# Quantitative Analysis of Drag Reduction Methods for Blunt Shaped Automobiles
## 2 Literature Review
### Aerodynamic Drag Reduction
#### 2.3.1 Active Systems  
For active systems to operate, power is required, which has to be less than the reduced propulsion demand to be effective. An active system can reduce the drag with different methods, but in most cases, it is injecting steady or pulsating flow in the rear wake region, even with wheel-vehicle interaction drag can be reduced, as [PERSON] et al. [29] showed on an Asmo model. For steady jet flow (SJF), active drag reducing systems flow can be transferred from the frontal high-pressure zone [46]. SJF systems were used in works by [PERSON] et al. [47], [PERSON] et al. [48], [PERSON] et al. [49], and [PERSON] et al. [15], where these were placed in an array [47; 48; 49] or along the trailing edges [15; 47; 50]. These methods simply reduced the pressure difference at the rear end. With higher flowrate (and more jets), larger DR was achieved. In work by [PERSON] et al. [10], high frequency (f = 6.7 kHz) plasma flow was induced.  
Figure 3: Generated vortices behind a notchback vehicle (based on [PERSON] et al. [35]).  
In their findings, it was concluded that at higher velocities the magnitude of the drag reduction was decreased, peak DR was achieved at 10 m s\({}^{-1}\).  
For pulsating flow generation, synthetic jet actuators (SJA) are used, these are low frequency flow control systems where a membrane generates a periodic minimal flow, which increases or decreases the Kelvin-Helmholtz instability effect [51]. For flow actuations, Strouhal number (St) is used in some analysis  
\[\text{St}=\text{f}\cdot\text{H}\cdot\text{v}^{-1} \tag{3}\]  
where H is the height of the vehicle. On a simplified (bullet) shape, [PERSON] [8] showed that SJA systems has some uncertainties. C\({}_{\text{D}}\) was constant above 10 m s\({}^{-1}\), when the velocity decreased from this value C\({}_{\text{D}}\) increased, in addition, when SJA was used, DR = 6% and DR = \(-22\%\) were achieved. This high fluctuation reduced at higher velocities along with the maximum DR. This peak DR as corresponded to St = 0.25, which was close to its natural shedding frequency [47]. On a similar D-shape, [PERSON] et al. [52] also concluded that the peak DR was close to this shedding frequency (St = 0.2). [PERSON] et al. [53] found a peak DR in 0-550 Hz frequency range (f = 500 Hz; H = 0.202m; v = 30 m s\({}^{-1}\)) this was at St = 3.6. This value is close to findings by [PERSON] et al. [14], yet the magnitude of DR was less. They also concluded that St has less significance and Re has stronger connection to DR. Close to this DR, on a comparable shape (Ahmed body), [PERSON] et al. [51] results peaked at St = 5.84. The SJA system flow direction was normal to the main flow.  
For on-road measurements, where the velocity is highly fluctuating, adaptive controlling is required [47; 54]. [PERSON] et al. [9], on a square back version of an Ahmed body, induced flow parallel to the top surface with high frequency St = 12.1, this DR was close to [PERSON] et al. [53] 18% and 20%, respectively.  
These experiments are done with PIV, for the active method numerical analysis is rare [55; 56; 57]. Low amounts can be due to the fact that the validation measurement already has enough information. There are promising projects that show new ideas and applications of active systems on other car geometries. For numerical modelling, [PERSON] et al. [57] used a combination of jet actuations and porous top layer on a Ahmed body; a remarkable DR = 31% was presented.  
Active systems can be used at higher velocities effectively when an active grille shutter control mechanism is used [58], or can supply other active DR systems when a small turbine is applied in the vehicle [59]. The advantage is that the utilized power is proportional to the speed. In an urban environment, where accelerating and braking are more frequent for electric vehicles, it could also help to power these systems [60].