relatively large,the flow field under the attack angle of incoming flow presents a triple vortex shedding mode.In the different attack angles cases,the critical spacing ratio of the flow field changes from the transition mode to the triple vortex shedding mode.(3)The numerical simulation of the flow around three rotating cylinders in equilateral triangle arrangements is carried out by using IB-LBM.The drag reduction effect of each cylinder is analyzed thoroughly in basis of distinct spacing ratio and attack angle,the internal mechanism of the rotation of the cylinders is analyzed by analyzing the flow force coefficient and flow field characteristics of the cylinders.The study found that when the spacing ratio Kd=1.2 and coming flow's attack angle??30~°,the wake state of the flow field will change from a regular single-row vortex street to a double-row vortex street,and finally an irregular tail flow status with the increase of the rotation speed.When?>30~°,the influence of attack angle of incoming flow on flow field is greater than rotation speed,and the wake of flow field in each condition presents regular two parallel vortex streets.When??30~°,the interaction between cylinder C1 and C2 will be stronger,and the strength of the vortex detached from the upper side of the cylinder C2 is significantly higher than that of the vortex shed from its inner side.When?>30~°,the interaction between cylinder C1 and C3 will be stronger.The counterclockwise rotation of the cylinder C2causes the vortex shedding of the cylinder C2 to be slightly affected by the rotation speed,and a strong vortex can still be formed at a higher rotation speed.When rotation speed??1.0,the vortices of cylinders C1 and C3 gradually disappear.When the distance ratio Kd?4.0,the mutual interference effect between the cylinders is weakened,and the vortex shedding of each cylinder approaches the single cylinder condition.Clockwise rotation is better than counterclockwise rotation in suppressing the shedding of cylindrical vortices.The inhibition effect of cylinder vortex shedding under?=0°cases is better than that under other inlet attack angle cases.