# Leading Edge Blowing to Mimic and Enhance the Serration Effects for Aerofoil +
## 4 Use of Leading Edge Blowing to Enhance the Serration Effects (Hybrid Device)
### Aerodynamic Results  
The evaluation of the hybrid device is now turned to the aerodynamic performance. Under the same experimental flow condition as that in Section 3.1, Figure 12a shows the C\({}_{\rm L}\) against the angles of attack \(\alpha\) produced by the baseline, serration-only (\(\lambda 45A30\)) and hybrid (\(\lambda 45A30\) + \(Q\)\({}^{\prime}\) = 1.0-4.5 L/min) leading edge device at \(U\) = 25 ms\({}^{-1}\). Figure 12b shows the corresponding distributions of the \(\Delta\)C\({}_{\rm L}\). Note that the definition of the \(\Delta\)C\({}_{\rm L}\) is the same as before, i.e., \(\Delta\)C\({}_{\rm L}\)(\(\alpha\)) = C\({}_{\rm L\;treated}\)(\(\alpha\)) - C\({}_{\rm L\;baseline}\)(\(\alpha\)), where a positive \(\Delta\)C\({}_{\rm L}\) denotes improvement of the C\({}_{\rm L}\) achieved by either the serration-only or hybrid devices. The C\({}_{\rm L}\) and \(\Delta\)C\({}_{\rm L}\) distributions for both the baseline and serration-only cases bear the same characteristics as those shown and discussed earlier in Figure 5. Interestingly, the response in C\({}_{\rm L}\) against the \(Q\)\({}^{\prime}\) for the hybrid device also exhibits a remarkably similar fashion as the far field radiation discussed in Section 4.1 (although one should take note that the flow conditions for the noise and aerodynamic tests are different). The most optimal blow rate is the same at \(Q\)\({}^{\prime}\) = 1.0 L/min, in which not only it increases the stall angle by about 1 degree, but it also produces the maximum C\({}_{\rm L}\) of 0.1 higher than the maximum C\({}_{\rm L}\) produced by the serration-only. From then \(\alpha\), a gradual increase of the \(Q\)\({}^{\prime}\) will affect negatively the C\({}_{\rm L}\) generation across the entire \(\alpha\) range investigated here. By examining Figure 12b, the distribution of the \(\Delta\)C\({}_{\rm L}\) exhibits a sinusoidal-liked pattern against \(\alpha\) for both the serration-only and hybrid devices. The decay rates at which the \(\Delta\)C\({}_{\rm L}\) changes with the \(Q\)\({}^{\prime}\), \(\rm d(\Delta\)C\({}_{\rm L})/d(Q\)\({}^{\prime}\)), at four representative \(\alpha\) are summarised in Table 2. From the table, it is clear that the decay rate is the greatest when the aerofoil is set at high loading condition (i.e., \(\alpha\) = 12\({}^{\circ}\)), while it is relatively mild at the pre-stall region at \(\alpha\) = 6\({}^{\circ}\). The \(\Delta\)C\({}_{\rm L}\) is the least sensitive to the \(Q\)\({}^{\prime}\) at the mild negative geometrical angle of attack (\(\alpha\) = -5\({}^{\circ}\)), before picking up the \(\rm d(\Delta\)C\({}_{\rm L})/d(Q\)\({}^{\prime}\)) again at large negative geometrical angle of attack (\(\alpha\) = -15\({}^{\circ}\)). As a summary of the hybrid device to the C\({}_{\rm L}\) generation, the effective blow rate only spans across a very narrow margin of \(Q\)\({}^{\prime}\). Exceeding this margin will easily negate the effectiveness of the serration-only configuration. Another observation for the use of hybrid device is that it creates the largest response in the C\({}_{\rm L}\) generation when the aerofoil is at high loading condition, which is applicable to both the nose up and nose down configurations. Interestingly, the hybrid device produces a more versatile performance in the drag coefficient \(\mathrm{C_{D}}\). Figure 13a,b show the \(\mathrm{C_{D}}\) and \(\mathrm{\Delta C_{D}}\) against the angles of attack \(\alpha\) produced by the baseline, serration-only (\(\lambda 45A30\)) and hybrid (\(\lambda 45A30+Q\)'= 1.0-4.5 L/min) leading edge device at \(U\) = 25 ms\({}^{-1}\), respectively. The \(\mathrm{\Delta C_{D}}\) is defined in a same manner as the \(\mathrm{\Delta C_{L}}\). Unlike the \(\mathrm{\Delta C_{L}}\) counterpart whose performance is underpinned by an optimal level, the hybrid device consistently shows an improvement of the \(\mathrm{\Delta C_{D}}\) as soon as the \(Q\)'is applied.