Numerical Simulation On Influence Of Web Slopes Of Box Girder On Aerodynamic Characteristics

Since the streamlined box girder was applied to Severn Bridge, UK in 1966, it has been widely accepted by bridge designers for its high torsional stiffness, favorable aerodynamic characteristics and good economy. The majority of highway long-span bridges in China adopted the streamlined box girders in the last decade. Similar to long-span bridges with the traditional truss girders, flutter instability and buffeting of a bridge with box girders are always the dominant issues in the design. The other challenge of bridges with box girders is the risk of vortex-induced vibration, which is one of inherent features for box girders.The streamlined box girder is a bluff body by aerodynamic standards and causes flow to be separated. The smaller web’s angle (0) is, the more the section is close to a flat plate, the better the dynamic characteristics are. However, too small angle of web will cause other problems such as the requirement of structural detail and accommodation of equipment. Wind tunnel tests of the Fourth Nanjing Yangtze River Bridge and other three long-span bridges with box girders showed that critical flutter velocity would increase significantly when the slope of web is smaller than 16°.In this paper, the deck section of Fourth Nanjing Yangtze River Bridge is taken as engineering example to build numerical model with a geometric scale of 1:20. The influence of section shape on aerodynamic stability and the mechanism of VIV mitigation is exploited by numerical simulation. Total 16 slopes of web (i.e.90°,75°, 45°,35°,25°,23° and 21°～12° with incremental 1°) are studied.(1) To investigate the effect of web’s angle on aerostatic force, the two-dimensional averaged Navier-Stokes equations and SST k-co turbulent model are employed to investigate the flow field. The steady calculation was adopted. The web’s angle has a great influence on drag and life coefficients, while little influence on moment coefficients.(2) The forced vibration method is adopted in the numerical simulation of flutter derivatives, which assumes model with pure vertical bending or pure torsion movement respectively. The results show that the minimum values of the flutter derivatives H1* and A2* appear when the angle is 17°. And the flutter stability of the deck is improved significantly at the angle of 17°.(3)The turbulence method of two-dimensional large-eddy simulation is used to capture the vortices accurately. In order to simulate the moving of bridge deck in the fluid field better, dynamic mesh is also applied by compiling UDF program to realize the fluid-solid coupling. It can be shown from the figures that vortex separation is reduced apparently when the web’s angle is smaller than 16°. The suppression of the formation of vortices avoids vortex shedding excitation in great extent.