## 2 Parametric Study Based on CFD Calculations
### Computational Results  
A parametric study has been carried out in order to find the optimal position of the MT on the airfoil DU91W(2)250. Table 1 illustrates the 13 cases studied in the current work. The first case studied is the one with no MT implemented and the other 12 cases with different sizes (\(\psi\)) and position of the MT from the leading edge of the airfoil (\(x\)). Each case has been studied for ten different degrees of angle of attack \(\alpha\), in the range from \(0^{\circ}\) to \(9^{\circ}\). Figure 7 illustrates the lift-to-drag ratio \(\mathrm{C_{L}/C_{D}}\) evolution for every angle \(\alpha\) and all MT cases. In the left column the evolution of the \(\mathrm{C_{L}/C_{D}}\) vs. the location of the MT from the airfoil leading edge \(x\) is represented. Right column plots illustrate the \(\mathrm{C_{L}/C_{D}}\) evolution against the three different heights of the MT \(y\) = 1, 1.5 and 2\(\%\)c. Both \(x\) and \(y\) parameters are represented in terms of percent of the airfoil chord length c. The Lift-to-drag ratio of the airfoil without MT for each AoA is represented by the black continuous line in each plot. Figure 7: \(Cont.\)For the range of angles of attack, from \(0^{\circ}\) to \(9^{\circ}\), the best values of the \(\rm{C_{L}/C_{D}}\) ratio are reached by the cases MT9510, MT9515 and MT9520. However, as can be seen in the left column plots of Figure 7, the highest values are reached around the \(x=95\%\) of the chord length. On the right column plots, it is clear again that the best values of the Lift-to-drag ratio are reached by \(x=95\%\) of \(c\), and the highest ones by the MT height of \(y=2\%\) of \(c\). Therefore, in the range of angles of attack studied in the present work, the best case in terms of Lift-to-drag ratio is the one defined by: \(x=95\%\) and \(y=2\%\) of \(c\). These results are in concordance previous studies of [PERSON] et al. [21] where it was found that the best place to situate the lower surface tab with respect to lift and drag was around \(95\%\)c. According to the classification of Table 1, that case corresponds to DU912250 MT9520. The \(\rm{C_{L}/C_{D}}\) ratio achieved in that case, keeps up to the ratio of the clean airfoil represented in Figure 7 with a continuous black line. Figure 8 illustrates the Lift-to-drag ratio values of the case DU912250 MT9520 in comparison with the ratios obtained for the clean airfoil. At low AoAs, the increase in the \(\rm{C_{L}/C_{D}}\) ratio due to the microtab MT9520 implementation is clearly visible. However, at the AoA of \(9^{\circ}\) the \(\rm{C_{L}/C_{D}}\) ratio stays below the \(\rm{C_{L}/C_{D}}\) ratio of the clean airfoil DU91W(2)250. Note that this behavior is repeated by the other cases with different MTs configurations, as shown in Figure 7e. The reason could be found in the fact that at \(9^{\circ}\) of AoA the airfoil is working near the stall conditions and the \(\rm{C_{D}}\) increases considerably. Since the computations were performed in 2D, three-dimensional effects were neglected. In the studies of [PERSON] et al. [26] and [PERSON] et al. [34], a detailed study on the three-dimensional shedding and spanwise flow can be found. Figure 9 represents the streamwise velocity distribution around the MT of the case with the best Lift-to-drag ratio: DU91W(2)250 MT9520. The presence of the tab changes the trailing edge flow development, the so-called Kutta condition, and consequently the effective c  
Figure 7: Lift-to-Drag ratio (\(\rm{C_{L}/C_{D}}\)) for the range of \(\alpha\) studied in the present work. Right side plots represent the \(\rm{C_{L}/C_{D}}\) evolution against the MT height \(y\). The evolution of the \(\rm{C_{L}/C_{D}}\) along the location of the MT from the leading edge \(x\) is represented in the left side. modified, providing in this case lift enhance. The MT jets the flow in the BL away from the airfoil surface, producing a circulation region behind the tab.