Numerical Simulation Of Bridge Aerodynamics Based On RANS/LES Hybrid Method

The wind resistance ability of bridge needs higher requirements due to the increasing bridge span.Over the years,wind tunnel test is a commonly used method in bridge wind engineering research.With the development of ultra-high-performance computers,numerical simulation technology shows great advantages,and has become another important method in wind resistance of bridges.Currently,research on bridge aerodynamics is mostly based on RANS and LES,there are some problems such as poor adaptability and high costs.The RANS/LES hybrid method effectively combines RANS and LES,it is superior to traditional wind tunnel test in terms of computational accuracy,efficiency,and costs.Applying the hybrid method to bridge aerodynamics research can avoid previous research shortcomings.The RANS/LES hybrid method has the ability to capture unsteady flow information,so it is a very useful tool to study aerodynamic admittance functions.However,the current aerodynamic admittance functions are identified mostly in wind tunnel test,and the integral scale is too small,it cannot be applied to real bridges.CFD can achieve large integration scale simulation similar to real bridges,so it can overcome many problems such as small integration scale identified in wind tunnel test.Based on the RANS/LES hybrid method,this paper conducts numerical simulation of bridge aerodynamics and aerodynamic admittance functions identified in large integration scales.The main research contents and conclusions are as follows:This paper first explores the adaptability of the RANS/LES hybrid method in some cross sections.The hybrid methods are SBES and IDDES,SAS method is used as a comparison.Three methods are applied to four cross sections in uniform flow field,including 5:1 rectangular section,streamlined box girder section,separated double box girder section,and steel concrete composite girder section.A comprehensive comparison is made from the average aerodynamic coefficients,surface pressure distributions,fluctuating pressure distributions,and flow field characteristics.The results show that the SBES method performs well in face of various flow phenomena,especially the fluctuating characteristics prediction is better than the other two methods,and is closer to the experimental values.The three-dimensional flow characteristics are also more obvious.The SAS method does not exhibit self-adaptability and shows twodimensional structural characteristics,possibly due to the excessive numerical viscosity that inhibits the fluctuating development.The IDDES method performs between SAS and SBES,it has the lowest computational efficiency because of too many parameters.In view of SBES method has better performance in flowing around sections,this method was combined with an improved turbulence synthesis method,and achieved large integration scale turbulence simulation similar to real bridge.In this flow field,three bridge sections aerodynamic admittances were identified and compared with the aerodynamic admittances identified in small integration scales and wind tunnel test.The results show that the aerodynamic admittances identified in large turbulent integral scale is greater than the results identified in small integral scale and wind tunnel test.It has obvious difference in the low frequency ranges.It means that applying the aerodynamic admittances identified in traditional small integral scale are used for real bridge buffeting analysis,the buffeting forces will be undervalued at the low frequency.