# Quantitative Analysis of Drag Reduction Methods for Blunt Shaped Automobiles
## 2 Literature Review
### Aerodynamic Drag Reduction
#### 2.3.3 Topology Modification
To solve this constantly changing parameter question, on a modified Ahmed body (curved rear part) [PERSON] et al. [91] attained DR = 12% with adjustable/motorized trapezoid shaped VGs. With his solution, the VG height question is solved for dynamic conditions, yet the optimal height, and the on-road boundary layer calculation method, still can be searched. For VG shape, the delta geometry was favoured in the work of [PERSON] et al. [92] over the bump shape. With a two-dimensional (2D) model, a sport utility vehicle (SUV) geometry was examined by [PERSON] et al. [93]. In their work, a delta shaped VG was placed on the top end of the vehicle. They concluded that when VG effect increased the velocity by 7.14 m s\({}^{-1}\), it reduced the \(\mathrm{C_{D}}\) by 10%. However, it has to be noted that for further conclusions, 3D and transient modelling is required. [PERSON] et al. [94] showed that delta shaped VGs at the top end of the SUV vehicle could reduce \(\mathrm{C_{D}}\) by 9.04%. It was also highlighted that, with the increase of velocity, the VG effects are enhanced. Figure 4: Delta shaped vortex generator types winglet (**a**); wing (**b**); flap (**c**) (based on [3; 78; 79]). [PERSON] et al. [95] examined different shapes of VGs. They estimated that when simulated values were compared to measurements (27% difference), it can be concluded that rectangular and triangular shapes are effective DR devices on notchback type vehicles. VG can be \"simple flaps\", as in [PERSON] et al. [96]'s work, it was applied at the rear end of an [PERSON] body, and on the side of the slant in the work by [PERSON] et al. [97]. [PERSON] et al. [78] examined different elliptic, rectangular, triangular shape flaps on a hatchback vehicle. Their work's main advantage is that three surfaces of the boat tail can be disregarded. They also examined the effect of a perforated surface; however, it did not show favourable results. [PERSON] et al. [98] used a vertical splitter plate at the rear and front of an [PERSON] body. Between the splitter plate and the vehicle wake region formed. The plate distance and height ratio were 0.9 in the favoured case; this was the highest examined ratio, which raises the question: do higher ratios increase DR or not? PIV measurements showed that in front of the plate the wake region almost disappeared. Between the splitter plate and the vehicle, the lower recirculation bubble was more dominant. Spoilers have similar effect as VGs, as [PERSON] et al. [19] and [PERSON] et al. [79] have presented; with validated numerical modelling, C\({}_{\text{D}}\) was reduced by 3.1% and 4.35%, respectively. With a combination of VG and a spoiler, further DR can be done, as [PERSON] [99] concluded. This leads to 6-12% fuel reduction, which is about the half of the DR.  
Small scale experiments should be evaluated carefully to match these numbers. Velocity has to be low; thus, increasing the risk of von Karman VG could compromise the results. At low velocity DR = 90% was achieved in the work by [PERSON] et al. [100].