## 4 Computational Fluid Dynamic (CFD)  
This section describes 2D computational fluid dynamics (CFD) calculation on the turbulence separation over the gable building model, carried out with the commercial code Fluent 16.1. The CFD gives an insight into the flow patterns that are difficult, and expensive to study using experimental methods. The choice of the method used is generally made based on the details of the flow to be obtained and the computing resources available [26]. The building model for the CFD calculation has a height of 1.36 m and a width of 0.74 m. The simulated streamlines are depicted in Figure 8. The flow-solver is based on the two-dimensional Reynolds-average Navier-Stokes equations (RANS), which formulate the principles of conservation of mass, momentum and energy in the form of partial differential equation. The shear stress transport (SST) \(k\)-\(\omega\) turbulence model was used in the simulation because this model could produce more accurate and reliable results [46]. The SST \(k\)-\(\omega\) model is also known to have reduced sensitivity to far field values of turbulence frequency, \(\omega\), and a more balanced performance for a wide range of flow types compared to other general purpose two equation model, as demonstrated by [PERSON] et al. [47]. The discretization is carried out with the first-order upwind scheme and the velocity-pressure flow field is determined using the SIMPLE model [48]. This is due to the computational efficiency and robustness in iterating the coupled parameters.  
A mesh independence study was carried out to determine the dependence of the flow field on the refinement of the mesh. Grid types on solution domain are performed near the building model with increasing intensity as shown in Figure 8b. The meshes are highly denser to capture the complex flow structure with lower expected error. To ensure attainment of grid-independent results, sensitivities of both grid numbers and grid distributions are tested and the mesh used is refined for the building model.  
Figure 8: (**a**) Velocity streamline for gable rooftop; and (**b**) 2 D mesh for the building model.  
The building is modeled with the dimensions 1.45 m (L) \(\times\) 0.74 m (W) \(\times\) 1.36 m (H), which is the same size as the prototype, and the pitch angle of the gable roof was 35\({}^{\circ}\) (see Section 5.2). The domain size for the CFD simulation is (H) 10,000 and (L) 28,000 mm. In the simulation, only wind velocity was considered. In this analysis, the pressure profile and multi-directional wind were not considered. Furthermore, the roughness level of the roof surface was ignored. In the simulation, the inlet wind speed was set as 4.5 m/s, this is to perform the simulation under the same condition as the experiment where the wind speed used for the actual test is 4.5 m/s. The simulated streamlines are depicted in Figure 8a.  
The boundary layer separates at the windward roof edge of the building and the flow forms a separation bubble on the roof (below the streamline above the roof). As a consequence, the velocity vector of the flow above the separation bubble makes an angle with the horizontal edge of the roof (Figure 8a). This angle is referred to as the skewed angle. The skewed angle is largest at the windward roof edge and decreases downwind [33]. From the simulation, the wind velocity along the centre line of the roof at \(Y\) = 100, 150, 200 and 250 mm was found to be 5.36, 5.8, 5.95 and 5.97 m/s respectively, it shows that the wind speed achieved high velocity at this points due to the accelerating effect on the building and comparing with the inlet wind speed, 4.5 m/s, the wind velocity at the above mention height of the CAWT increases by 19%, 29%, 32%, and 33% with a wind speed augmented factor \(f\) = 1.19, 1.28, 1.32, and 1.33, respectively.