Abstract
The influence of axial and azimuthal source coherence on the noise radiation has been investigated. For jet noise at small Helmholtz numbers results of a theoretical source model have been compared with measured data. It is found from the noise data that the axial coherence length scales of the sound-radiating turbulence is excited artificially. Furthermore, are relatively large and do increase when the turbulence is excited artificially. Furthermore, the influence of the azimuthal source coherence length scale on the sound radiation from a source ring has been discussed. It is found that the sound field depends remarkably on the azimuthal source coherence, and that a completely incoherent source ring does not radiate sound. If the jet shear noise-source term is modelled, then in the source ring plane the second azimuthal sound component dominates at small Helmholtz numbers, as observed in jet experiments.
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