## 4 Results and Discussion
### Varying \(C_{\text{sep}}\) Coefficient  
The numerical predictions of the global aerodynamic loading on the aircraft model, for varying \(C_{\text{sep}}\) coefficients, are reported in Figure 6, in terms of lift curve and drag polar, compared to WT data [26]. Here, the lift and drag coefficients are normalized by the experimental values of \(C_{\text{L,max}}\) and \(C_{\text{D,min}}\), respectively, while the angle of incidence is normalized as \(\alpha^{*}=(\alpha-\alpha_{\text{min}})/(\alpha_{\text{max}}-\alpha_{\text{ min}})\). To perform a systematic analysis, four different calculations were carried out for each incidence by selecting different values for the \(C_{\text{sep}}\) coefficient in the recommended range, namely \(C_{\text{sep}}=0.7\), \(1,1.75\), and \(2\) (see Table 1). Moreover, a few (costly) SBES solutions were obtained for the stall regime, namely for \(0.8<\alpha^{*}\leq 1\), demonstrating the conformity between the present SRS results and experimental data. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Solution** & **\# Cells** & \(\Delta C_{L}\) & \(\Delta C_{D}\) **(Counts)** \\ \hline Coarse & \(70\) M & \(-0.203\) & \(+200\) \\ Medium & \(110\) M & \(-0.132\) & \(+130\) \\ Fine & \(150\) M & \(-0.034\) & \(+25\) \\ SBES & \(150\) M & \(-0.027\) & \(-16\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Number of FV cells and errors for the predicted aerodynamic force coefficients at \(\alpha=19^{\circ}\) against the experimental data for varying mesh resolution. Figure 5: Normalized sectional pressure (**left**) and skin-friction (**right**) coefficients at \(\eta=0.63\), for \(\alpha=19^{\circ}\), and varying mesh resolution compared to SBES. Figure 6: Global aerodynamic loading on the aircraft model: lift curve (**left**) and drag polar (**right**), for different values of \(C_{\text{sep}}\), compared to SBES and experimental data. In the linear lift regime, the predicted lift curve shows good agreement with the experiments regardless of the \(C_{\text{sep}}\) level, while marked discrepancies are observed at high angles of attack. Apparently, both the lift curve slope \(C_{La}\) and the zero-lift angle \(\alpha_{0}\) result in being quite accurate. The errors against experimental values for these aerodynamic parameters, together with that for the zero-lift drag coefficient \(C_{\text{D0}}\), are summarized in Table 3. Furthermore, by inspection of the drag polar, the drag force is evidently overestimated by choosing the default value \(C_{\text{sep}}=1.75\), especially at high incidences. Note that near the maximum lift condition, the variation of \(C_{\text{sep}}\) affects both drag and lift forces. As expected, the \"separation\" coefficient \(C_{\text{sep}}\) plays a key role for adjusting the predicted aerodynamic performance of the aircraft model. As it will be demonstrated in the following, this strong dependence has to be attributed to the more or less accurate prediction of the separated turbulent flow on the main wing of the aircraft. In fact, the free parameter \(C_{\text{sep}}\) directly affects the onset and evolution of the separated BL flow on the suction-side of the main wing, which becomes crucial at high incidences. Therefore, we focused our attention on the GEXO prediction of the cruise wing aerodynamics at the stall regime by considering three different angles of attack, namely \(\alpha=17^{\circ}\), \(19^{\circ}\), and \(21^{\circ}\). First, let us note that due to the three-dimensional character of the wing BL, the choice of \(C_{\text{sep}}\) also influenced the spanwise behavior of the aerodynamic loading. This is clearly illustrated, for instance, by looking at the contours of the skin-friction coefficient on the suction-side of the main wing, which are depicted in Figure 7, for the three different incidences and two different values of \(C_{\text{sep}}\). Apparently, the choice of the higher value for this parameter, which is \(C_{\text{sep}}=2\), alters the BL flow evolution along the wing, providing the early onset of flow separa