# Leading Edge Blowing to Mimic and Enhance the Serration Effects for Aerofoil +
## 3 Results
### Aerodynamic Forces
For the \(\lambda^{\prime}\) = 10 mm, the blowing rates vary between \(3.2\leq Q^{\prime}\leq 6.12\) L/min. It is worth mentioning that the lower limits of \(Q^{\prime}\) for both the \(\lambda^{\prime}\) = 5 mm and 10 mm leading edge configurations (\(\lambda^{\prime}\)5Q '1.6 versus \(\lambda^{\prime}\)10Q '3.2) stem from the same bulk mass blowing rate \(Q\) into the aerofoil. The same applies to the upper limits (\(\lambda^{\prime}\)5Q '3.06 versus \(\lambda^{\prime}\)10Q '6.12). After analysing from a large experimental dataset, it can be concluded that a lower blow rate \(Q^{\prime}\) tends to increase the C\({}_{\rm L}\) and stall angle. The effect of the blow rate to the C\({}_{\rm L}\) can also be affected by the \(\lambda^{\prime}\). For example, in Figure 5 the \(\lambda^{\prime}\)5Q '1.6 is found to produce the best resemblance to the \(\lambda^{\prime}\)10Q '3.2 in terms of their C\({}_{\rm L}\) curves. This can be confirmed by another example where the \(\lambda^{\prime}\)5Q '3.06 matches the best C\({}_{\rm L}\) curve with the \(\lambda^{\prime}\)10Q'6.12 (not shown here for brevity). However, it should be noted that the \(dC_{\rm L}/d\alpha\) produced by the different combinations of \(\lambda^{\prime}\) and \(Q^{\prime}\) can still be very different. Within the \(Q^{\prime}\) range described above, it is possible to match some of the C\({}_{\rm L}\) curves produced by the leading edge blowing against those produced by the serrated aerofoil. The comparison can also be found in Figure 5. First, a trend is discernible whereby an increase of the serration amplitude from \(A\) = 15 mm to 30 mm can re-produce somewhat very similar C\({}_{\rm L}\) variations with the \(\alpha\) by an increase of the blowing rate from \(Q^{\prime}\) = 1.6 L/min to  
Figure 6: Comparison of the drag coefficient C\({}_{\rm D}\) and \(\Delta\)C\({}_{\rm D}\) as a function of angle of attack \(\alpha\) produced by serrated and blowing leading edges at \(U\) = 24 ms\({}^{-1}\). 2.61 L/min (when \(\lambda^{\prime}\) = 5 mm), or 3.2 L/min to 4.24 L/min (when \(\lambda^{\prime}\) = 10 mm). In other words, a doubling of the serration amplitude corresponds to a \(\Delta Q^{\prime}\approx\)1 L/min. Interestingly, a second trend concerning the correlation between the serration wavelength \(\lambda\) and the leading edge blow rate \(Q^{{}^{\prime}}\) also emerges. In order to match the same C\({}_{\rm L}\) curves, a doubling of the serration wavelength from \(\lambda\) = 5 mm to 10 mm will also correspond to a doubling of the \(Q^{{}^{\prime}}\). Both trends suggest that the manipulation of the C\({}_{\rm L}\) distributions and the mitigation of boundary layer stall at high angles of attack by the leading edge serration and leading edge blowing are governed by similar flow mechanisms. More importantly, the \(\lambda^{\prime}\) and \(Q^{{}^{\prime}}\) can indeed mimic the \(\lambda\) and \(A\), respectively, to produce the same aerodynamic performances. This is a significant finding. Further corroboration can be made in Figure 6 for the variations of the drag coefficients C\({}_{\rm D}\), as well as the \(\Delta\)C\({}_{\rm D}\), against the \(\alpha\). Similar to their C\({}_{\rm L}\) counterparts, each paring between the serrated leading edge and blowing leading edge for the C\({}_{\rm D}\) across -15\({}^{\circ}\leq\alpha\leq 15^{\circ}\) is also very well matched. All curves have roughly the same point of interception with C\({}_{\rm D}\approx\)0.03 at \(\alpha\) = 0\({}^{\circ}\). Majority of the (\(\lambda\), \(A\)) and (\(\lambda^{\prime}\), \(Q^{{}^{\prime}}\)) pairing, when compared with the baseline leading edge, would exhibit a reduction of C\({}_{\rm D}\) across -15\({}^{\circ}\leq\alpha\leq 0^{\circ}\), but an increase of C\({}_{\rm D}\) across 0\({}^{\circ}<\alpha\leq 15^{\circ}\). The increase of the C\({}_{\rm D}\) at the post-stall regime is noticeable for both the blowing leading edge and serrated leading edge.