# Effect of Topography Truncation on Experimental Simulation of Flow over Complex Terrain
## 1 Introduction  
Flow over complex terrain remains a formidable issue that plays an important role in many practical applications, including wind power micrositing, pollution dispersion as well as wind loading on bridges and transmission towers. Flow separation and reattachment regions induced by topographies such as ridges, cliffs and exsperments not only lead to misconceptions about flow characteristics but also cause difficulties in wind tunnel testing and numerical simulation. Flow characteristics in a region close to a topographic feature and to what extent the flow is influenced by the terrain geometry are not exactly specific [1]. An in-depth understanding of complex turbulent flows is essential for greatly improving simulation accuracy, especially for numerical simulations including the large eddy simulation (LES) and the unsteady Reynolds-averaged Navier-Stokes (URANS). Significant effort has been devoted in recent years to gaining a better understanding of flow characteristics in complex terrains with wind tunnel testing [2, 3, 4, 5, 6]. Furthermore, numerical simulation can be compared to validate results [7]. [PERSON] et al. [8] investigated the effect of inflow conditions on the characteristics of flow over Bolund Hill. Bolund Hill isa well-studied test case for wind power misting and numerical simulations, as its steep geometry poses a challenge for numerical models to resolve [9]. [PERSON] et al. [10] studied the mean flow behavior and turbulence intensity when flowing over Bolund Hill under a round or sharp escarpment leading edge; wind energy production was also analyzed. To further study the unsteady flow behavior around Bolund Hill, a 1/115 scale model was employed in the wind tunnel experiment by [PERSON] et al. [11]. Several field measurements had also been carried out to verify the experimental and numerical models [12; 13; 14]. In addition, early studies such as Askervein Hill [15] and Kettles Hill [16] are classic cases for studying the characteristics of flow over complex terrain. The above-mentioned studies primarily focused on terrains such as regular independent hills, slopes and steep escarpments to reveal the mechanism of the flow over different terrains. There was little attention devoted to real complex terrains where the topography and geometry are more intricate. Due to the limitations of the wind tunnel test and of the numerical computational domain, complex terrain models should be truncated. The bottom of a terrain model is determined by the lowest elevation in the real terrain. Because the elevation differences of real topography such as deep-cutting gorges and valleys are often extreme, the edges of the model are higher than the bottom side of the wind tunnel and computational domain. This phenomenon, named \"artificial cliff\" [17], leads to flow separation and baffling flow at the leading edge, changing inflow conditions and inaccuracies in the test. Several methods have been proposed to address the \"artificial cliff\"[18; 19; 20]. For example, a transition section, which was used to connect the floor of the wind tunnel or numerical computational domain and the terrain model edge, was proposed to avert flow separation. There are various forms of transition sections, such as ramps, bicubic transition curves, and Witoszynski transition curves. [PERSON] and [PERSON] [21] implemented 3.2 mm thick masonite sheets at the model front and sides to smoothen the height transition from the wind tunnel floor to the model edge. [PERSON] et al. [22] used a ramp transition section with a slope of 5.7\({}^{\circ}\) to simulate the wind characteristics of a mountainous terrain by numerical simulation. Although the form of the ramp transition section is simple and easy to use, its applicability still needs to be verified. For topography with large elevation differences, the length of the ramp transition section may be too long to be suitable for a wind tunnel simulation because of the limitation of the test section.