Study On Wind Characteristics At Bridge Site In A Deep-Cutting Gorge By Wind Tunnel Test And CFD Method

Wind characteristics at bridge site are the prerequisites to the bridge wind-resistent research, and the uncertainty of wind parameters is the most important factor to influence the accuracy of the bridge wind-resistent research. Long-span bridges located in the mountain area typically straddle the deep-cutting gorge, however, the understanding of strong wind characteristics above the complex terrain such as the deep-cutting gorge is still very superficial. Therefore, the research on wind characteristics above the complex terrain becomes the key issues and leads the research direction in the wind engineering community. In this dissertation, the understanding of the wind characteristics at the bridge site in a deep-cutting gorge is taken as the goal. The wind characteristics above the deep-cutting gorge and around the bridge main beam and tower are studied in detail, mainly by the wind tunnel test and the CFD numerical simulation methods.In addition, the theoretical analysis and surveying data from the field measurement are also considered.Firstly, based on the SST k-ω turbulence model which performs well in the problem of flow around the bluff bodies, a numerical simulation method to simulate the equilibrium atmospheric boundary layer was proposed. It was validated that the accuracy of the actual numerical simulation was greatly improved based on the already achieved results of the equilibrium atmospheric boundary layer, which laid the foundations for the numerical simulation on the wind characteristics above the ideal gorge terrain with high accuracy. To study the boundary transition section shape of the terrain model for the bridge site in the mountainous gorge terrain, one kind of transition section curves was theoretically derived, and a three-dimensional gradual transition section was developed for the actual complex terrain model. Secondly, to discover the universal laws of the wind characteristics above the gorge, the wind characteristics above an ideal gorge terrain including the mean wind speed, turbulence intensity, integral length scale and power spectra were studied by wind tunnel tests. The effects of different incoming flow fields and different incoming flow directions on the wind characteristics above the ideal gorge terrain were analyzed. To solve the problem that the inlet boundary conditions can not be precisely given in the numerical simulation, the method of changing the coefficient of inlet turbulence kinetic energy to a variable was proposed. Then the effects of different angles and different heights of the ideal gorge terrain on its wind characteristics were further investigated. Finally, the Longjiang Bridge which straddles the deep-cutting gorge is employed as a typical example, and the terrain model centered on the bridge site, with a diameter of 15km, was made in the wind tunnel using a 1/1000 scale. Then the effects of different incoming flow fields and different incoming flow directions on the wind characteristics around the bridge main beam and tower were studied. The variation law of wind characteristics along the deep-cutting gorge were analyzed, and the universal laws of the wind characteristics above the ideal gorge terrain and the particular laws of the wind characteristics above the actual deep-cutting gorge terrain were discussed.The results on the wind characteristics above the ideal gorge terrain by the wind tunnel test and the CFD numerical simulations show that when compared with the incoming flow, the wind speed in the gorge is larger, but the turbulence intensity and mean square deviations of wind speed become smaller. In addition, the power spectra values in the low frequency part decrease, but keep mostly unchangeable in the high frequency part. The incoming flow turbulence can increase the wind speed in the gorge and when the angle between the incoming flow direction and the gorge longitudinal direction is within 10°, the wind characteristics above the gorge are almost invariant. The angle of the gorge has remarkable effects on its wind characteristics, while the height of the gorge has little effects on them. Wind tunnel test studies on the wind characteristics above the actual deep-cutting gorge terrain shows that the wind characteristics around the bridge main beam are different from those around the bridge tower. The relationships of the mean wind speed, turbulence intensity and mean square deviations of wind speed in and outside the deep-cutting gorge terrain are similar to those in and outside the ideal gorge terrain. Furthermore, the relationships between the power spectra in the deep-cutting gorge terrain or on the mid-span and the incoming flow are almost the same as those in the ideal gorge terrain and the corresponding incoming flow. The incoming flow turbulence can increase the wind speed at the heights near the main beam. For the heights near the ground, the effects of different incoming flow directions on the wind characteristics above the deep-cutting gorge terrain are similar to those on the wind characteristics above the ideal gorge terrain. However, for the heights far from the ground, the effects of different incoming flow directions on the wind characteristics above the two kinds of gorge terrains are different.