## 3 Results and Discussions
### Active Flow Control
#### 3.2.1 Effect of the Different Active Flow Control Types
[28] proposed another way for aerodynamic drag analysis, which is determined based on the parameters measured in the near-wake region. These parameters include total pressure loss, size of the wake section, and streamwise backward flow behind the model. Accordingly, the aerodynamic drag reduction involves decreasing the size of the wake section, reducing pressure loss and limiting streamwise backward flow. Figure 12 shows that the streamlines with flow control devices deviated with respect to the separated lines, which results in a reduction in wake region size. This phenomenon can be clearly observed in the actuated case with a steady blowing jet (Figure 12a), and an unsteady jet (Figure 12c). The total pressure loss is investigated using the pressure coefficient \(\text{C}_{\text{p}}\) in the vicinity of the rear surface of the model. The vertical pressure coefficient distributions at position x/H = 0.1 in the symmetry plane (z = 0) of flow around the model with and without a control device are shown in Figure 14. In all actuated flows, the pressure coefficient \(\text{C}_{\text{p}}\) is increased compared with unactuated flow (\(\text{C}_{\text{pmin}}=-0.215\) for baseline flow, \(\text{C}_{\text{pmin}}=-0.189\) with a synthetic jet, \(\text{C}_{\text{pmin}}=-0.159\) with a steady blowing jet, and \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Steady Blowing Jet** & **Synthetic Jet** & **Unsteady Jet** \\ \hline \(\text{C}_{\text{D}}\) & 0.183 & 0.246 & 0.229 \\ \(\Delta\text{C}_{\text{D}}\) (\%) & 26.51 & 1.2 & 8.03 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Drag coefficient reduction with different actuator types. Figure 13: Contour of TKE behind of model, on the symmetry plane with different actuator types. (a) Baseline flow; (b) a steady blowing jet; (c) a synthetic jet; (d) an unsteady jet. \(\mathrm{C_{pmn}}=-0.15\) using an unsteady jet) which demonstrates that the application of control jets increases the static pressure directly behind the model, and reduces pressure loss. The pressure coefficients \(\mathrm{C_{p}}\) obtained by using a steady blowing jet and an unsteady jet are quite similar, and larger than that when using the synthetic jet. The backward flow in the near-wake region, another parameter affecting aerodynamic drag on the model, is evaluated based on the vertical streamwise velocity profiles with and without the control device at position \(\mathrm{x/H}=0.5\) in the symmetry plane, as shown in Figure 15. The numerical results indicated that a decrease in the negative streamwise velocity is observed with a steady blowing jet (\(\mathrm{u_{min}}=0.2\mathrm{U_{0}}\)) and with an unsteady jet (\(\mathrm{u_{min}}\) = 0.3\(\mathrm{U_{0}}\)). On the contrary, the vertical streamwise velocity distributions of the baseline flow and the flow using a synthetic jet are quite similar, in which the backward velocity has no significant change over the entire rear height of the model. This seems to be due to the existence of large recirculation zones in the wake region in both cases. The analysis of the streamlines, pressure coefficient distributions and backward velocity profiles in the wake region, as shown in Figures 12-15,  
Figure 14: Vertical pressure coefficient profiles measured at \(\mathrm{x/H}=0.1\) in the symmetry plane, with different actuator types. Figure 15: Vertical normalized streamwise velocity profiles measured at \(\mathrm{x/H}=0.5\) in the symmetry plane, with different actuator types. of drag coefficient \(\mathrm{C_{D}}\) on the model with different flow control techniques (Table 1). A maximum coefficient drag reduction is achieved when a steady blowing jet is applied, based on the reduction in the wake section area due to deviation of the streamlines, the recovery of static pressure, and the limitation of the backward flow behind the model.