# 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)  
### Acoustic Results  
The serrated leading edge has been demonstrated as a powerful passive device for the reduction of the turbulence-leading edge broadband interaction noise. Some recent studies [1; 2; 3; 4; 5; 6] report that the far field reduction are underpinned by the reduction of the surface pressure fluctuations around the tip and mid-regions of the serration, as well as the phase interference and destruction effect between the serration peak and the mid-region of the oblique edge. These suggest that the serration trough region remains effective in the radiation of the interaction noise. The results provided thus far for the leading edge blowing, as an active device, strongly indicate that the produced exit air jet can counteract with the inflow turbulence, and successfully mimic the leading edge serration effect to reduce the level of the broadband interaction noise radiation. A new concept is developed here to use the leading edge blowing to enhance the serration effects. The rationale to investigate such hybrid device is based on the hypothesis that when the orifices are placed strategically across a serrated leading edge, e.g., at the tip and trough regions of the serration, the exit air jets can further reduce the level of turbulence interaction. Figure 10 provides an illustration for the hybrid device. In the current study, the serrated leading edge is of the type \(\lambda 45A30\). The relatively large serration wavelength used here, instead of a prefer choice of smaller serration wavelength that is known to be a favourable choice for leading edge noise reduction, is to mitigate the manufacturing complexity of the internal air plenum chamber for the aerofoil. Under exactly the same experimental procedures and inflow turbulence level, a comparison of the \(\Delta\)PWL as a function of normalised frequency \(f\) 'produced by (1) the \(\lambda 45A30\) only, and (2) hybrid device (\(\lambda 45A30\) + blowing \(Q\) ') at \(U\) = 40 ms\({}^{-1}\) and \(\alpha\) = 0\({}^{\circ}\) is shown in Figure 11a. Note that the \(\Delta\)PWL is the difference in the Sound Power Level between the baseline and either the \(\lambda 45A30\) or the hybrid device. Similarly, a positive value of \(\Delta\)PWL denotes noise reduction against the baseline, and the opposite is true. The values of \(Q\) 'indicated in the figure refer to the flow rate at each orifice. From the figure, the \(\lambda 45A30\) serration-only case is shown to achieve a maximum noise reduction of \(\sim\) 7 dB at \(f\)'\(\approx\) 7. After facilitating a small blow rate of \(Q\)'= 0.5 L/min for the hybrid device, the noise reduction performance is enhanced throughout \(f\)'\(<\) 7 where a further 1 dB  
Figure 10: Isometric drawing of the hybrid device featuring a serrated leading edge with orifices at the serration troughs and tips for leading edge blowing. Note that the sketching of the exit air jets is not to scale, is for illustration only and may not represent the real flow behaviors. improvement can be observed across the lower frequency. Further level of improvement is achieved when \(Q\)'is increased to 1.0 L/min, where the peak noise reduction is about 8.5 dB at a lower frequency of \(f\)'= 5. Compared to the serration-only case, the addition of leading edge blowing can cultivate a further 2 dB level of noise reduction throughout \(f\)'< 5. Interestingly, the sensitivity of the noise radiation against \(Q\)'for the hybrid device can be optimised. This is because when \(Q\)'is increased beyond the level of 1.0 L/min, the overall level of \(\Delta\)PWL begins to drop across the frequency. In a relatively high blow rate \(Q\)'= 4.5 L/min, the corresponding \(\Delta\)PWL is lower than the serration-only case, essentially means that this particular high blow rate hybrid device is detrimental. The results in Figure 11a thus indicate that there is an optimum value of \(Q\)'in the hybrid device, and the maximum noise reduction is generally shifted to a lower frequency.