# A Fluid-Structure Coupling Analysis of a Far-Field Flat Mirror for AliCPT-1 Telescope Calibration
## 3 Fluid Field Calculation
### Boundary Conditions  
Ali is an area characterized by harsh natural conditions in Tibet, falling under a high-altitude frigid monsoon climate. The AliCPT-1 observatory site experiences frequent strong winds, with approximately 115 windy days per year. The extreme wind speed is 53 m/s from the southwest, so the installation of a windshield reduces the wind speed to less than 10 m/s in the direction of 220\({}^{\circ}\) to 315\({}^{\circ}\). An on-site meteorological station is installed on the rooftop of the observatory building to monitor weather conditions. This station is positioned near the installation site of the FFF mirror. After the installation of the windbreak, the flow field was analyzed. When the wind blows toward the windshield, the wind speed at the meteorological station is 12.1 m/s, while at the location of the FFF mirror, it is 12.8 m/s. Therefore, it can be concluded that the wind speeds at the two locations are similar and that the wind speeds at the meteorological station can be used. Observational data were collected from November 2019 to July 2020, with a specific focus on daily instantaneous wind speeds and wind directions, as shown in Figure 9. From these data, it was determined that the max wind speed reached 17.5 m/s, with a prevailing wind direction from the northwest. The FFF mirror experienced wind from the forward direction at a bearing of 104\({}^{\circ}\) with a maximum wind speed of 7.5 m/s. In contrast, wind from the back direction occurs at a bearing of 290\({}^{\circ}\) with a maximum wind speed of 5.2 m/s. A cumulative histogram shown in Figure 9 demonstrated that wind speeds most frequently fell within the range of 6 to 14 m/s, accounting for approximately 90% of the total occurrences. We take a maximum wind speed of 17.5 m/s as an extreme wind load to analyze the wind pressure on the reflector. Regarding boundary conditions within the computational  
Figure 8: Optimized mesh of the simulation model. Figure 9: The wind speed data statistics. (**a**) Wind rose chart depicting wind speed magnitude and direction. (**b**) Cumulative histogram illustrating the occurrence of wind speed magnitudes. domain, the fluid inlet velocity was set to 17.5 m/s, the fluid outlet pressure was set to 0 Pa, and we specified the no-slip boundary conditions on the surrounding walls. Currently, the wall approach primarily utilizes the function approach and the viscous sublayer resolving approach. Wall functions are based on the law of the wall, valid for attached flow with favorable or zero pressure gradient and for relatively simple flow geometries such as flat plate or channel flows. The viscous sublayer resolving approach uses a highly precise grid to resolve steep profiles within the viscous sublayer. This approach is more accurate for heat transfer simulations, natural convection, cases with flow impingement, and cases requiring accurate prediction of skin friction. Since the FFF mainly involves natural convection and flow impingement, this study employs the viscous sublayer resolving approach within the wall approach. In Fluent, the k-\(\epsilon\) model is selected, and the near-wall treatment is set to enhanced wall treatment. Fluent automatically selects the viscous sublayer resolving approach. Mesh resolution requirements are expressed in terms of y+. By reviewing the analysis results of the reflective mirror, it is found that the y+ value is 19.9, which falls within the range of 10-110 as required. Upon reviewing the analysis results of the reflector, the y+ value of 19.9 falls within the range of 10-110, which is considered acceptable. A realizable k-\(\epsilon\) turbulence model was employed for turbulent simulation, with model constants set to C\({}_{2}\epsilon\) = 1.9, \(\alpha_{k}\) = 1, and \(\sigma_{\epsilon}\) = 1.2. For the inlet boundary conditions for the turbulence model, the specification method employs the intensity and viscosity ratio method.