Experimental and Analytical Investigation of the Tonal Trailing-Edge Noise Radiated by Low Reynolds Number Aerofoils

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.

Profile vortex shedding from low-speed axial fan rotor blades: a modelling overview

“…Moreover, for zero angle of attack, they found that the maximum tone can be detected if the BL reattaches just at the TE, and the decay of the tones is closely related to the laminar-toturbulent transition of the BL. Recently, Yakhina et al [10] also represented that the formed separation bubble plays an important role in tonal TE noise radiated by low-Reynolds-number airfoils. Nash et al [11] proposed a new mechanism for PVS based on a VS process.…”

Noise from a Model-Scale Vertical-Axis Wind Turbine

“…A possible explanation is that the instabilities lock on to the duct resonances. This would be plausible behavior, given the nature of the underlying generation mechanism [22][23][24][25] and the presence of multiple duct modes in the relevant frequency range.…”

“…Laminar-instability noise can be eliminated by forcing transition in the relevant boundary layer. This may lie on either the pressure or the suction surface [22]. The situation is further complicated for a VAWT blade, as the pressure and suction surfaces alternate between outside and inside as the rotor circle is traversed.…”

“…from 53.6 dB to 48.6 dB in TSR 3 contra-flow at 4 kHz). However, the possibility of coupling between the underlying laminar instability on one airfoil surface and the boundary layer on the other is now well established (cf., for example, [22][23][24]); while the conditions allowing the instability to develop are local, those governing its final level are not. Hence modification of such coupling by the outer-surface trip seems the most likely explanation for the apparent anomaly.…”

Theoretical Study of the Acoustic Installation Effects in Closed-Vein Wind Tunnels for the Experimental Characterization of Trailing Edge Noise

“…Other TE noise sources exist, such as the vortex shedding entailed by a blunted trailing edge (which also generates a strong tonal, dipolar acoustic emission), or the flow separation occurring in the early stage of stall (as opposed to the deep stall one, which rather results in noise radiating from the entire airfoil). Given its multiple components and complex physics, the airfoil trailing edge (TE) noise is still only partly understood [1][2][3], despite half a century of investigations using either experimental [1,[4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19], theoretical [15][16][17][18][19], or computational [20][21][22][23][24][25] means.…”