| Long span cable-stayed bridge and suspension bridge are important options when crossing obstacles such as canyons.They are sensitive to wind load,and the strong wind characteristics of construction site are the key factors to determine their wind-induced response.Compared with the wind field in coastal and plain areas,the wind field in mountainous areas has strong gust,high turbulence intensity,prominent non-stationary and complex spatial distribution.At present,there are many researches on the average wind characteristics of bridge site,especially the wind speed standard is studied by CFD numerical simulation method,but the research on the turbulence characteristics is less.Therefore,in this paper,the key technology of CFD simulation of complex wind field turbulence characteristics in bridge site is studied.On this basis,the wind field turbulence characteristics of a bridge site under the most unfavorable flow condition are simulated.Firstly,the sub grid stress model,the cut-off frequency of inlet turbulence sequence,the mesh size and the calculation time step are given in Chapter 2.The correctness of the method of "using CIRFG to generate the fluctuating sequence satisfying the turbulence characteristics of the target wind field,then superimposing the average wind speed profile to form the incoming wind speed,and attaching it to the inflow boundary by UDF program in FLUENT software" is verified.Then,in the third chapter,the flow characteristics of streamline of flow around Rankin body,hyperbolic tangent function and square sine function as the transition curve are compared and studied by numerical simulation method,and a new principle of boundary transition curve construction of terrain model is proposed.In the fourth chapter,a method is proposed to solve the problem of boundary step caused by terrain interception by piecewise fitting the terrain boundary curve and giving the analytical expression.Based on the numerical simulation of the wind field in a bridge site with different terrain range and different entrance boundary setting methods,the reasonable terrain range and the corresponding entrance boundary setting method in LES of complex wind field in bridge site are determined.Finally,the average wind characteristics and turbulent wind characteristics of the bridge site under the most unfavorable inflow conditions are studied.The results show that:(1)the cut-off frequency and calculation time step of the synthetic pulsation series at the entrance boundary be determined by the frequency range concerned in the wind resistance research and the grid size that can be divided.(2)The step problem caused by terrain interception can be solved by adding boundary transition section or fitting the boundary curve and giving analytical expression.(3)The following principles should be followed when constructing the transition curve of terrain model: first,the transition curve itself should satisfy the continuity of the second-order derivative function,and the first-order derivative function should satisfy the continuity at the connecting boundary;second,the slope of the transition curve should be evenly distributed to avoid large narrow-band peaks.(4)Fourier series or sine sum function should be used in terrain boundary curve fitting.(5)In the complex wind field simulation of bridge site,the terrain range of flow simulation should not be too small or too large,and the terrain range of 20 km square with the bridge site as the center should be adopted.When the transition section method is used to solve the problem of terrain steps,the terrain range to be simulated should be expanded;when the terrain boundary curve fitting method is used,the terrain range to be simulated can be appropriately reduced.(6)In the turbulent fluctuation time history of a certain point at the bridge site,the low-frequency component is mainly caused by the topographic fluctuation of the bridge site,and the turbulent fluctuation at the entrance boundary has a greater contribution to its high-frequency component. |
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