Airfoil noise reduction by edge treatments

Abstract

The general aim of this thesis is to investigate experimentally airfoil trailing edge noise reduction using various trailing edge geometries. The work presented in this thesis is part of the FP7 European Project FLOCON. This thesis focuses on sawtooth serrations and a detailed study is conducted in which thirty seven sawtooth trailing edges are tested for reducing the noise at various flow velocities and angles of attack. Broadband noise reductions of up to 5 dB are obtained below some critical frequency above which the noise is increased. The mechanisms by which the noise is changed in the presence of sawtooth serrations are also investigated experimentally by measuring the changes introduced in the unsteady surface pressure near the edge, the turbulence in the boundary layer and in the near wake, and also using Howe's model [66] as a reference for comparisons. Generally, it is shown that noise reductions occur due to an attenuation of the interaction between incident and scattered pressures, which results in a decrease of up to a half of the phase speed along the edges compared with the corresponding straight edge. The noise increase is shown to be caused by a cross-flow being forced through the valleys of the serrations by the pressure difference between the two sides of the airfoil near the trailing edge.

Four novel trailing edge geometries are also tested to address the high frequency noise increase observed with sawtooth serrations. These are the slits, the sawtooth with holes, the slitted sawtooth and the random trailing edges. The slitted sawtooth are shown to provide a good alternative to sawtooth serrations, and afford similar levels of noise reductions while limiting the high frequency noise increase to no more than 1 dB. Random trailing edges also show reasonable levels of broadband noise reductions of up to 3 dB and no increase at high frequencies.

Finally, serrations are used simultaneously at the trailing edge of an upstream airfoil and at the leading edge of a downstream airfoil to reduce trailing edge noise and interaction noise of the airfoils in a tandem configuration. Broadband reductions of up to 8.5 dB are obtained using the slitted sawtooth trailing edge and the leading edge serrations designed by ONERA. It is shown that most of the noise reduction is provided by a reduction of the airfoil leading edge response due to the leading edge serrations, but that sawtooth slitted serrations provide up to about 3.5 dB additional broadband noise reductions due to a reduction in its wake turbulence.

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