The dominant aeroacoustic mechanisms of serrated leading edges, subjected to highly turbulent inflow conditions, can be compressed to spanwise decorrelation effects as well as effects of destructive interference. For single aerofoils, the resulting broadband noise reduction is known to follow spectral scaling laws. However, transferring serrated leading edges to rotating machinery, results in noise radiation patterns of significantly increased complexity, impeding to allocate the observed noise reduction to the underlying physical mechanisms. The current study aims at concatenating the scaling laws for stationary aerofoil and rotating-blade application and thus at providing valuable information on the aeroacoustic transferability of leading edge serrations. For the pursued approach, low-pressure axial fans are designed, obtaining identical serrated fan blade geometries than previously analyzed single aerofoils, hence allowing for direct comparison. Highly similar spectral noise reduction patterns are obtained for the broadband noise reduction of the serrated rotors, generally confirming the transferability and showing a scaling with the geometrical parameters of the serrations as well as the inflow conditions. Continuative analysis of the total noise reduction, however, constrains the applicability of the scaling laws to a specific operating range of the rotors and motivates for a devaluation of the scaling coefficients regarding additional rotor-specific effects.

At high-turbulent inflow conditions, the blades of axial fans are known to emit a significantly increased amount of leading edge broadband noise. Recent research has firmly confirmed leading edge serrations to be an effective passive treatment in noise reduction. Up to now, leading edge serrations are mainly analysed in rigid test settings. With the aim of transferring leading edge serrations from the single aerofoil approach to the full rotor, an experimental rig was developed where a ducted low-pressure fan is installed. The fan blades are designed according to NACA65(12)-10 profiles, which were already analysed extensively in previous experimental and numerical studies. The rig allows to gather the aerodynamic and aeroacoustic data. Altogether, five different parameters can be analysed, namely the serration amplitude and wavelength, the stagger angle, the inflow turbulence and the rotational speed. The presented work focuses on a quantitative aerodynamic and aeroacoustic analysis of the designed rotor, where straight leading edges are compared to five sets of serrated blades, covering the full fan performance characteristics. A clear aeroacoustic sensitivity towards different incoming turbulence intensities was detected where a noise reduction can be observed especially for high serration amplitudes and large wavelengths. The aerodynamic performance primary scales with the ratio of serration wavelength and amplitude.

Leading edge serrations are well known for their ability to reduce turbulence-induced noise of single aerofoils while also providing aerodynamic advantages under certain operating conditions. Continuatively, applying leading edge serrations to rotating machinery such as axial fans proved the validity to generally transfer the obtained aeroacoustic benefits of single aerofoils. However, for the rotating applications, besides the discrete and broadband characteristics of the investigated machines, the noise reduction potential highly depends on the point of operation. The current work aims at assessing the aeroacoustic effects of serrated leading edges under the increased geometrical complexity of the fan blades through blade skew, a combination of blade sweep and blade dihedral. Therefore, the question is whether combining two potentially effective noise-reducing treatments through blade skew and leading edge serrations results in leveraging or obstructing effects. By varying the skew angle from 0 deg to 38 deg, four different prototypes of the fan impeller are designed, manufactured and tested experimentally in a test rig according to ISO 5136 and ISO 5801, allowing for simultaneous assessment of the aerodynamic and acoustic performance of the machines. All configurations are tested with original blades of straight leading edges plus five sets of serrations each, parameterised by the serration amplitude and the serrations wavelength. The intensity of the incoming turbulence ranges from 2.6% to 12.1%. The results obtained show the skewed blades to improve both the aerodynamic performance and the noise radiation after exceeding an initial skew angle, complemented by a significant onset of stall. Moreover, no contraindication between blade skew and serrated leading edges is encountered, showing the potential to further extend the noise reduction capabilities by combining effects of blade skew and leading edge treatment.

Leading edge serrations are well known for their ability to reduce turbulence-induced noise of single aerofoils while also providing aerodynamic advantages under certain operating conditions. Continuatively, applying leading edge serrations to rotating machinery such as axial fans proved the validity to generally transfer the obtained aeroacoustic benefits of single aerofoils. However, for the rotating applications the noise reduction potential highly depends on the point of operation. The current work aims at assessing the aeroacoustic effects of serrated leading edges under the increased geometrical complexity of the fan blades through blade skew. Therefore, the question is whether combining two potentially effective noise-reducing treatments through blade skew and leading edge serrations results in leveraging or obstructing effects. By varying the skew angle from 0 deg to 38 deg, four different prototypes of the fan impeller are tested experimentally in a test rig according to ISO 5136 and ISO 5801. All configurations are tested with original blades of straight leading edges plus five sets of serrations each, parameterised by the serration amplitude and the serrations wavelength. The intensity of the incoming turbulence ranges from 2.6% to 12.1%. The results obtained show the skewed blades to improve both the aerodynamic performance and the noise radiation after exceeding an initial skew angle, complemented by a significant onset of stall. Moreover, no contraindication between blade skew and serrated leading edges is encountered, showing the potential to further extend the noise reduction capabilities by combining effects of blade skew and leading edge treatment.