*2.4. Flow Field and Boundary Conditions*

As shown in Figure 2, the flow field calculation domain is divided into the external static domain and internal rotation domain. The rotational domain is used to define the rotation of the wind turbine relative to the external domain under the action of the inflow wind. The origin of the coordinate system is located in the center of the hub. The rotational domain is a cylinder with a diameter of 140 m and a height of 8 m. Considering the vastness of the sea area, the calculation domain should be divided large enough to reduce the influence of the boundary on the calculation accuracy [35]. The external flow field is a combination of a hemisphere with a diameter of 6.3 R (R is 63 m radius of the wind turbine rotor) and a cuboid with a width of 90 m. The inlet distance is 3.2 R from the rotation domain, and the outlet distance is 8 R from the rotation domain.

**Figure 2.** Computational domain of wind turbine flow field: (**a**) external domain; (**b**) rotational domain.

Due to the nested motion in the blade rotation domain, it is necessary to use the embedded sliding mesh to define its rotation. The area where the inner and outer domains are in contact with each other is set as the interface. There is a relative slip between the interfaces and the flow field information is transmitted. The inlet and outlet of the external flow field are set as velocity inlet and pressure outlet, respectively. Additionally, the surface of the blade and the surrounding boundary of the external flow field are set as non-slip walls.

#### *2.5. Computational Mesh*

The internal and external flow field of the wind turbine is divided into unstructured grids by using MESH software. Figure 3 presents the grid generation in the overall computational domain. As the blade structure of the wind turbine model is complex and the tip position of the blade is too sharp, the grid size of the rotation region close to the blade needs to be set smaller, and the surface grid of the blade is further refined, which meets the requirements of the SST *k-w* turbulence model. As shown in Figure 3a, there are a total of 8.27 million grids in the whole flow field, of which the internal flow field grid is 6.21 million and the external field grid is 2.06 million.
