# Leading Edge Blowing to Mimic and Enhance the Serration Effects for Aerofoil +
## 5 Conclusions  
According to classical theory on aerofoil noise [1], injection of mass flow from the leading edge (against the incoming flow) can potentially be an effective mechanism to reduce the turbulence intensity, hence the level of turbulence-leading edge interaction noise radiation can also be reduced. The leading edge blowing may also displace the stagnation point, subsequently lessen the level of impingement between the incoming turbulent eddies and the leading edge. After the injection of air from the leading edge orifices and mixed with the incoming flow, the mass flow can be entrained back and propagated as streamwise vortices in the downstream direction along the aerofoil surface. These streamwise vortices generated indirectly as a result of the leading edge blowing could also be effective in the suppression of boundary layer separation. On the other hand, leading edge serration is now a well-established effective passive control device for the reduction of turbulence-leading edge interaction noise and mitigation of the boundary layer separation. It is envisaged that leading edge blowing, an active flow control approach, could produce the same mechanisms as those produced by a serrated leading edge. NACA 65(12)-10 is chosen as the baseline aerofoil in this experimental study. Four types of serrated leading edge in different combinations of serration wavelength \(\lambda\) and amplitude \(A\), as well as straight leading edge with different spanwise orifice spacing \(\lambda^{\prime}\) and blow rate \(Q^{\prime}\) were investigated. From the results, there exists an explicit relationship between the \(A\) and \(Q^{\prime}\), as well as the \(\lambda\) and \(\lambda^{\prime}\) for the aerodynamic lift and drag coefficients produced across a wide range of angle of attack. In terms of the aeroacoustics performance, it can also be concluded that there exists a correlation between the \(A\) and \(Q^{\prime}\) for the turbulent-leading edge interaction broadband noise reduction. Unlike the serration counterpart, the effectiveness of the leading edge noise reduction by blowing is found to be very sensitive to the choice of \(Q^{\prime}\), and to some extents, the \(\lambda^{\prime}\). Although  
Figure 14: Comparison of the (**a**) \(\rm C_{L}/C_{D}\) as a function of \(\alpha\) produced by the baseline and hybrid (\(\lambda 45A30+blowingQ^{\prime}\)) leading edge device at \(U=25\) ms\({}^{-1}\). The leading edge blowing range covers 0.0 (serration only) \(\leq Q^{\prime}\leq 4.5\) L/min, and (**b**) The corresponding \(\rm\Delta C_{L}/C_{D}\) as a function of \(\alpha\), where \(\rm\Delta C_{L}/C_{D}\) is the difference in \(\rm C_{L}/C_{D}\) between the hybrid (\(\lambda 45A30+Q^{\prime}\) = 0.0–4.50 L/min) and the baseline. more investigations are still needed, particularly on the flow mechanisms, the current paper contributes in a positive outcome for a proof-of-concept where the serration effect on aerofoil can be mimicked by the leading edge blowing. A new concept is investigated for the addition of the leading edge blowing to enhance the serration effect. The rationale is based on the hypothesis that when the air orifices are placed strategically across a serrated leading edge, the exit air jets can further reduce the level of turbulence interaction. This hybrid device can indeed cultivate further level of noise reduction at low to medium frequency. In the context of the overall Sound Power Level performance, the most optimal \(Q\)'that achieves the largest level of reduction (\(\sim\)2.5 dB more than the serration-only case) is to apply an exit jet velocity of between 10 and 13% of the main inflow velocity. However, the preliminary indication is that such criterion is only valid between the low and medium inflow velocity range (less than 45 ms\({}^{-1}\)). In terms of the lift coefficients, the effectiveness of the \(Q\)'spans across a narrow margin. Exceeding this margin can negate the effectiveness of the serration-only configuration.