Comparison of Jet Noise Models

Acoustic analogy theories for Jet Noise Prediction are formulated from the Navier Stokes equations using Green functions, describing wave propagation, and semi-empirical modeling of the turbulent statistics that act as noise sources. In a departure from this standpoint the Tam and Aurialt use an ad-hoc kinetic theory type argument that has been successful in predicting the 90 degree acoustic spectrum. In this paper we reassess the underlying assumptions of the Tam and Aurialt model. It is shown, while the Tam and Aurialt model can be interpreted as the integration of a source with a derivative of a Green function; the particular derivative is discontinuous across a thin shear layer, resulting in a sound power spectrum that is sensitive to the position of the discontinuity even when the acoustic wavelength is large. This is a disadvantage because there is some arbitrariness in the radial position of maximum shear, if a parallel shear flow is used to represent a developing jet. Comparison is made between predictions of the far-field sound from acoustic analogies and the Tam and Aurialt model.