# Blockage Effects in Wind Tunnel Tests for Tall Buildings with Surrounding Buildings
## 4 Blockage Effect of Grouped Models  
### When the Surrounding Model Is Half the Height of the Target Pressure Model  
In this section, the surrounding model is half the height of the target pressure model. According to the test results, the mean pressure coefficient of the windward surface is less affected by the blockage effect, and the relative error of pressure coefficient is generally less than 15%. However, when the surrounding models are located upstream of the target pressure model, the relative error of the mean wind pressure of the windward surface cannot be ignored. For example, the \(\Delta\mu_{layer}\) of the medium model of test condition 2 reached 30% (see Figure 9, which shows the layer pressure coefficient and relative error of the layer pressure coefficient of test condition 2). Similarly, the \(\Delta\mu_{layer}\) of the windward surface of the medium model of test condition 6 was close to 40% (see Figure 10, which shows the layer pressure coefficient and the relative error of the layer pressure coefficient of test condition 6). Obviously, this is because the wind velocity flowing through the upstream surrounding models is increased and this phenomenon, with the model creating a larger blockage ratio, is more obvious. The results were substantially different from the existing study results of the blockage effect of the single model. Figure 8: The drag coefficient varying with the blockage ratio. Figure 9: The wind tunnel test results of test condition 2. (**a**) Layer pressure coefficient; (**b**) relative error of layer pressure coefficient. Compared with the wind pressure coefficient of the windward surface, the blockage effect significantly affected the negative pressure area of the target pressure model including the lateral and leeward surfaces. Moreover, the phenomenon of the blockage effect of the negative pressure area is very interesting as it showed the following characteristics:  
The \(\Delta\mu_{layer}\) of the negative pressure area of test condition 1 was close to 30% (see Figure 11). Compared with test condition 1, two surrounding models were added upstream for test condition 2, and then the \(\Delta\mu_{layer}\) increased slightly (see Figure 9b, whose \(\Delta\mu_{layer}\) of the medium model was close to 35%). The resulting effect shows that when surrounding models are added in the upstream of the surrounding models at both sides of the target pressure model, the blockage effect increases slightly, even though the blockage ratio is constant. The surrounding models for test condition 3 were located upstream of the target pressure model (see Figure 12). The \(\Delta\mu_{layer}\) of the lateral and leeward surfaces of the medium model were about 20%, and the \(\Delta\mu_{layer}\) of the lateral and leeward surfaces of the large model was about 50%. In fact, the projection blockage ratio of test condition 3 was the same as that of test condition 1 and test condition 2 (see Figures 9 and 11), but the blockage effect was quite different because of the difference in the relative position of the surrounding models and the target pressure model. Figure 11: Wind tunnel test results of test condition 1. (**a**) Layer pressure coefficient; (**b**) relative error of layer pressure coefficient. Figure 10: The wind tunnel test results of test condition 6. (**a**) Layer pressure coefficient; (**b**) relative error of layer pressure coefficient. Compared with test condition 1, two surrounding models were added downstream in test condition 4 (see Figures 11 and 13). From the wind pressure results, there was little difference between the two test conditions. This result shows that when there are surrounding models at the sides of the target pressure model, the blockage effect caused by the surrounding model at the downstream of the side surrounding model can be ignored. Compared with test condition 1, two surrounding models were added at both sides of the target pressure model in test condition 5 (see Figures 11 and 14).