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Etude expérimentale du bruit de bord de fuite à large bande d'une grille d'aubes linéaire et de sa réduction par dispositifs passifs

Abstract : Broadband trailing edge noise is one of the main contributors to modern turbofan noise. The current need for both understanding and reducing those sources motivated the present work. This study focuses on the cascade effect which is produced by the scattering of acoustic waves on neighbouring blades. A seven blade linear cascade is set up for acoustic measurements in the downstream sector with varying speed and angle of attack. Broadband trailing edge noise is thus the main noise source in the facility on a wide frequency range. Acoustic resonances in the cascade and far field interferences as well as specific velocity dependence are proofs of the sought blade interactions. To give a more quantitative insight in the cascade effect, Amiet isolated airfoil trailing edge noise model is first used. Its input data which are a statistical description of the turbulent boundary layer are directly measured on the suction surface of the center blade. The noise levels are fairly well predicted suggesting that the cascade effect only moderately affects the far-field acoustic spectra. Glegg's cascade model is then modified to fit the experimental set-up and used with the same input data. The estimates differ from the isolated airfoil predictions from ±3dB confirming the preceeding conclusions. However far field interferences are well recovered by Glegg's model. Noise reduction is then assessed in this study. First, brushes are inserted in an isolated airfoil trailing edge and a broadband noise reduction of 4,5 dB is obtained. A hot wire coherence study is carried out in the near wake of the brush showing that spanwise decorrelation could be partly responsible for the observed noise reduction. Trailing edge serrations are finally applied on the cascade trailing edges and the same reduction potential than on isolated airfoil with the same device is recovered. This shows that the cascade effet has little influence on the noise reduction process. This mechanism is more likely to be threefold. Particle image velocimetry measurements show that the suction side boundary layer is thrown out from the airfoil surface which could result in smaller induced surface pressure. Secondly, the vorticity sheet shed in the wake because of the Kutta condition is necessarily less coherent in the spanwise direction with the serrations than with the straight edge. Last, the reduced relative angle between the flow and the local trailing edge could also theorically reduce the far-field noise. This has been investigated analytically by means of a modification of Amiet's model to account for the sweep angle of the blade.
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Contributor : Arthur Finez <>
Submitted on : Tuesday, September 25, 2012 - 10:49:48 AM
Last modification on : Wednesday, July 8, 2020 - 12:42:05 PM
Long-term archiving on: : Wednesday, December 26, 2012 - 4:35:12 AM

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  • HAL Id : tel-00735034, version 1

Citation

Arthur Finez. Etude expérimentale du bruit de bord de fuite à large bande d'une grille d'aubes linéaire et de sa réduction par dispositifs passifs. Acoustique [physics.class-ph]. Ecole Centrale de Lyon, 2012. Français. ⟨NNT : 2012-07⟩. ⟨tel-00735034⟩

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