Characteristics And Identification Of The Aerodynamic Admittance Of Bridge Sections Based On CFD And Active Wind Tunnel Test

With the constantly increasing length of long-span bridges,the wind-induced buffeting represents a problem of remarkable importance.The aerodynamic admittance functions are the key parameters in the calculation of long-span bridges buffeting response.For bluff bridge section,due to the complex effect of incoming flow and the difficulty in accurately control the inflow properties,the existing aerodynamic admittance recognition results often show large dispersion and low repeatability.The development of higher-precision admittance functions procedure has become an urgent problem need to be solved in the refinements study of long-span buffeting analysis.In this study,based on the CFD numerical simulations and active grids wind tunnel test,the controllable harmonic gusts and broad turbulence were simulated.Moreover,the admittance functions of rectangular cylinders with different aspect ratio and a twin-box steel deck with different gap-width ratio were studied.The reliability of the identified admittance function in the calculation of long-span bridges buffeting response were verified by the comparison with field measurements.In this dissertation,the main research contents are given as follows:(1)The controllable harmonic gusts and broad-band turbulence wind fields were accurately simulated in CFD domain.In particular,a new harmonic inflow theoretical model by introducing vortex number and cosine function was proposed.Such model respects the continuity Euler equations and the boundary conditions,thus solved the amplitude attenuation problem and can be used to well control the amplitude and simulate multi-frequency harmonic inflows in vertical and along-wind directions.Based on PRFG~3(Prescribed-wavevector Random Flow Generator based on Three-dimensional spectra)theory and Open FOAM software,a synthetic inflow turbulence procedure for Large Eddy Simulation(LES)was programmed.Such procedure can be used to well simulate the target turbulence properties including wind spectral,turbulence intensity and integral length scale.Moreover,its reliability is verified by the LES simulation of the measured anisotropic turbulent wind fields.(2)Based on the harmonic inflow theoretical model,the complex admittance function of rectangular cylinders with aspect ratio 4,6,10 and 16.67 were studied by means of URANS and LES.Moreover,focus on the rectangular cylinder with aspect ratio 4,the effect of input harmonic inflow parameters and motion states on the admittance function were analyzed.Meanwhile,the superposition of multi-frequency buffeting force and the superposition of buffeting force and self-excited force was discussed.(3)Based on the PRFG~3 procedure,firstly five homogenous isotropic turbulent inflows with different properties were synthetically generated and imposed on the inflow boundary of LES.Then the effect of turbulence intensity and turbulence integral length scale on the lift and pressure admittance of rectangular cylinder with aspect ratio6 were studied by means of LES.Especially,the probability density function weighted method was used to consider the nonlinear characteristic of lift coefficients.Results show that when the integral length scale is close to the section width,the lift admittance functions identified in broad-band turbulent inflow shows good agreement with the values in harmonic gusts.(4)An active grids experimental technique was developed to generate the vertical harmonic gusts at the wind tunnel.Based on the measurement of pressure and force section model wind tunnel test,the effect of active grids vibration frequency,vibration angle amplitude,mean wind attack angle as well as attached railings on the admittance function of rectangular cylinder with aspect ratio 6 were studied.Moreover,the reasons for the change of admittance function were explained by means of pressure coefficient distribution.Wind tunnel tests results show that a larger wind attack angle and active grids angle amplitude can reduce the lift admittance functions.(5)Through active grid wind tunnel test and CFD simulation,the admittance functions of twin-box steel girder with different gap-width ratio were studied.Especially,the capabilities of URANS and LES based simulation in the evaluation of the admittance function were investigated.Moreover,the changes of admittance function for twin-box girder in harmonic inflows and measured turbulent fields were analyzed.Results shows that the lift buffeting force of twin-box girder is mainly provided by the upstream box.The lift admittance functions are larger than Sears function and show less sensitive to the variation of inflow properties.(6)Based on the measured strong wind turbulence properties,the finite-element-based frame work for buffeting analysis of Xihoumen bridge were carried out in time-domain utilizing different admittance functions.The identified admittance function applied to the long-span buffeting analysis was verified by comparing the calculated results with the measured values.Moreover,the effects of turbulence properties and aerodynamic parameters on the buffeting response were analyzed,which can provide references for the buffeting design of long-span bridges.