Understanding the Impact of a Serrated Trailing Edge on the Unsteady Hydrodynamic Field

This paper discusses the elements that make up a theoretical model for predicting the noise generated by a serrated trailing edge, and in particular, the required input to such a model; the wavenumber frequency spectra of the turbulence at the edge. It is proposed that this input, which is often modeled empirically, varies between a straight edge and a serrated edge and this fundamental difference in turbulence structure at the trailing edge leads to inaccurate theoretical predictions of noise reduction. Experimental measurements are therefore taken for straight and serrated trailing edges, with a particular focus on measuring the quantities which arise in typical wavenumber-frequency spectra models such as the TNO model. Whilst elements like the boundary layer thickness, shear profile, and skin friction velocity are found to be similar across the straight and serrated edges, the normal velocity wavenumber spectra is observed to vary substantially between the straight edge, and different locations along the serrated edge. In particular, the high-frequency decay rate of this spectra is reduced for a serrated edge versus a straight edge. This observed difference is then incorporated into a simple Chase-style empirical wavenumber-frequency spectrum; theoretical predictions which take account of a weaker high-frequency decay rate for a serrated edge agree much better with the experimental far-field noise predictions than theoretical predictions which assume an identical turbulent structure for both straight and serrated edges.

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Analytic Prediction of Rotor Broadband Noise with Serrated Trailing Edges with Applications to Urban Air Mobility Aircraft

Li,

Lee

2024

j am helicopter soc

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Theoretical predictions for broadband noise generated by rotors with serrated trailing edges are presented. This study extends Lyu and Ayton's model for serrated trailing-edge noise to account for factors such as finite span length, the accurate acoustic attenuation rate of trailing-edge noise, modified spanwise wavenumber, and the proper scattering factor. This new model is implemented in the rotorcraft broadband noise tool, UCD-QuietFly. The validation results demonstrate a satisfactory level of agreement with the experimental data for serrated trailing-edge noise of a wing section, a hovering rotor, and a forward-flight rotor. Next, the effects of serration parameters, such as the number of serrations, serrations starting radial location, serration geometry, and serration height, on rotor broadband noise are investigated. For example, it is observed that sine-wave, chopped peak, or sawtooth serration shapes provide the most significant noise reduction among the various shapes considered. Finally, broadband noise reduction through serrations is applied to two urban air mobility aircraft scenarios: a six-passenger quadrotor and a quiet helicopter. For the quadrotor, serrated trailing edges show a noise reduction potential of over 9 dB, with higher reductions observed at higher tip speeds and fewer blades. A quiet helicopter equipped with serrated blades in forward flight achieves a maximum noise reduction of over 10 dB in the forward direction.

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Understanding the Impact of a Serrated Trailing Edge on the Unsteady Hydrodynamic Field

Cited by 3 publications

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