## 3 Results and Discussions
### Active Flow Control  
#### 3.2.1 Effect of the Different Active Flow Control Types  
The first computational attempt of actuated flow around the model focused on evaluating the influence of different control techniques on the flow structures and the obtained aerodynamic forces. Three flow control devices, i.e., steady blowing, synthetic and unsteady jets, with a jet velocity of \(\mathrm{U_{j}}=\mathrm{U_{0}}\) and jet angle of \(\beta=45^{\circ}\) were used. The control frequency of \(\mathrm{f_{c}}=34\) Hz was applied for the synthetic and unsteady jets. The time-averaged normalized streamwise velocity contour and streamlines in the symmetry plane (z = 0) of the three actuated flows are presented in Figure 12. Compared with the baseline flow, the streamlines in the actuated cases deviate from the horizontal plane, which is clearly observed with the steady blowing jet (Figure 12a) and the unsteady jet (Figure 12c). In contrast, the streamlines in the synthetic jet case are not significantly inclined (Figure 12b). The deviation of the streamlines in the actuated cases is explained by the introduction of momentum from the control jets, which then interact with the separated flow from all rear edges of the model and deviate the streamlines. This introduced momentum also affects the recirculation zones in the near-wake region, in which the lower recirculation is displaced away from the rear surface of the model, leading to an asymmetrical near-wake structure (Figure 12a,c). The more intensive influence of the control jet on the lower vortex than on the upper vortex seems to be due to the interaction with the complex flow derived from the underbody of the model. On the contrary, the structure of the recirculation zones in the case using a synthetic jet displayed an insignificant difference compared to baseline flow without a control device (Figure 12b). In addition, Figure 12a also shows that the recirculations become smaller when a steady blowing jet is used, which leads to a considerable reduction in the aerodynamic drag force obtained on the Ahmed model. The numerical results shown in Figure 12 reveal that the application of the control jets has significantly modified the near-wake structure behind the model, especially in the cases with the steady blowing and unsteady jets. This seems to be the main source that leads to a reduction in vehicle aerodynamic drag. The effect of active flow control devices on the turbulent kinetic energy (TKE) is presented in Figure 13. A maximum TKE value of 50 m\({}^{2}\)/s\({}^{2}\) is obtained in the near-wake region in all cases, with and without a control flow device. The high value of TKE corresponds to the recirculation zones behind the model. For all control cases, the maximum value of TKE is still located in the recirculation zones (Figure 13b-d), but the values of TKE are strongly reduced compared to the baseline flow case (Figure 13a). In particular, the high value of TKE completely disappeared at the upper part of the wake region when using a steady jet (Figure 13b). The high reduction in TKE value by applying the control flow devices demonstrates the effect of stabilization and the restriction of fluctuations in the flow field of these control methods. Figure 12: Time-averaged streamlines behind of model, on the symmetry plane with different actuator types. (**a**) A steady blowing jet; (**b**) a synthetic jet; (**c**) an unsteady jet. The time-averaged drag coefficients obtained on the model with three distinct flow control techniques are presented in Table 1. The maximum drag coefficient reduction of \(\Delta\text{C}_{\text{D}}=26.51\%\) is achieved when using a steady blowing jet, while it is \(8.03\%\) and \(1.2\%\) for an unsteady and a synthetic jet, respectively. Although the evolution of the aerodynamic drag coefficient obtained in Table 1 is relatively consistent with the analysis of the near-wake structure in Figure 12, the origin of the aerodynamic drag reduction has not been clearly explained. [PERSON] et al.