No abstract

No abstract

On présente une description spatio-temporelle de la diffusion d'ondes acoustiques par une zone de mélange turbulente. Les mesures ont été effectuées dans la soufflerie à veine ouverte de l'Institut von Karman dont le diamètre initial du jet libre est 3 m et la vitesse maximum de 60 m/s. Dans le cas d'ondes incidentes monochromatiques, l'analyse spectrale de la pression sonore met en évidence un élargissement des raies présentant une structure fine. L'étude des corrélations spatio-temporelles et des spectres croisés entre les signaux directs ou les modulations permet de caractériser le défilement du champ acoustique diffusé et d'estimer la vitesse de convection des tourbillons diffuseurs dans la zone de mélange

The present study is dedicated to the analytical modeling of the broadband noise radiated by subsonic fans, such as encountered in HVAC or automotive engine cooling applications. A fan noise prediction scheme is proposed on the basis of single-airfoil linearized unsteady aerodynamics theories. Two basic spanwise distributed mechanisms are considered, namely the noise from the impingement of upstream turbulence, and the trailing edge noise associated with turbulence boundary layer scattering. The associated analytical models are validated by comparing the theoretical results with experimental data collected in open-jet anechoic wind tunnels, and existing numerical computations found in the literature. Validations involve single-frequency directivity, single-direction spectra and radiation maps in a large range of flow conditions over different mock-ups in the midspan plane. The transfer function between the wall-pressure statistics and the far-field sound should be invariant according to the analytical model and is experimentally found to be within ±5 dB with respect to flow conditions encountered on an isolated airfoil. Finally, a first application of the model to a highly twisted, tapered and swept automotive engine cooling fan is presented. In view of the results, trailing-edge noise is found to be significant contributor, at least in the high frequency range beyond 4 kHz for a chord-based Reynolds number of about 2 x 10 5 and Mach number of about 0.1.

No abstract