# Computational Fluid Dynamics-Aided Simulation of Twisted Wind Flows in Boundary Layer Wind Tunnel
## 3 Preparation Stage
### Wind Tunnel Setup  
A series of tests were conducted in the Wind Tunnel Laboratory at the School of Civil Engineering, Chongqing University. The test section is 2.4 m \(\times\) 1.8 m (breadth \(\times\) height) in size, and the turntable is 2.0 m in diameter. Multiple passive devices were employed in the wind tunnel to generate the target TWF profile that follows the [PERSON] spiral model. Different combinations of spires and ground roughness elements, with the dimensions indicated in Figure 3, were available to generate the ABL wind profiles. In this study, the target profiles of wind speed and turbulence intensity are expressed as follows:  
\[u(z)=\left(\frac{u}{u_{g}}\right)^{\alpha} \tag{4}\]  
\[I_{u}(z)=\left(\frac{I_{u}}{I_{u,g}}\right)^{-\alpha-0.1} \tag{5}\]  
where \(u_{g}\) and \(I_{u,g}\) denote the longitudinal wind speed and turbulence intensity at the gradient height, respectively, and \(\alpha\) denotes the power-law exponent, which is taken as 0.12 in this study. On the other hand, the TWF was generated using the guide vane system shown in Figure 4. This system comprises two identical vanes made of 10 mm thick fiberglass molded into a single piece. Each guide vane is of a total height of 1.5 m and consists of two sections, as plotted in Figure 5, namely, a twist section ranging from ground level to 1.0 m in height and a transition section from 1.0 m to 1.5 m. The leading edge of the twist section is straight, whilst the trailing edge is curved to form a guide angle varying with height to redirect the approach flow. Since the Ekman spiral model has been selected in  
Figure 2: Comparison of common twist angle curve models. this study as discussed above, the trailing edge is curved following Equation (3) with a twist angle of 30\({}^{\circ}\) at ground level and 0\({}^{\circ}\) at a height of 1.0 m. The transition section is in the shape of a straight board to avoid altering the flow direction and to prevent the unfavorable eddies generated at its leading edge from reaching the downstream area. At the preparation stage, the two vanes were positioned as shown in Figure 6a, i.e., one vane was installed on the right side of the wind tunnel (as observed in Figure 4) with its trailing edge 850 mm from the side wall, and the other vane was installed on the left side with a separation of 900 mm between the vanes. Figure 4: Guide vanes for wind tunnel simulation of twisted wind flow. Figure 3: Dimensions of spires and ground roughness elements. To comprehensively measure the flow field generated in the wind tunnel, a Cobra probe (Turbulent Flow Instrumentation Pty. Ltd., Victoria, Australia) was deployed to measure the winds over five different locations within the model test region (i.e., turntable), indicated as Points A to E in Figure 6a. The winds were measured at 13 different heights  
Figure 5: Dimensions of guide vanes: (**a**) front view; (**b**) top view. Figure 6: Positions of guide vanes and measurement points: (**a**) plan view; (**b**) side view. ranging from 50 to 1050 mm, as shown in Figure 6b, i.e., the heights from 50 to 250 mm with intervals of 50 mm and from 250 to 1050 with intervals of 100 mm. By setting the geometric scale as 1:500, the above height range in the model scale corresponds to 25 to 525 m in full scale. The measurements were performed for 30 s at each height. The sampling frequency was set at 625 Hz, which is adequately high to capture the turbulence characteristics of the flow. To obtain accurate mean wind speed and direction profiles, the measurements were repeated three times at each height, and the average values were adopted. Figure 7 depicts the twist angle profiles measured over Points A to E. These profiles generally match each other, suggesting that the TWF generated by the vanes was distributed over the model test region in a uniform manner (i.e., flow speed and direction at the same height are similar within the region).