With the objective of reducing the broadband noise from the interaction of highly turbulent flow and airfoil leading edge, sinusoidal leading edge serrations were investigated as an effective passive treatment. An extensive aeroacoustic study was performed in order to determine the main influences and interdependencies of factors, such as the Reynolds number, turbulence intensity, serration amplitude and wavelength as well as the angle of attack on the noise reduction capability. A statistical-empirical model was developed to predict the overall sound pressure level and noise reduction of a NACA65(12)-10 airfoil with and without leading edge serrations in the range of chord-based Reynolds numbers of 2.5•10 5 ≤ Re ≤ 6•10 5. The observed main influencing factors on the noise radiation were quantified in a systematic order for the first time. Moreover, significant interdependencies of the turbulence intensity and the serration wavelength, as well as the serration wavelength and the angle of attack were observed, validated and quantified. The statistical-empirical model was validated against an external set of experimental data, which is shown to be accurate and reliable.

Leading edge serrations are identified as an effective passive treatment for reducing fan broadband noise due to high turbulent inflow conditions. This paper aims to investigate the isolated effect of serrated applications in a rotating frame, covering the aerodynamic and aeroacoustic performance. With this purpose, a serration design, previously analyzed in the rigid domain, is transferred to the rotating frame, following a successive approach in form of a continuous increase of the fan blade number. This is considered as a feasible way to isolate the serration effects and to provide information on fan blade interaction and possible masking effects. Comparing blades with straight and serrated leading edges by analyzing the spectral noise reduction and the overall level result in deep insights in the underlying noise reduction mechanisms. Furthermore, analysis of phase differences by means of the wall pressure fluctuations leads to the identification of rotating flow phenomena, nonsynchronized with the rotor speed. The results obtained indicate an efficient noise reduction by the serrations in the vicinity of the design point. By use of the presented successive approach, noise reduction phenomena observed with the full rotor could be identified to be of either aeroacoustic or aerodynamic nature. A reduced noise is observed for the full rotor case, showing a reduction of blade interaction effects. At reducing flow coefficients, an improved stall margin of the serrated rotor is identified that also affects the aeroacoustic signature.

Recent research confirmed leading edge serrations to be an effective passive noise reduction treatment for aerofoil broadband noise at high-turbulent inflow conditions. Therefore, reducing leading edge broadband noise while maintaining acceptable aerodynamic aerofoil performance represents a pressing task for future applications. In this context, an extensive aeroacoustic study, analysing a NACA65(12)-10 aerofoil, was continued towards defining an aeroacoustic optimum between aerofoil noise radiation and noise reduction due to serrated leading edges in order to provide ideal design parameters for low-noise serrations. On this basis, part of the aeroacoustically analysed experimental space was extracted and analysed in terms of aerodynamic performance parameters, defined by lift and drag coefficients. This was carried out both, numerically and experimentally. The main parameters of interest were a variation of the Reynolds number, the angle of attack and the serration design parameters, namely the serration amplitude and the serration wavelength. The aerodynamic study showed a good match between experimental and numerical results in the pre-stall regime. Slight deviations occurred in a precise determination of the stall-angle and the maximum lift coefficients which mainly could be assigned to differing boundary conditions. However, for the serrations slight improvements of the maximum pre-stall angles as well as high post-stall lift coefficients were observed, which could be linked to specific separation pattern on the aerofoil suction side. An increase of the serration wavelength showed an increased lift performance, which could not be linked solely to a change in the aerofoils surface. Combining aeroacoustic and aerodynamic results showed that the aerodynamic trends towards a maximum lift performance compete aeroacoustic maximum-performance findings. Finally, defining a polyoptimum of the multi-parameter system in terms of maximum noise reduction effects while maintaining an acceptable aerodynamic performance provides a deepened insight into the relations between aerodynamics and aeroacoustics, where the presented data pool might give assistance for future design processes.

With the aim of analysing the efficiency of leading edge serrations under realistic conditions, an experimental rig was developed where a ducted low-speed fan is installed that allows to gather data of both, aerodynamic and aeroacoustic nature. Turbulent inflow conditions were generated via biplane-square grids, resulting in turbulence intensities of different magnitude and of high isotropic character that were quantified by use of hotwire measurements. The fan blades were designed according to the NACA65(12)-10 profile with interchangeable features and an independently adjustable angle of attack. Altogether, five different parameters can be analysed, namely the serration amplitude and wavelength, the angle of attack, the inflow turbulence and the rotational speed. In addition, the blade design allows for a variation of the blade skew, sweep and dihedral as well. The presented work focusses on validating and optimising the test rig as well as a detailed quantification of the turbulent inflow conditions. Furthermore, first aerodynamic and aeroacoustic results of fan blades with straight leading edges are compared to those of serrated leading edges. The aerodynamic performance was found to be mainly affected by the serrations as a function of the serration amplitude. Aeroacoustically, a clear sensitivity towards different incoming turbulence intensities and serration parameters was detected, showing significant broadband noise reduction below 2 kHz with an overall noise reduction of ΔOASPL = 3.4 dB at maximum serration amplitudes and minimum wavelengths.

This paper presents experimental results on the aeroacoustic performances of a NACA 65(12)-10 aerofoil subjected to serrated leading edges. The serration patterns of these leading edges are formed by cutting into the main body of the aerofoil, instead of extending the leading edges. Therefore these serrated leading edges, when attached to the main body of the aerofoil, will always result in the same overall chord length. The experiment was performed in an aeroacoustic wind tunnel facility. These serrated leading edges were investigated for their effectiveness in suppressing four different types of noise sources: laminar instability tonal noise, leading edge separation bubble noise, turbulence-leading edge interaction noise and trailing edge self-noise. Streamwise vortices produced by an optimised serrated leading edge can suppress the separation bubble at the trailing edge, thereby reducing the instability laminar tonal noise significantly. It is found that the most effective serration configuration is the one with the largest serration amplitude and smallest serration wavelength. Without even relying on the streamwise vortices, the sawtooth geometry of the serration itself can already be sufficient to suppress the leading edge separation bubble. Due to the special geometry of the NACA 65(12)-10, it is very effective in the production of laminar separation bubble noise at the leading edge. The use of serrated leading edge can therefore be an effective passive device to suppress this particular noise source. Similarly, the most effective serration geometry in the reduction of turbulenceleading edge interaction noise is the one with the largest serration amplitude and smallest serration wavelength. However, this configuration is also prone to generating superfluous noise at high frequency. Extensive boundary layer and very near wake measurements were performed to investigate the flow structures on the NACA 65(12)-10 aerofoil with a large serration amplitude leading edge. It can be concluded that the serrated leading edge is very disruptive to the hydrodynamic growth of the turbulent boundary layer at the trailing edge. Evidences on the reduction of boundary layer low-frequency turbulence at the trailing edge could support the hypothesis of a reduction in the low-frequency far field noise. This remains to be confirmed in the future studies.