# Leading Edge Blowing to Mimic and Enhance the Serration Effects for Aerofoil +
## 3 Results
### Aerodynamic Forces  
Figure 5 compares the aerodynamic lift coefficient C\({}_{\text{L}}\) generated by the aerofoil as a function of angle of attack \(\alpha\). Three cases are examined. First is the baseline leading edge. Second is when the aerofoil is subjected to leading edge serration, and third is the leading edge blowing. In all cases the planform areas (C \(\times\)\(S\)) are the same, which enable an unambiguous definition of the C\({}_{\text{L}}\). Four types of serrated leading edges are used in this study: \(\lambda\)5\(A\)15, \(\lambda\)10\(A\)15, \(\lambda\)5\(A\)30 and \(\lambda\)10\(A\)30. The figure also includes the \(\Delta\)C\({}_{\text{L}}\), which is defined as follow:  
\[\Delta\text{C}_{\text{L}}\left(\alpha\right) =\text{C}_{\text{L}\left(\text{treated}\right)}\left(\alpha \right)-\text{C}_{\text{L}\left(\text{baseline}\right)}\left(\alpha\right) \tag{3}\] \[\Delta\text{C}_{\text{D}}\left(\alpha\right) =\text{C}_{\text{D}\left(\text{treated}\right)}\left(\alpha\right) -\text{C}_{\text{D}\left(\text{baseline}\right)}\left(\alpha\right) \tag{4}\]  
Figure 4: (**a**) Isometric drawing of the NACA 65(12)-10 aerofoil model with blowing leading edge; (**b**) streamwise development of the velocity profiles produced by the leading edge blowing under the configuration of \(\lambda\)’ = 5 mm, \(Q\) = 208 L/min, and quiescent condition. where a positive value of \(\Delta\rm{C_{L}}\) denotes a higher lift coefficient generated by either the serrated leading edge or blowing leading edge, at a particular \(\alpha\), compared to the baseline aerofoil. The opposite is true. The difference in the drag coefficient \(\Delta\rm{C_{D}}\) (to be shown in Figure 6) is also defined in a similar fashion, i.e., a positive \(\Delta\rm{C_{D}}\) means that a larger drag coefficient is produced by the treated aerofoil, and vice versa. It can be seen that for the baseline case the corresponding \(\rm{C_{L}}\) curve follows largely the classical thin aerofoil theory where a lift curve slope \(d\rm{C_{L}}/da\)\(\approx\)\(2\pi\) is evident at \(-6^{\circ}\leq\alpha\leq 8^{\circ}\). At \(\alpha>8^{\circ}\), the aerofoil starts to stall and the lift coefficient begins to drop. On the other hand, at \(\alpha<-6^{\circ}\) the \(\rm{C_{L}}\) does not vary much at increasingly negative angle of attack. Generally, it is observed that a smaller serration wavelength (i.e., \(\lambda\) = 5 mm), regardless of the serration amplitude \(A\), can preserve the lift curve slope of \(2\pi\) at \(-6^{\circ}\leq\alpha\leq 8^{\circ}\). For a larger serration wavelength, such as the \(\lambda\) = 10 mm case, a smaller value of lift curve slope (\(d\rm{C_{L}}/da\)\(\approx\)0.08) is produced by the serrated aerofoil. With regard to the effect of the serration amplitude, it can be concluded that a smaller serration amplitude (i.e., \(A\) = 15 mm) is more effective than the larger serration amplitude (\(A\) = 30 mm). The effectiveness is measured in two phenomena. First, the stall angle is usually larger. Second, the values of the \(\rm{C_{L}}\) as a function of \(\alpha\) are larger. This trend remains true at the post-stall regime as clearly demonstrated in the \(\Delta\rm{C_{L}}\) distributions. Based on the above observation, the most superior serrated aerofoil in terms of the lift generation are the \(\lambda 5A15\) and \(\lambda 10A15\). Figure 5: Comparison of the lift coefficient \(\rm{C_{L}}\) and \(\Delta\rm{C_{L}}\) as a function of angle of attack \(\alpha\) produced by serrated and blowing leading edges at \(U\) = 24 ms\({}^{-1}\). The analysis now focuses on the aerofoil with leading edge blowing. As mentioned earlier, the spanwise distributions of the leading edge orifices are defined by \(\lambda^{\prime}\) = 5 mm and 10 mm. These two spacing correlate with the serrated aerofoil with \(\lambda\) = 5 mm and 10 mm, respectively. For the \(\lambda^{\prime}\) = 5 mm leading edge configuration, the blowing rates would vary between \(1.6\leq Q^{\prime}\leq 3.06\) L/min.