Comptes Rendus
Shear-layer acoustic radiation in an excited subsonic jet: experimental study
Comptes Rendus. Mécanique, Volume 333 (2005) no. 10, pp. 746-753.

The subharmonic acoustic radiation of a tone excited subsonic jet shear-layer has been investigated experimentally. Two jet velocities Uj=20 ms−1 and Uj=40 ms−1 were studied. For Uj=20 ms−1, the natural boundary-layer at the nozzle exit is laminar. When the perturbation is applied, the fluctuations of the first and the second subharmonics of the excitation frequency are detected in the shear-layer. In addition, the first subharmonic near pressure field along the spreading jet is constituted of two strong maxima of sinusoidal shape. The far-field directivity pattern displays two lobes separated by an extinction angle θ at around 85° from the jet axis. These observations follow the results of Bridges about the vortex pairing noise. On the other hand, for Uj=40 ms−1, the initial boundary-layer is transitional and only the first subharmonic is observed in the presence of the excitation. The near pressure field is of Gaussian shape in the jet periphery and the acoustic far-field is superdirective as observed by Laufer and Yen. The state of the initial shear-layer seems to be the key feature to distinguish these two different radiation patterns.

Le rayonnement acoustique sous-harmonique de la couche de cisaillement d'un jet subsonique excité tonalement a été étudié pour deux vitesses de jet, Uj=20 ms−1 et Uj=40 ms−1. Pour Uj=20 ms−1, la couche limite naturelle en sortie de buse est laminaire. En présence du forçage, les fluctuations du premier et du second sous-harmonique de la fréquence de l'excitation sont détectées dans la couche de cisaillement. Par ailleurs, le champ proche du premier sous-harmonique de la pression mesuré le long du jet est constitué de deux maxima de forme sinusoïdale. Le diagramme de directivité en champ lointain possède deux lobes séparés par un angle d'extinction θ autour de 85° de l'axe du jet. Ces observations sont en accord avec les résultats de Bridges concernant le bruit d'appariement tourbillonnaire. En revanche, pour Uj=40 ms−1, la couche limite initiale est transitionnelle et seul le premier sous-harmonique est observé lorsque l'excitation est utilisée. Dans ce cas, le champ de pression proche en périphérie du jet est de forme gaussienne et le champ acoustique lointain est superdirectif, comme observé par Laufer et Yen. L'état initial de la couche de cisaillement semble être le facteur déterminant pour distinguer ces deux rayonnements très différents.

Received:
Accepted:
Published online:
DOI: 10.1016/j.crme.2005.09.003
Keywords: Fluid mechanics, Excited jet, Subharmonic, Pairing noise, Superdirective radiation
Mot clés : Mécanique des fluides, Jet excité, Sous-harmonique, Bruit d'appariement, Rayonnement superdirectif

Vincent Fleury 1; Christophe Bailly 1; Daniel Juvé 1

1 Laboratoire de mécanique des fluides et d'acoustique, UMR CNRS 5509 & École centrale de Lyon, 36 avenue Guy de Collongue, 69134 Ecully cedex, France
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Vincent Fleury; Christophe Bailly; Daniel Juvé. Shear-layer acoustic radiation in an excited subsonic jet: experimental study. Comptes Rendus. Mécanique, Volume 333 (2005) no. 10, pp. 746-753. doi : 10.1016/j.crme.2005.09.003. https://comptes-rendus.academie-sciences.fr/mecanique/articles/10.1016/j.crme.2005.09.003/

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