## 1 Introduction
[27] used an inclined boundary transition section (BTS) to connect the ground and the top of the terrain model. [PERSON] et al. [28] proposed a curved transition section for complex terrain and concluded that this kind of transition section has a better flow transition performance compared with the traditional inclined one. [PERSON] et al. [29] combined a terrain model with three-dimensional transition sections in the wind tunnel. [PERSON] et al. [30] evaluated speed-up ratios, mean vertical attack angles, and mean exceeding turbulence intensities of different transition curves to determine the optimal transition section for terrain models. [PERSON] et al. [31] established two different BTS in the computational domain for comparison purposes and the results show that the updated curved BTS has a better flow transition efficiency than those reported previously. [PERSON] et al. [32] proposed an improved BTS to modify the inlet boundary by combining the Witozinsky curve and straight line. With the development of computational fluid dynamics (CFD), many scholars ([PERSON] and [PERSON] [33]; [PERSON] et al. [34]; [PERSON] et al. [35]; [PERSON] et al. [10]; [PERSON] et al. [36] and [PERSON] et al. [37]) investigated wind characteristics over complex terrain with CFD method. [PERSON] et al. [26], [PERSON] et al. [38] and [PERSON] et al. [39] confirmed the availability of the \(k-\varepsilon\) model and RNG \(k-\varepsilon\) model. Although there is some research focusing on wind characteristics over complex terrain by CFD simulation, few studies on transition sections on the edge of terrain models have been found recently. Due to diversity and complexity of complex terrain, it is necessary to conduct further research on the BTS of mountainous terrain models. The Xiangjiang Bridge in Guizhou, China, was employed as an engineering example to study the wind characteristics at bridge site over complex terrain and effects of the improved BTS. The mid-span of the cable-stayed bridge was regarded as the origin. The terrain model with a geometric scale of 1:1500 (as suggested by [PERSON] [25]) was fabricated to simulate a region with a diameter of 8 km. Wind tunnel tests were conducted with and without the proposed BTS in order to investigate the BTS effects on wind characteristics ([PERSON] et al. [32]). To verify the feasibility of the improved BTS numerically, wind characteristics over the scaled mountainous terrain model with and without the proposed BTS were predicted by CFD simulation. The wind characteristics, such as mean cross-bridge wind speed and wind attack angles at the main deck level, longitudinal wind speed profiles at L/4, L/2 and 3L/4 of the bridge length were studied in detail to verify the accuracy of CFD simulation and effects of improved BTS. This paper is organized as follows: Section 2 introduces the numerical method, including governing equations of fluids, turbulence model, optimization of terrain model with BTS, the and numerical model. Section 3 describes the details of wind tunnel test setup. Section 4 investigates the cross-bridge wind speed ratios and wind attack angles at main deck level, longitudinal wind speed ratios profiles at L/4, L/2 and 3L/4 of bridge length of the terrain model with/without BTS under different cases. Section 5 presents conclusions of the study.