# Quantitative Analysis of Drag Reduction Methods for Blunt Shaped Automobiles
## 2 Literature Review
### Aerodynamic Drag Reduction
#### 2.3.3 Topology Modification  
When an additional object is placed on the vehicle, its topology is modified. This modification could alter the flow around the body. These objects can be categorized into smaller categories, such as winglets, wings, flaps, spoilers, and splitter plates. If these modifications could generate longitudinal vortices or delay flow separation, it can be categorized to VG. In more specific terms, VG is a small surface that alters a flow along a larger surface. As it was mentioned, VG has advantageous properties in heat transfer as the vortices modifies the temperature and velocity distributions [3]. In the automotive industry, VG is applied to reduce the wake regions behind the vehicle. [PERSON] et al. [77] examined VG with both numerical models and wind tunnel tests. Its advantageous effects, besides drag reduction, were that they have found that it also reduces the overturning risk, slightly. Firstly, VG DR effect is presented by its individual vortex size reduction side and, secondly, when these are applied on vehicles. VG could have different types of shapes (see Figure 4); delta and rectangular shapes are the most well-known. If the connecting VG edge is perpendicular to the flow, it is categorized to wings (see Figure 4a), and when it is parallel, winglets (see Figure 4b). There is also a third case when it is attached to the edge of the vehicle, these are known as flaps (see Figure 4c). The offset between two delta shaped winglets was examined in the work by [PERSON] et al. [80]. They concluded that the maximum drag reduction was achieved at 5 offset distance/VG height ratio. This study was made for wind turbines, although, thanks to the subsonic velocity regime, it could also be applied to ground vehicles. In their other [81] work, they studied the height and boundary layer ratio for wind turbines. The requirements and goals were different, but they concluded that the drag was reduced by the most (84.9-83.2%) when the height of the VG height was between the 66-100% of the total height of the boundary layer, while the installation angle did not affect the \(\mathrm{C_{D}}\)[82]. Rectangular, triangular, and symmetrical NACA0012 shaped VG were examined on a flat plat in the work of [PERSON] et al. [83]. In their work, they presented that the shape had strong connection to the vortex size generated after the VG. Streamlined shapes had smaller vortex sizes than rectangular ones. The effects on winglets under adverse pressure gradient, numerically, was looked at by [PERSON] et al. [84]; in their experiment, the VG was placed on a ramp. For flat plate examinations, the drag coefficient cannot be concluded; therefore, other values should be examined, such as the wall shear stress. This value can predict the flow separation, as [PERSON] et al. [85; 86] concluded. It was shown that VG can be modelled by simple Reynolds-Averaged Navier-Stokes (RANS) turbulence models. With the combination of polyhedral mesh and k\(-\omega\) SST, and Spalart-Allmaras turbulence models, good convergence was concluded [87]. There is no accepted unified height for the VG; however, it has a strong connection with the boundary layer where the equipment is applied. Two sizes of VGs were installed at two different locations on a heavy truck by [PERSON] et al. [88]. It was concluded that VG scaled up to 150%, reduced the wake area by about 3%. VGs at the front end were considerably effective in point of wake size reduction (10.7%-12.19% compared to 2.8%-5.8%); \(\mathrm{C_{D}}\) was not calculated in that work. [PERSON] et al. [89] concluded that the VG optimal height is the boundary layer height. The VG was placed to delay the flow separation, and for that reason, it was moved to the trailing edge. At 0.6 height of the boundary layer, the wall shear stress peaks, and the value reduces when this value changes, as [PERSON] et al. [90] stated. The boundary layer height, even for one vehicle, is constantly changing with the velocity and the wind.