# Influence of Porosity on Drag Coefficient of Perforated Road Signs
## 3 Results and Discussion  
### Drag Coefficient of Perforated Road Signs According to the Porosity  
To calculate the drag coefficient of the perforated road sign with a porosity less than 30%, CFD analysis was conducted on perforated road signs with porosities of 0, 5, 10, 20, and 30%. The wind pressure and wind speed distribution of the perforated road sign according to the porosity is shown in the following figures. Figures 6 and 7 show the results at a wind speed of 10 m/s when the aspect ratio was 2.0 and the clearance ratio was 0.5.  
region also decreased. This phenomenon can be attributed to the fact that the pores in the perforated road sign affect the pressure and wind speed by generating airflow around the perforated road sign. If the area of this separation region decreases, the drag coefficient decreases. This correlation between the area of the separation region and the drag coefficient was verified through a study by [PERSON] et al. [27]. The CFD analysis results showed that the drag coefficient decreased as the porosity of the perforated road sign increased. In Figure 7, the wind speed distribution around the perforated sign can be identified by contour. The wind pressure difference between the front and rear of the road sign was the lowest in the perforated road sign with a porosity of 30%.  
Figure 8 shows the drag coefficient of the perforated road sign according to the porosity. Although the drag coefficient of the perforated road sign differed with changes in the aspect and clearance ratios, the reduction in the drag coefficient of the perforated road sign according to the porosity exhibited a consistent trend. The drag coefficient decreased as the porosity increased, and it converged to 1.2 for the perforated road signs with a porosity of 20% or higher.  
The reduction effect of the drag coefficient of the perforated road sign according to the porosity was analyzed. The drag coefficient reduction ratio according to the porosity is expressed as the ratio of the drag coefficient of the road sign with the 0% porosity to that of the perforated road sign. Figure 9 shows the drag coefficient reduction effect of perforated road signs according to the porosity. The following characteristics were observed. First, for perforated road signs, the drag coefficient reduction effect increases as the porosity increases. Compared with the road sign with a porosity of 0%, the drag coefficient reduction effect was found to be 0.88, 0.86, 0.85 and 0.85 times for the road signs with a porosity of 5%, 10%, 20% and 30%, respectively. Second, the drag coefficient reduction effect of perforated road signs is more affected by the clearance ratio than the aspect ratio. Finally, the perforated road signs close to the ground have a higher drag coefficient reduction effect than those far from the ground. The drag coefficient reduction effect of the perforated road  
Figure 7: Wind speed distribution of perforated road sign by porosity.  
sign with a clearance ratio of 0.2 was analyzed to be 0.9 times, while that of the perforated road signs with clearance ratios of 0.3 and 0.5 was found to be 0.85 times that of the road sign with a porosity of 0%.