Abstract
An experimental and analytical study of the tonal trailing-edge noise of a symmetric NACA-0012 aerofoil and of a cambered SD7003 aerofoil has been achieved. It provides a complete experimental database for both aerofoils and improves the understanding of the underlying mechanisms. The analysis stresses the high sensitivity of the tonal noise phenomenon to the flow velocity and the angle of attack. Several regimes of the noise emission are observed depending on the aforementioned parameters. The contributions of the pressure and the suction sides are found to vary with the flow parameters too. A special attention has been paid to the role of the separation bubble in the tonal noise generation. Hot-wire measurements and flow visualization prove that the separation bubble is a necessary condition for the tonal noise production. Moreover, the bubble must be located close enough to the trailing edge. Several tests with small-scale upstream turbulence confirm the existence of the feedback loop. Analytical predictions with a classical trailing-edge noise model show a good agreement with the experimental data; they confirm the cause-to-effect relationship between the wall-pressure fluctuations and the radiated sound. Finally, previously reported works on fans and propellers are shortly re-addressed to show that the tonal noise associated with laminar-boundary-layer instabilities can take place in rotating blade technology.
Highlights
Tonal trailing-edge noise refers to the loud, annoying whistling sometimes produced lifting surfaces at low Reynolds numbers and moderate angles of attack in a clean flow
As reported by Pröbsting [18] the emission area for the NACA-0012 aerofoil features two sub-regions: at low Reynolds number or velocity, tones are produced by the suction-side boundary layer whereas at higher flow velocity they are produced by the pressure-side boundary layer
According to most broadband trailing-edge noise models [38], the power spectral densities (PSD) of the far-field sound pressure is proportional to the spanwise correlation length ly defined from the coherence γ2 by the expression y(ω) =
Summary
Tonal trailing-edge noise refers to the loud, annoying whistling sometimes produced lifting surfaces at low Reynolds numbers and moderate angles of attack in a clean flow. A peak at much higher frequencies than the rotational noise has been reported for a low-speed fan by Longhouse [1], whereas a signature very similar to those of rectangular isolated aerofoils with multiple tones has been observed during the wind-tunnel testing of model propellers by Grosche and Stiewitt [2]. Both examples are discussed later on in the paper.
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