# Leading Edge Blowing to Mimic and Enhance the Serration Effects for Aerofoil +
###### Abstract
The level of broadband noise reduction is a strong function of the serration amplitude (longitudinal distance between the peak and trough), but it is less sensitive to the serration wavelength (spanwise distance between successive peaks). There have also been many studies using high-fidelity numerical flow simulation to provide a physical insight of the noise reduction mechanisms by the serration [4; 5; 6]. These studies show that the surface pressure fluctuation and the far field noise on a serrated leading edge are de-correlated by the serrated leading edges. In particular, the noise source at the mid-region of the oblique edge becomes ineffective across the mid to high frequency range. The serration could cause a significant decrease in the surface pressure fluctuations around the tip and mid-regions of the serration, which directly results in a broadband noise reduction. Another noise reduction mechanism is attributed to the phase interference and destruction effect between the serration peak and the mid-region of the oblique edge. Accordingly, the serration trough region remains effective in the noise radiation. Now if we focus on the aerodynamic performances, [PERSON] and [PERSON] [7] show that the humpback whale benefits from tubercl along the leading edge of its flippers, which would resemble remarkably well with the serrated leading edge of owls. The context is the enhancement of the maneuverability by delaying flow separation. However, it is noteworthy that, despite the delayed aerodynamic stall and improved post-stall lift performance, the serrated leading edges also reduce the maximum lift coefficient and increase the drag coefficient, resulting in lower lift-to-drag ratio. [PERSON] et al. [8] perform experimental evaluation of the effects of various wavelength and amplitude configurations on the aerodynamic force characteristics of a full-span NACA 634-021 aerofoil at a Reynolds Number of 183,000. Despite the increased post-stall lift up to 50%, the stall angle pertaining to the serrated aerofoil is actually reduced. Detrimental effect is also observed for both the lift coefficient and drag coefficient, which decrease and increase, respectively, against the baseline aerofoil. [PERSON] et al. [9] replicate several aerofoil models whose the tapered wing possesses the aspect ratios similar to that of the humpback whale flipper. Following the wind tunnel experiments, they observe an increase above the baseline configuration of the maximum lift by 6%, and a delay of the stall angle by about 40% for the flipper model with leading edge bumps. Despite some discrepancies in the literatures, the general consensus of the serrated leading edge is that, though the stall angle and the maximum lift coefficient can be increased by an optimised serrated-configuration, the lift coefficients at the pre-stall regime will be lower than those produced by an unmodified, baseline leading edge. The momentum exchange owing to the mixing between the streamwise vortices and the otherwise separated boundary layer is regarded as one of the main mechanisms to prevent/delay the boundary layer separation, and subsequently improve the post-stall performance by a serrated aerofoil. The streamwise vortices in the form of counter-rotating pairs are predominantly generated at the troughs of the serration as a direct consequence of the serration geometry [10]. [PERSON] et al. [11] highlight that the streamwise vortices can be induced by the spanwise pressure gradient where the near wall low-momentum fluid is transported away from the serration peak towards the trough. Conversely, high-momentum fluid from the freestream is drawn to re-energies the boundary layer downstream of the serration peaks. Focusing on the evolution of vorticity near the serration troughs, [PERSON] et al. [12] observe that the external pressure force transforms the initial vorticity near the leading edge into pairs of counter-rotating streamwise vortices via a three-dimensional process (stretching, tilting and diffusion).