## 2 Materials and Methods
### Establishment of Geometric Model  
To conduct wind field research at the NSC based on real scenarios, it is necessary to establish a real geometric model of the NSC. The process of establishing the geometric model is shown in Figure 2. In addition, in order to show the semi-enclosed structure of the venue and the interaction of its indoor and outdoor air, Figure 3 was drawn. Figure 2: Schematic diagram of basic geometric model establishment. The NSC covers a total area of 18.7 hectares, and has a large span. The three-dimensional geometric model of the NSC established based on BIM technology is relatively complex and highly fragmented. However, CFD calculation simulation does not need to consider overly detailed architectural details, so the existing three-dimensional BIM model needs to be simplified. Considering the characteristics of the venue model, with a large span, a large area, a complex model, and a high degree of fragmentation, it is difficult to simulate the wind field and windward resistance of the entire venue and all track positions. To effectively reduce the amount of model calculation and improve the efficiency of the model simulation, this study extracted typical areas and typical track sections of the NSC for wind field simulation, and simulated and analyzed the windward resistance of typical track sections that might affect athletes' performance. Firstly, four typical areas were selected. The tracks and some buildings in these four areas are relatively complex in shape. They are key areas that directly affect the athletes' performance, including Starting area 1, the S-curve section, the 360-degree spiral curve section, and the end area (as shown in Figure 3). Starting area 1 is the starting area for bobsleigh, skeleton, and men's huge events [32]. Starting area 1 is where the push-start phase of the skeleton competition occurs. The time spent in this phase is one of the most important factors affecting the performance of the competition [33; 34]. Therefore, Starting area 1 was selected as the first typical area of the NSC, called Area I. The S-curve section is an important curve section of the NSC. The skiing level and skills at the S-curve are important factors affecting the athletes' performance. Therefore, this paper selects an S-curve section as the second typical area, called Area II. The 360-degree spiral curve section is a characteristic curve of the NSC. The NSC is the only bobsleigh track in the world with a 360-degree spiral bend, so the 360-degree spiral bend section was selected as the third typical area, called Area III. The end area is the uphill section of the track and the deceleration area for skiers. This is obviously different from other sections of the track, so the end area was selected as the fourth typical area, called Area IV. Secondly, the four typical areas were simplified. The detailed building components, such as window railings, internal equipment, and irrelevant internal walls, of the above four typical area models were removed. Then, CFD pre-processing was performed on the geometric models of the above typical areas to check and deal with their geometric errors. Thirdly, simplified human body models of athletes were arranged on the track of the four typical area models. The simplified human body models of the athletes were in a prone  
Figure 3: Interaction between indoor and outdoor airflow fields. position, which is the reference posture of the skeleton athlete.