## 4 CFD Simulation  
### Geometrical Models and Numerical Methods  
Seven typical window types that are widely used in buildings were selected in this study: casement window opening inwards (CWI), casement window opening outwards (CWO), top-hung window opening inwards (THWO), bottom-hung window opening inwards (BHWI), bottom-hung window opening outwards (BHWO), and sliding window (SW). Figure 4 presents diagrams of the seven windows.  
At present, room models established by researchers in CFD simulations of natural ventilation are mostly classified into two types: the single-opening model and the double-opening model with windows on opposite walls. The first model is suitable for SV in this study, whereas the latter model is relatively rare in actual buildings. When the influence of wind direction on natural ventilation is considered, the windward and leeward sides cannot be distinguished because of the geometric symmetry. Therefore, a room model with a corridor was proposed for CV in this study. The layout of the model room was the same as that of the experimental room. As the wall thickness had a minor effect on the discharge coefficient of windows [20], and when the upper, lower, left, and right adjacent rooms open the windows, it has little impact on the indoor ventilation [36]. Therefore, according to the above similar studies, the wall thickness and adjacent rooms were not considered in the CFD models. The model conforms to a common office structure. It can distinguish the windward and leeward sides, and it avoids setting a complex model, which simplifies  
Figure 4: Diagrams of the seven windows: (**a**) CWI; (**b**) CWO; (**c**) THWI; (**d**) THWO; (**e**) BHWI; (**f**) BHWO; and (**g**) SW.  
the simulation process. Figure 5 shows the models of the two ventilation modes. The size of the model room is 8.1 m \(\times\) 4.8 m \(\times\) 2.7 m (L \(\times\) W \(\times\) H). The window-opening size of the room is 0.45 m \(\times\) 1.18 m (W \(\times\) H). In the CV model, the corridor size is 4.8 m \(\times\) 1.6 m (L \(\times\) W), the window-opening size of the corridor is the same as that of the room, and the door size is 0.9 m \(\times\) 2.34 m (W \(\times\) H). The outfield area was set outside the room. Referring to the relevant research [37], the distances from the left, right, front, rear, and top sides of the outfield to the corresponding surfaces of the room model were set as 6H, 6H, 3H, 3H, and 3H, respectively (H is the height of the model room).  
In this study, an industry-leading fluid simulation software, ANSYS Fluent [38], was used, which can accurately solve the widest range of CFD problems. Three-dimensional steady-state Reynolds-averaged Navier-Stokes models [39] were used because of their low requirements for computer memory space and computing speed. As the natural ventilation problem in this study belonged to a flow with a high Reynolds number and high shear rate, it was suitable to adopt the realisable k-\(\varepsilon\) turbulence model [40,41]. A pressure-based solver, which is applicable to the low-speed incompressible flow problem in this study and has a fast convergence speed, was adopted, and the SIMPLE algorithm was employed for the coupling of the pressure and velocity fields.