# Fast Prediction and Optimization of Building Wind Environment Using CFD and Deep Learning Method
## 3 Results and Discussions
### Flow Field Analysis  
In this paper, due to the consideration of multiple wind speed values, the non-dimensional velocity \(K\) is regarded as an indicator of the flow field conditions which is shown in Equation (9), where \(U\) represents the incoming wind speed, while \(U_{0}\) denotes the wind speed at a point in the building wind field.  
\[K=\frac{U}{U_{0}} \tag{9}\]  
Figure 8 displays the contour lines of non-dimensional velocity for four different building spacings and four different wind directions. It can be observed that, at a wind direction angle of \(0^{\circ}\), because of the obstruction of the buildings, airflow converges at the two windward corners of the buildings. Clear acceleration zones are formed on both sides of the buildings, and a low-velocity weak region formed behind the buildings. As the building spacing increases, the area of the acceleration zone between the two buildings (highlighted in the red mark) increases, and the length of the low-speed region behind the buildings also increases. At a wind direction angle of \(30^{\circ}\), significant wind acceleration zones are observed at the two relative corners of the buildings, and the area of the wind acceleration zone between the buildings increases with increasing building spacing. Observations indicate that a small localized low wind velocity zone is formed behind the upper structure, while a larger area of low wind velocity is formed behind the lower structure. At a wind direction angle of \(60^{\circ}\), the acceleration zones are primarily located at the lower corner of the upper building, with small acceleration zones observed at the upper corner of the lower building and between the two buildings. Similar to the case at a \(30^{\circ}\) angle, both the upper and lower structures create smaller and larger low wind velocity zones behind them. However, there is no significant trend observed in the acceleration zones with increasing building spacing,  
Figure 7: Comparison between CFD simulation and model predictions results of mean velocity.  
while the area of the low-speed region behind the upper building shows an increasing trend. At a wind direction angle of 90\({}^{\circ}\), similar to the previous cases, acceleration zones are observed at the two windward corners of the buildings, and a low-speed region is formed behind the buildings. It can be observed that the downstream buildings are essentially submerged in the wake of the upstream buildings. No significant changes are observed in the acceleration zones and low-speed regions as the building spacing increases.  
It can be observed in Figure 9 that as the wind speed increases, the flow characteristics in the same building layout also vary. At a wind direction angle of 0\({}^{\circ}\), compared to the flow field at lower wind speeds, a small region of increased velocity appears in the low-speed region behind the building in high-speed flow. Similarly, at a wind direction angle of 45\({}^{\circ}\), both low-speed regions behind the two buildings exhibit small regions of increased flow velocity. At a wind direction angle of 60\({}^{\circ}\), with the increase in flow velocity, it is found that, in addition to the generation of high-speed regions within the low-speed region behind the building, two additional high-speed regions are observed above the rear of the buildings. These phenomena may be attributed to the instability of the flow field caused by the increase in wind speed. At a wind direction angle of 90\({}^{\circ}\), apart from the increased length of the acceleration region at the windward corner of the building, as the incoming flow velocity increases, it can be observed that the width of the low-velocity region behind the downstream structures also increases, leading to an increase in the overall area, which is the cause of increased flow separation at the windward corners of the buildings.