## 3 Results
### Flow Discharge for Isolated Spoiler  
Figure 7 shows the aerodynamic drag and downforce coefficients for the discharge positions of 0.25C, 0.50C, and 0.75C when Vb is 40% of the vehicle driving speed. For the discharge positions of 0.25C and 0.50C, the downforce is reduced; however, the aerodynamic drag is increased. In addition, it can be seen that discharging at 0.75C can reduce the drag and downforce by an average of 4.5% and 1.8%, respectively, in the speed range of 100-300 km/h. To analyze the effect of flow discharging on the vehicle aerodynamic performance, the surface pressure distributions for the various discharge positions at the vehicle driving speed of 300 km/h were compared. Figure 6: Comparison of the flow fields around the spoiler at various driving speeds: (**a**) Normalized velocity magnitude (**b**) Surface pressure distributions: Section A: Suction peak and pressure recovery section (before maximum thickness); Section B: Pressure recovery section; Section C: Flow separation section. Figure 8 shows the comparison of the surface pressure distributions for the three discharge positions when the vehicle speed is 300 km/h. In the case of discharging at 0.50C, a nearly constant pressure distribution appears after the discharge point, suggesting that flow separation has occurred. As a result, the suction peak is drastically decreased, and the pressure at the lower side of the spoiler increases dramatically, which indicates that the downforce is reduced. The pressure increase in Section B is favorable for drag reduction; however, the overall drag increases due to the decrease in the suction peak in Section A and the pressure decrease in Section C. Furthermore, when discharging at 0.25C and 0.75C, the surface pressure distributions are nearly similar to that without flow discharging. However, the pressure in the Section B is slightly increased, and the suction peak in the Section A is not large, which results in a slight increase in the drag and decrease in the downforce. In the case of discharging at 0.75C, the suction peak in Section A is slightly larger than that when discharging at 0.25C, resulting in a drag reduction effect. The effect of discharge direction was investigated by changing the discharge directions tangentially and normally to the vehicle driving direction. The discharge position was 0.75C, and the vehicle driving speed varied from 100-300 km/h. As shown in Figure 9, the aerodynamic drag and downforce were reduced in the case of tangential discharge. For the discharge in the vertical direction, flow separation appears immediately after the discharge position, and the pressure distribution is similar to that of the discharge at 0.50C, as shown in Figure 8.  
Figure 8: Comparison of surface pressure distributions for various discharge positions at U = 300 km/h and Vb/U = 0.4. Figure 7: Comparison of aerodynamic coefficients for various discharge positions at Vb/U = 0.4 (a) Aerodynamic drag coefficient; (b) Downforce coefficient. To investigate the effect of the discharge speed, a case study was performed by changing Vb in the range of 10% to 100% of the vehicle driving speed (U). As shown in Figure 10, the aerodynamic drag tended to decrease with increase in the discharge speed, resulting in a maximum drag reduction of 4.6% at Vb/U = 0.5, followed by an increase in the drag. The downforce also decreases with an increase in the discharge speed. After decreasing by 4.5% at Vb/U = 0.3, the downforce begins to increase. Figure 10: Effect of discharge velocity when the discharge position is 0.75C and the discharge is tangential to the vehicle driving direction; (**a**) Aerodynamic drag coefficient; (**b**) Downforce coefficient. Figure 9: Effect of discharge direction when the discharge position is 0.75C and Vb/U = 0.4. Comparison of (**a**) drag coefficient; (**b**) downforce; and (**c**) surface pressure distributions for flow discharge in the normal and tangential directions.