# Wind Field Characteristics of Complex Terrain Based on Experimental and Numerical Investigation
## 1 Introduction  
In recent years, renewable energy has played an important role in environmental and climate governance by relying on its environmental protection characteristics, and the proportion of wind energy as a member of renewable energy has gradually increased [1]. Wind energy has great potential in coastal islands, which can provide energy supply for local production and life. In order to obtain wind energy, it is necessary to build specific wind energy equipment, such as windmills or wind farms. In general, wind farms will be built in areas where there is a great deal of wind energy. However, these landforms often have the characteristics of high wind attack angle and increased turbulence [2]. The atmospheric boundary layer will become more complex due to the non-uniformity of the terrain. The interaction with the wind turbine will lead to more complex flowfield characteristics, and even shorten the service life of the equipment [3,4]. In addition, the complex terrain has a major impact on the wind characteristics, and the load on the independent wind turbine or the wind turbine in the wind farm is determined by the terrain [5]. Considering the safety of wind turbines and other equipment, accurate prediction of wind fields is the main work. In the early stage of wind field research, a large number of theoretical analysis models were verified by two-dimensional and three-dimensional simple Hill models [6,7]. Many scholars focused on predicting the position and size of the maximum acceleration factor [8], and later researchers developed a series of models considering roughness or other factors on the basis of the original model [9, 10, 11]. However, these theoretical models are often not applicable on mountains with large slopes, which will produce significant errors [12]. In this case, researchers often use numerical simulation and wind tunnel tests to explore the wind field on simple or real complex terrain. Studying the wind field of a simple hill can better reveal the mechanism and easily verify its accuracy. Some good results can be obtained by using the Reynolds-Averaged Navier-Stokes (RANS) turbulence model to study two-dimensional or three-dimensional simple hills [13,14]. However, the simulation results of fluid separation areas such as a leeward slope will be quite different, and the results obtained by different RANS models are also very different [15, 16, 17]. The results obtained by the large eddy simulation (LES) model are more accurate. [PERSON], [PERSON] and [PERSON] [18,19] used the LES method to simulate the air flow on a two-dimensional steep slope, and the numerical simulation results with the wind tunnel test results were compared. The comparison results show that the LES model can accurately simulate the average wind speed distribution on the two-dimensional steep slope. It is worth noting that the wake behind the mountain can be accurately simulated by LES, and the mountain flow separation areas with different smooth and rough surfaces can be given [20,21]. [PERSON] [22] studied the average velocity distribution and turbulence characteristics of airflow passing through smooth two-dimensional mountains and three-dimensional mountains through a large eddy simulation. The results show that LES can well predict the separation point and reattachment point, which is in good agreement with the experimental results. Then they carried out the research on the flow field on the rough three-dimensional hill. Different from other studies, the research gave the three-dimensional characteristics of the wind field, and the results showed that the three-dimensional characteristics of LES were in good agreement with the wind tunnel test [23]. It was found that when the air flow passes through the two-dimensional slope, the turbulence characteristics on the leeward side of the mountain change significantly using the wind tunnel tests [24,25]. The acceleration factor of the mountain with a rough surface is greater than that of the mountain with a smooth surface at the top.