## 3 Result and Discussion
### Variation in Drag Coefficient with Separation Distance  
#### 3.1.1 Small SUV as the Following Vehicle  
As shown in Figure 2a, the fluctuations in the drag coefficient are consistent, without being considerably affected by the type of leading vehicle. When the separation distance is 1 m, the drag coefficient undergoes the most significant reduction; as the separation distance increases, the drag coefficient also increases. Furthermore, the drag coefficients under separation distances of 15 m and 20 m do not change significantly. This is likely because the overall height of the small SUV is less than those of the other leading vehicles; hence, it is not affected directly by the wake at separation distances of 15 m or more. Figure 2: Variation in drag coefficient with respect to the leading (front) vehicle type when the following (rear) vehicle is the (**a**) small sport utility vehicle (SUV), (**b**) medium SUV, (**c**) purpose-based vehicle (PBV), and (**d**) truck. 3.1.2. Medium SUV as the Following Vehicle  
As shown in Figure 2b, similar trends were observed for the drag coefficient, except when using the PBV as the leading vehicle. In these cases, such as when the leading vehicle is the small SUV, the change in the drag coefficient under separation distances of 15 m and 20 m was small owing to the effect of the intensification of the wake of the leading vehicle. 3.1.3. PBV as the Following Vehicle  
As shown in Figure 2c, compared to that for other vehicles, the reduction in the drag coefficient when using PBV as the following vehicle is not significant. However, owing to the curved surface of the PBV, the resulting stationary pressure at the front of the PBV is considerably low. A reduction in the separation distance results in a decrease in the air flow at the front of the PBV, thereby increasing the drag coefficient. In particular, the most significant increase in the drag coefficient under platooning was observed when the leading vehicle was the PBV. Furthermore, when the leading vehicle is the truck, the drag coefficient remains low owing to the low pressure generated in the wake, which was less than those for the other vehicles. 3.1.4. Truck as the Following Vehicle  
As shown in Figure 2d, there was no significant change in the drag coefficient, except when the leading vehicle was also a truck with a separation distance of 15 m. When the leading vehicle was the medium SUV, the drag coefficient exceeded that when the leading vehicle was the truck. It has affected the spoiler at the front of the truck by increasing the upwash from the rear of the vehicle, provided that the separation distance increased. Consequently, the intensification of the wake due to the upwash improved the effect of the spoiler, thereby reducing the drag coefficient. The wake of the PBV reduces drag coefficient due to the diffusion of low pressures; however, it was not affected significantly by the large projected front area of the truck. 3.2. Analysis of the Effect of Drag Coefficient on Separation Distance and Vehicle Type  
For different leading vehicles, the effects of the following vehicles on their drag coefficient with respect to the separation distance was determined. In general, the larger the area of the leading vehicle, the lower is the drag coefficient of the following vehicle; however, owing to the shape of the rear of the leading vehicle, different trends of the drag coefficient reduction can be observed when using the PBV and medium SUV. As shown in Figure 3c, compared to that when using the PBV alone, the drag coefficient varied by \(-40.23\%\) during the platooning between PBV and PBV with a separation distance of 1 m. As shown in Figure 3d, at a separation distance of 20 m between the medium SUV and truck, compared to that when using the truck alone, the drag coefficient was 8.5% lower. This resulted in a greater drag coefficient reduction because the rear of the leading vehicle was raised to create an upwash, and the wake acted on the front spoiler of the truck.