# Blockage Effects in Wind Tunnel Tests for Tall Buildings with Surrounding Buildings
## 3 Blockage Effects of Isolated Model Wind Tunnel Test
### Discuses about the Cause of the Drag Coefficient Difference
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**Research** & \(\mathbf{Re}\) & **Model Size** & **Tunnel Size** & **Blockage Ratio** & \(\mathbf{C_{D}}\) & **3D or 2D** \\ \hline Yang, 2021 [14] & 0.7–4.1 \(\times\) 10\({}^{5}\) & 0.2m \(\times\) 0.2 m & 3.0 m \(\times\) 3.0 m & 6.7\% & 2.10 & 2D model \\ \hline [PERSON], 2015 [15] & 1–5 \(\times\) 10\({}^{4}\) & 0.029 m \(\times\) 0.29 m & 0.6 m \(\times\) 0.8 m & 4.8\% & 2.10 & 2D model \\ \hline [PERSON], 2020 [16] & 1–4 \(\times\) 10\({}^{5}\) & 0.12 m \(\times\) 0.12 m & 2.2 m \(\times\) 2.0 m & 5.4\% & 2.30 & 2D model \\ \hline [PERSON], 2017 [17] & 0.7–4 \(\times\) 10\({}^{5}\) & 0.2 m \(\times\) 0.2 m & 3.0 m \(\times\) 3.0 m & 2.2\% & 2.21 & 2D model \\ \hline [PERSON], 1990 [18] & 2.2–4.4 \(\times\) 10\({}^{4}\) & 0.05 m \(\times\) 0.05 m & 1.0 m \(\times\) 0.7 m & 5.0\% & 2.05 & 2D model \\ \hline [PERSON], 2016 [19] & 1–4.8 \(\times\) 10\({}^{5}\) & 0.2 m \(\times\) 0.2 m & 3.0 m \(\times\) 2.5 m & 4.0\% & 2.15 & 2D model \\ \hline [PERSON], 2018 [20] & 3–4.5 \(\times\) 10\({}^{4}\) & 0.025 m \(\times\) 0.25 m & 0.3 m \(\times\) 0.3 m & 8.3\% & 2.20 & 2D model \\ \hline [PERSON], 2021 [21] & 4.1–8.2 \(\times\) 10\({}^{4}\) & 0.06 m \(\times\) 0.06 m & 0.45 m \(\times\) 0.45 m & 7.0\% & 1.81 & 3D model \\ \hline [PERSON], 1992 [22] & 2.7 \(\times\) 10\({}^{4}\) & 0.05 m \(\times\) 0.05 m & 1.2 m \(\times\) 1.0 m & 4.2\% & 2.00 & 2D model \\ \hline [PERSON], 1975 [23] & 1.76 \(\times\)10\({}^{5}\) & 0.165 m \(\times\) 0.165 m & 4.58 m \(\times\) 1.53 m & 3.6\% & 2.20 & 2D model \\ \hline [PERSON], 1977 [24] & 1.4 \(\times\) 10\({}^{6}\) & 0.102 m \(\times\) 0.102 m & 1.83 m \(\times\) 1.83 m & 5.6\% & 2.19 & 2D model \\ \hline [PERSON], 1987 [25] & 3.3 \(\times\) 10\({}^{4}\) & 0.038 m \(\times\) 0.038 m & 0.91 m \(\times\) 0.68 m & 4.1\% & 2.04 & 2D model \\ \hline [PERSON], 1987 [26] & 5.52 \(\times\) 10\({}^{4}\) & 0.042 m \(\times\) 0.042 m & 0.40 m \(\times\) 0.43 m & 9.8\% & 2.38 & 2D model \\ \hline [PERSON], 1993 [27] & 1.3 \(\times\) 10\({}^{4}\) & 0.02 m \(\times\) 0.02 m & 1.80 m \(\times\) 1.25 m & 4.7\% & 2.16 & 2D model \\ \hline [PERSON], 1999 [28] & 3 \(\times\) 10\({}^{4}\) & 0.05 m \(\times\) 0.05 m & 1.0 m \(\times\) 0.8 m & 5.0\% & 2.10 & 2D model \\ \hline [PERSON], 2008 [29] & 2 \(\times\) 10\({}^{4}\) & 0.03 m \(\times\) 0.03 m & 0.4 m \(\times\) 0.4 m & 7.0\% & 2.18 & 2D model \\ \hline [PERSON], 2011 [30] & 4.7 \(\times\) 10\({}^{4}\) & 0.042 m \(\times\) 0.042 m & 0.3 m \(\times\) 1.2 m & 3.5\% & 2.15 & 2D model \\ \hline [PERSON], 2011 [31] & 2.1 \(\times\) 10\({}^{4}\) & 0.02 m \(\times\) 0.02 m & 0.5 m \(\times\) 0.5 m & 4.0\% & 2.06 & 2D model \\ \hline [PERSON], 2006 [32] & 2.14 \(\times\) 10\({}^{4}\) & 0.04 m \(\times\) 0.04 m & 0.39 m \(\times\) 0.56 m & 7.0\% & 2.13 & 2D model \\ \hline [PERSON], 2010 [33] & 6 \(\times\) 10\({}^{4}\) & 0.03 m \(\times\) 0.03 m & 0.4 m \(\times\) 0.4 m & 7.8\% & 2.00 & 2D model \\ \hline [PERSON], 2000 [34] & 1.7 \(\times\) 10\({}^{4}\) & 0.025 m \(\times\) 0.025 & 0.4 m \(\times\) 0.4 m & 6.3\% & 2.13 & 2D model \\ \hline [PERSON], 1994 [35] & 3.4 \(\times\) 10\({}^{4}\) & 0.05 m \(\times\) 0.05 m & 1.0 m \(\times\) 0.6 m & 5.0\% & 2.20 & 2D model \\ \hline [PERSON], 2006 [36] & 0.3 \(\times\) 10\({}^{4}\) & 0.013 m \(\times\) 0.013 m & 0.6 m \(\times\) 0.6 m & 2.1\% & 2.00 & 2D model \\ \hline [PERSON] seen in Table 4, the drag coefficient results of the 2D model were much bigger than the 3D model. Regardless of whether it was a 2D or 3D square cylinder, if the blockage ratio is constant, the drag coefficient changes slightly when the Reynolds number is changed by changing the wind speed (see Figure 6). For example, the drag coefficients in many studies were all close to 2.1-2.2 [15; 17; 18; 23; 25; 28; 30; 31]. In contrast, the drag coefficient corresponding to different blockage ratios was quite different, and the drag coefficient increased with the increase in the blockage ratio (see Figure 7).