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Communication Dans Un Congrès Année : 2020

Experimental evidence of sound amplification by stable shear layers in a corrugated pipe

Résumé

Flow through a corrugated pipe can cause whistling in favorable conditions. Indeed, the shear layers over the cavities act as amplifiers for sound waves at certain frequencies. If the losses in the system (radiation losses and visco-thermal losses) are not larger than this amplification at a given frequency, a sound wave of this frequency propagating in the pipe will be amplified until non-linear phenomena limit its amplitude. This causes whistling in the pipe. To act as an amplifier, it is generally considered that the shear layer should be unstable, that is, that a Kelvin-Helmholtz (K-H) instability should be present in the shear layer. Such an instability is only possible for small shear-layer thicknesses. Indeed, a stability analysis of the shear layer shows that the growth factor becomes negative for Strouhal number based on the momentum thickness &_theta; larger than a value of 0.04 (Michalke JFM 1965). On the other hand, a shear layer acts as an amplifier for a grazing acoustic field at a Strouhal number based on the cavity width W typically between 0.2 ? 0.5. This means that, if the K-H instability is required to obtain amplification, it is only possible for thin shear layers compared to the cavity width (&_theta;/W < ~0.1). In this paper, we present measurements of sound propagation in three corrugated pipes of inner diameter of 30 mm, with cavity widths of 0.2 mm, 0.6 mm and 1.2 mm. In presence of flow, it is shown that sound waves are amplified at the expected Strouhal number, even for the very narrow cavities of 0.6 mm and 0.2 mm. It is furthermore observed that the sound amplification by the relatively thick shear layer remains linear for larger amplitude of the incident waves than for thin shear layers.
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Dates et versions

hal-03229438 , version 1 (21-05-2021)

Identifiants

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Joachim Golliard, Stefan Belfroid, Thomas Humbert. Experimental evidence of sound amplification by stable shear layers in a corrugated pipe. e-Forum Acusticum 2020, Dec 2020, Lyon, France. pp.791-792, ⟨10.48465/fa.2020.0326⟩. ⟨hal-03229438⟩
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