# The Spatial Structure of Passively Simulated Atmospheric Boundary Layer Turbulence
## 3 Experimental Setup  
All the tests were conducted in a closed circuit-type boundary layer wind tunnel with a 22.5 (width) \(\times\) 4.5 m (height) test section (XNJD-3). The traditional passive method was used to simulate the atmospheric boundary layer (i.e., upstream spires and the surface roughness on the wind tunnel floor). This study simulated three types of typical atmospheric boundary layer flows, where BL1, BL2, and BL3 denote the ground categories over open country terrain, suburban areas, and urban landforms, respectively (GB50009-2012, Architectural Industry Press of China [36]).  
Near the station where the measurements were to be taken, a rounded rod was fixed across the tunnel to support two Cobra Probes for lateral coherence tests. Cobra Probes are a type of wind speed measurement instrument that is frequently used in wind tunnel testing applications, where airflow conditions are intentionally and regularly changed. To increase the spacing combination number along with the lateral separation distance, seven observation points were established. The lateral separation between two adjacent points ranged from 0.05 to 0.45 m, with the greatest separation distance reaching 1.3 m. The effects of height on lateral correlation were investigated using three different observation points at 0.6 m, 0.9 m, and 1.2 m in height. Eleven measurement points were marked along the height in the vertical direction, with vertical spacing ranging from 0.03 to 1.62 m. The simulation and test of the turbulent wind field are shown in Figure 1. With point 1# taken as the starting point, the measuring point layout is shown in Table 2.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline
**Probe Layout** & **1\#** & **2\#** & **3\#** & **4\#** & **5\#** & **6\#** & **7\#** & **8\#** & **9\#** & **10\#** & **11\#** \\ \hline Lateral direction & 0 & 0.45 & 0.6 & 0.65 & 0.73 & 0.95 & 1.3 & - & - & - & - \\ Vertical direction & 0 & 0.05 & 0.25 & 0.48 & 0.71 & 0.91 & 0.96 & 1.11 & 1.14 & 1.31 & 1.62 \\ \hline \hline \end{tabular} \end{The flow field characteristics were calibrated by a Cobra Probe, and the wind fluctuations in three directions were measured simultaneously. A frequency response of more than 2000 Hz indicates that the Cobra Probe is stable and reliable when measuring turbulence. The wind velocity measurement uncertainty was about 0.18 m/s, the Cobra Probe was in good condition, and the test method was reasonable. The sampling frequency of turbulence in this study was 256 Hz, and the sampling period was 120 s, allowing the data length to be up to 30,720 and meeting the requirements of the coherence analysis [18; 23; 26; 31; 35]. The measured average wind velocity (\(U_{2}\)) and the turbulence intensities (\(I_{u}\)) of three incident turbulences at various heights are shown in Figure 2, where \(z_{0}\) is the reference height, \(U_{0}\) is the corresponding average wind velocity, and \(z\) is the height of the probe. It is demonstrated that the exponent model can reasonably fit the profile of the average wind velocity. Because of the large size of the test section, a larger-scale wind field can be simulated, and the turbulence characteristics will be closer to atmospheric boundary layer turbulence, which has certain advantages in analyzing the turbulent spatial structure.  
Figure 1: Turbulent wind field simulation and testing.  
Figure 2: Average wind velocity and turbulence intensity profiles.