Fluent-Based Method For Identifying Aerodynamic Parameters Of Bridges

Advances in technology have led to rapid advances in computer performance,which has led to the use of CFD numerical simulation as an aid to bridge structural wind resistance.Compared to wind tunnel tests,CFD is able to identify the aerodynamic parameters of bridge sections quickly and efficiently,allowing a large number of calculations to be carried out simultaneously to investigate the internal regularities and mechanisms.For light weight,low damping bridge structures,wind induced vibrations are an important part of limiting the development of bridge spans,so it is necessary to study the aerodynamic parameters such as aerodynamic three-part force coefficients,chattering derivatives and vortex vibrations of bridge sections based on Fluent commercial software.In this paper,the following work is carried out based on Fluent:1)Aiming at the problem of static flow around bridge section,a two-dimensional grid model is established by using flat steel box girder.Realizable k-εturbulence model,RNG k-εturbulence model,standard k-εturbulence model,standard k-ωturbulence model and SST k-ωturbulence model are adopted.These five turbulence models are selected to solve the static force coefficients at different angles of attack,and the SST k-ωturbulence model is the best by comparing the error with the wind tunnel test value and calculating efficiency.The standard k-ωturbulence model has the highest computational efficiency and the second simulation accuracy.The standard k-εturbulence model is the third in simulation accuracy and calculation efficiency.SST k–ωturbulence model,standard k–εturbulence model and standard k–ωturbulence model with good performance in flat steel box girder are selected to carry out numerical simulation of static force coefficients of blunt closed box girder at different attack angles.The results show that SST k–ωturbulence model has the highest simulation accuracy and efficiency.The above work can provide numerical wind tunnel reference for practical engineering.2)A two-dimensional mesh model of single rib arch and parallel rib arch with different D/B values is established to solve the structural stability problem of long-span arch bridge in the construction stage of parallel rib arch.The static three-component force coefficient under different wind attack angles is solved.The dimensionless aerodynamic interference factor is defined to discuss the variation of resistance coefficient C_D,lift coefficient C_L and moment coefficient C_M with D/B value and wind attack angle.The results show that the upstream rib arch C_D and C_L are strengthened by aerodynamic force,and C_M is weakened by aerodynamic force.The downstream rib arch is subjected to C_Daerodynamic weakening effect,and C_L aerodynamic weakening effect is not obvious.C_Mis not sensitive when D/B≥0.4,and the aerodynamic weakening effect is gradually strengthened when D/B>0.4.When C_D≤0.6,the trend of downstream rib arch is consistent with that of single rib arch.When C_D>0.6,the trend is opposite.C_L and C_M show more than 3 times difference compared with single rib arch.3)In view of the flutter aerodynamic instability of flat steel box girder,a two-dimensional grid model is established with an ideal plate.The flutter derivative identification method based on the sub-state is written into the UDF self-compiled program.The influence of the diffusion smoothing method and the dynamic grid method combined with linear elasticity and grid reconstruction on the flutter derivative identification accuracy is discussed.The results show that the diffusion smoothing method has better grid distortion,tearing and smoothness in the forced vibration process,and the identification accuracy is better.In the numerical wind tunnel,the influence of different turbulence parameters and forced vibration amplitude on the flutter derivative identification of flat steel box girder is discussed.The results show that different turbulence parameters have nothing to do with flutter derivative identification.The difference of different vibration amplitudes is sensitive to the identification of flutter derivatives.When the vertical bending amplitude h₀/B is between 0.024 and 0.073,the identification accuracy of flutter derivatives is high.When the torsional amplitude is between 1°and 3°,the identification accuracy is good,and when the torsional amplitude exceeds 3°,it is not good.It is suggested that the torsional vibration amplitude should be controlled within 3°.4)For the vortex-induced vibration phenomenon of flat steel box girder,the UDF of free vibration based on Runge-Kutta method is compiled.Taking the rectangular section as the verification example,the static flow around it and the vortex-induced vibration are solved.By comparing with the literature,the correctness of the program is verified.The influence of the initial excitation from B/100 to B/700 on the vortex amplitude was discussed.The results show that in a reasonable range,the larger the initial excitation is,the faster the structure can achieve vortex-induced resonance.It is recommended to use the initial excitation under B/300,which will speed up the calculation efficiency.In view of the structural vibration of a flat steel box girder caused by the temporary guardrail,the static flow around the main beam section is carried out,and the lifting coefficient is transformed by FFT,so as to find the locking range of the vortex-induced vibration of the main beam.The results show that the temporary fence of the structure is the direct reason for the increase of the amplitude of vortex-induced vibration,and the unidentified vibration of the structure is explained from the perspective of CFD.