# Study of Airfoil Deflections for Unsteady Aerodynamics Optimization in Pitching Airfoils +
## 3 Results and Discussion
### Deflected Airfoil Results  
As stated earlier, the computational results of performance with angle of attack based on coefficient of lift, drag, and lift over drag were the starting point for the experimental design. The upper and lower limits of the pitching domain were increased to ensure dynamic stall was seen through all configurations. Further testing was performed from 22\({}^{\circ}\) to \(-\)10\({}^{\circ}\), and it was found that dynamic stall initiates from 14 to 16\({}^{\circ}\) for all airfoils, with full stall occurring between 18\({}^{\circ}\) and 22\({}^{\circ}\). The results are presented as phase-averaged velocity field plots (magnitude contour, vectors, and streamlines) for representative deflection cases and the baseline and pitching angles. It is noted that in the global view presented here, the LE location is hidden due to obstruction of the view from the wing, and thus, the actual flow around the LE is not visible. The velocity fields below have been non-dimensionalized with respect to their average free stream velocity:  
\[V^{*}=\frac{V}{V_{\infty}} \tag{3}\]  
The axis coordinates x and y have been non-dimensionalized by the chord (c) as x/c and y/c, respectively. It is noted that the center of pressure (and pivoting/pitching), which is at 1/3 of the chord from the LE, is chosen in the following plots to be at coordinates x/c = 0.2 and y/c = 0.2. This choice allows the full chord range from 0.0 to 1.0 to be displayed in most plots. In the extended views of the tandem arrangement, which were obtained by combining two camera views, the non-dimensional coordinates match x/c = y/c = 0.2 for the first (upstream) airfoil and match x/c = 3.2 and y/c = 0.2 for thesecond (downstream) airfoil; this coordinate arrangement permits displaying the full view of four chords captured with the PIV cameras consistently. Results are presented for three angles of attack: the maximum (22\({}^{\circ}\)) (Figure 9) and intermediate pitching down 16\({}^{\circ}\) (Figure 10) and 10\({}^{\circ}\) (Figure 11). In these figures, the actual airfoil geometries are shown. These selections correspond to pitching down motion and reflect most prominently flow separation and stall characteristics. The main indicators of performance are separation regions and wake properties, as they directly relate to lift and drag. Details such as starting point, size, extent, and direction of these regions can be measured from the velocity field. From the velocity field, prominent separation regions are revealed by the lower velocity magnitude, flow reversal, and recirculation regions. In line with this reasoning, it can be seen that the best-performing static airfoil is the 6\({}^{\circ}\) deflected LE, consistent with previous experimental [5] and computational [17; 28] findings from the literature. The deflection effect is less noticeable at the full stall angle \(\alpha\) (22\({}^{\circ}\), max), such as those seen in Figure 9, although the patterns are different for each case, based on the size and shape of each stall bubble and when compared with the baseline. The benefits of LE deflection are quite revealing in the 16\({}^{\circ}\) pitching down motion (Figure 10), showing that this LE configuration is the only one without a recirculation and with a smaller wake; its comparison with baseline using streamlines is shown in Figure 12. The leading and trailing edge deflected airfoil performs similarly at the lower \(\alpha\) values. The worst-performing airfoil would be the one with only trailing edge deflection. Figure 10: Velocity fields for 16\({}^{\circ}\)\(\alpha\) pitching down (\(\downarrow\)) for deflected airfoil variations: (**a**) baseline, (**b**) LE deflected, (**c**) TE deflected, (**d**) LE&TE deflected. Center of pressure and pitching is at x/c = y/c = 0.2. A distinguishable difference can also be detected in terms of performance associated with the four variations for the 10\({}^{\circ}\) pitching down (Figure 11).