Investigation On Aerodynamic Admittance Of Bridge Decks By Large Eddy Simulation And Experiments

With the growing increase of the bridge main span,the structures are more sensitive to dynamics of natural wind,hence,investigateion on buffeting response of long-span bridge is of great importance.The effective identification of aerodynamic admittance function(AAF)plays a key role in reasonable estimation of buffeting response of bridge.The present studies focus on random generation of turbulent wind field,wind tunnel test of AAF of benchmark mo del,the large eddy simulation(LES)of flow around the bridge main girder,as well as the identification of the AAFs of typical bridge girders through LES modelling.The major works and corresponding conclusions are as follows.In order to generate turbulent wind field at inlet that satisfies the LES requirement for bridge wind engineering,the discretizing and synthesizing random flow generation(DSRFG)method is used,which is capable of producing anisotropic turbulent flow matching the target turbulent intensity,integral scale,wind spectrum and spatial coherence.Based on measured wind characteristics at bridge site of the Great Belt East Bridge,turbulent flow is generated at the inlet of computational domain on the scale level of sectional model of w ind tunnel tests.The reasonable value of major parameters,as used in producing turbulent flow by DSRFG is also studied.Based on self-developed User Define Functions in software Fluent,the generated turbulent wind is then imposed on the inflow boundary of computational domain,and the variation of the turbulent wind along the computational domain is investigated under consideration of various grid scales and different time step sizes.Results show that the turbulent wind field,which satisfies the LES requirement on turbulent characteristics of flow around a bridge girder,can be synthesized by DSRFG,and the statistical characteristics of both the spectra and velocity components agree well with the target values.Furthermore,the characteristics of inflow turbulence are reasonably maintained along the computational domain.Meanwhile,the simulated spectra of turbulent flow are in accordance with the target spectrum at lower frequency range,while it slightly decays at higher frequency band,which can be improved by employed finer grids and smaller time steps.As regards to the scale level of sectional model wind tunnel tests,a non-dimensional grid system of 0.02 between the inlet boundary and the model in computational domain,and a non-dimensional time step size of no more than 0.01 are suggested to well maintain the turbulence properties in the computational domain.Sectional model wind tunnel tests are carried out for the stiffening girder of the Great Belt East Bridge,under various wind speeds,typical wind angles of attack,as well as smooth flow and turbulent flow.The distributions of mean and RMS value of pressure coefficients on girder surface,as well as the sectional aerodynamic coefficients are presented.Their variation and vortex shedding pat tern,as presented by the effects of side and central barriers are also evaluated.Meanwhile,the frequency-domain characteristics and the spatial coherence of fluctuating pressure are also discussed.The results show that the distributions of mean and RMS value of pressure coefficients on the upper surface are significantly affected by the windward side barrier.Under the typical wind angle of attack,the bare girder presents only one vortex shedding St number,and it is significantly larger than that of g irder with barriers,in which complicated vortex shedding features,including difference in spanwise direction and multiple-orders vortex shedding,appears due to the attachment of deck barriers.The primary vortex shedding dominates the distribution of fluctuating pressure of the bare girder,while deck barriers weaken the coherence of fluctuating pressure on girder surface,and interrupt the vortex shedding around the girder in spanwise direction.The AAFs of the stiffening girder of the Great Belt East Br idge are investigated through wind tunnel tests,and the spanwise correlation of buffeting forces is discussed.The AAFs of the girder are identified by various methods,and the effectiveness of those methods is also evaluated.The effects of wind angles o f attack and deck barriers on the AAFs are also investigated.The results indicate that the wind angles of attack and deck barrier only show minor effects on the s panwise correlation.The integral length scale of drag is close to that of longitudinal velocity component,while the integral length scales of lift and moment are siginificantly larger than that of vertical velocity component.The colligated residue least square method of auto and crosss pectra(CRLSMACS)method can efficiently separate the contribution on the AAFs from the different velocity components,particularly,the reproduced spectra of buffeting forces based on the identified AAFs can effectively represent the spectral characteristics of buffeting forces acting on the bridge girder.Under the considered wind angles of attack,it does not present significant effects on the AAFs of girder.In addition,the AAFs of girder with barrier are slightly larger than that of bare girder.Based on the inflow turbulent flow generated by the DSRFG method,the flow field around three typical bridge girder sections,including the thin plate,the stiffening girder of the Great Belt East Bridge,as well as the main girde of the Danjiang Reservoir Bridge,are numerically solved by using the LES,and their AAFs are identified by the CRLSMACS method and were compared to each other.Fairly good agreement between the identified AAFs of the thin plate and the Sears function validates the feasibility of the LES in estimation of the AAFs.The simulated AAFs of the stiffening girder of the Great Belt East Bridge agree well with the test results,which also confirm the effectiveness of the present method.Since the flow pattern and pressure distributions around the?-shaped main girder of the Danjiang Reservoir Bridge girder are significantly different from those of around the thin plate and the stiffening girder of the Great Belt East Bridge,the computed||² and||² of the former is higher than those of the latter two.Consequently,it will be significantly overestimated or underestimated the buffeting response of long-span bridges if the Sears function is employed.The present study can provide important information to generate the turbulent wind at inlet and to identify the AAFs of bridge girders using the LES.