# Quantitative Analysis of Drag Reduction Methods for Blunt Shaped Automobiles
## 2 Literature Review
### Aerodynamic Drag Reduction
#### 2.3.2 Shape Optimization  
Shape optimization is a highly researched field, both in the field of fluid mechanics and other fields of the automotive industry [61; 4; 62]. While active system jets create vortices, on passive systems a modified shape can create or reduce the wake region. The simplest shape optimization method for drag reduction is when the wake regions are covered with fairings. At the frontal edge of the vehicle, corner vortices could appear typically at blunt shapes, especially for light trucks on the top side, where vortex shedding could occur, though on most vehicles, vortex separation occurs at the rear end.  
[PERSON] et al. [63] carried out 1000 two-dimensional numerical simulations for shape optimizations. Their work showed a 6% difference between measurements, the lowest C\({}_{\text{D}}\) was 0.186 and the highest was 0.556.  
For heavy trucks, the major wake regions (see Figure 2) are the upper cab, between the trailer wheels and rear end regions. Drag created by the wheels in the wake region could be reduced by using only flap-type side skirts for heavy trucks in the work of [PERSON] et al. [17]. This reduction was 5.3% while the difference between the experiment and numerical model was 3.7%. For the rear end, boat tails can be applied, as work by [PERSON] et al. [64] and [PERSON] et al. [65] shows. By mimicking a seal face on a truck cab, DR = 22.5% was concluded [66]. By covering the gap between the truck and trailer, DR = 16.4% was evaluated by [PERSON] et al. [67]. [PERSON] et al. [68] stated that the frontal aerodynamic enhancement influenced the rear end wake region. In work by [PERSON] et al. [69], where side skirts and boat tails were applied, the DR decrease led to an estimated 13.4% fuel saving. [PERSON] et al. [22] have examined various shapes of trucks used in the Indian subcontinent (typically large C\({}_{\rm D}\) vehicles) that generate large wake regions above and behind the vehicle. By using fairings compared to the baseline construction, the C\({}_{\rm D}\) was reduced by 12%. The other constructions showed that the generated wake zone that highly reduces the DR can be as low as 14-22% compared to the base line model. This proves that the vortices on the top side could have negative effects.  
To reduce the lower recirculation bubble, \"rear under-body slice\" shape optimization was carried out by [PERSON] et al. [70]. [PERSON] et al. [71] have shown that, with only hood inclination change, the drag could be reduced considerably.  
When [PERSON] body was tested for the first time by [PERSON] et al. [7] in 1984, a shape optimization was made by examining the flows at different slant angles. In that paper, it was concluded that the lowest C\({}_{\rm D}\) was at 10\({}^{\circ}\) slant angle. Much later, in 2018, with adjoint shape optimization, the base pressure was increased at the rear end of the vehicle. This method validated (0.6% difference) that the drag was reduced by 12.1% in the work by [PERSON] et al. [72].  
In smaller vehicles, such as the Windsor body, by changing the pitch angle by 1\({}^{\circ}\), [PERSON] et al. [30] reduced the C\({}_{\rm D}\) by 4.9%, showing high sensitivity to the DR. With a rear tapered shape of the Windsor body, [PERSON] et al. carried out around DR = 10%.  
On bluff bodies, numerical models could show large DR, such as DR = 50% in the works of [PERSON] et al. [73], [PERSON] et al. [74], and [PERSON] et al. [75]; however, the results have to be treated carefully since experimental validation is required.  
For the advanced shape optimization method, [PERSON] et al. [76] used shape-adjustable quadrilateral surfaces for optimization. This work emphasizes and describes the optimization method, yet results have not been validated by experimental measurements.