# Blockage Effects in Wind Tunnel Tests for Tall Buildings with Surrounding Buildings
## 6 Proposed Calculation Method of Equivalent Blockage Ratio
### Equivalent Blockage Ratio  
Based on the above analysis, the drag coefficient error caused by the blockage effect of the target pressure-measuring models can be calculated according to Equation (9), and the drag coefficient error caused by the blockage effect of the surrounding models can be calculated according to Equation (10); the added amount of these two values is the total error caused by the blockage effect. For the convenience of Equations (9) and (10), this paper proposes the concept of a equivalent blockage ratio. Taking the influence coefficient of the blockage effect of a target pressure-measuring model in Equation (9), which was 2.5, as the benchmark, the influence coefficient of the blockage effect of the surrounding models in Equation (10) was divided by 2.5, thereafter, the equation of the equivalent blockage ratio can be expressed as follows:  
\[R_{r}=R_{t}+1.52R_{ue}+1.72R_{le}+0.56R_{de}-0.56R_{ae}+1.28R_{uh}+1.48R_{lh}+0. 6R_{dh}-0.64R_{oe} \tag{11}\]  
Thus, the relationship between the equivalent blockage ratio \(R_{r}\) and \(\Delta C_{D}\) is as follows:  
\[\Delta C_{D}=2.5R_{r} \tag{12}\]  
Figure 20 shows the test results of \(\Delta C_{D}\) and the results calculated by Equation (12). It can be seen from Figure 20 that the results of Equation (12) are in agreement with the test data. As is known, the essential meaning of the blockage ratio is to evaluate the severity of the blockage effect to ensure the reliability of the wind tunnel test data. In this paper, the meaning of the equivalent blockage ratio by Equation (11) can directly serve for the wind tunnel tests of tall buildings. In other words, if the drag coefficient is taken as the control object, the calculation method of a blockage ratio should be the equivalent blockage ratio instead of the traditional method when determining the scale ratio for test models.  
As the actual wind tunnel test of high-rise buildings may be very different from the test conditions in this paper, the practical application method of Equation (11) and Table 8 is described as follows:  
For an actual wind tunnel test for a tall building with surrounding buildings, the drag coefficient error caused by the blockage effect of the target pressure-measuring model can be calculated easily, whose influence coefficient is 2.5, while the drag coefficient error caused by the surrounding buildings should be calculated according to the heights of the surrounding buildings. If the heights of the surrounding buildings are about 1/3 to 2/3 of the pressure-measuring building, the influence coefficient of the drag coefficient error can be approximately determined according to the 'half height' conditions in Table 8, and if the height of the surrounding buildings exceeds 2/3 of the height of the pressure-measuring building, the influence coefficient can be approximately determined according to 'equal height' conditions in Table 8. Of course, this is only an approximate method, and further research is necessary to cover other actual possible situations.  
Figure 20: The relationship between \(\Delta C_{D}\) and the equivalent blockage ratio.