Modelling self- and shear-noise mechanisms in inhomogeneous, anisotropic turbulence

Abstract

A statistical jet noise model which includes the effects of mixing layer inhomogeneity and anisotropy is presented. The model adapts the spatial and temporal correlation function models frequently used in jet noise prediction, so that the axial, radial and lateral integral space and time scales are included to model flow anisotropy, the spatial structure of the Reynolds stress field being used to account for flow inhomogeneity. These flow properties have been estimated from single and multi-point LDV measurements performed in the mixing layer of an isothermal jet with a Mach number of 0.75. The model is used to assess acoustic contributions from the constituent self- and shear-noise quadrupoles. Results highlight the very different nature of the self- and shear-noise source mechanisms and identify those aspects of the flow structure on which their sound production efficiency depends. The dependence of the shear noise on the radial distance over which the turbulence is correlated illustrates how an isotropic model will overestimate this term. The model demonstrates how sound generation is largely dominated by axially aligned longitudinal quadrupoles and shows the self-noise mechanisms to dominate the shear by a factor of about 2.5 when the flow is anisotropic. A moving-axis temporal correlation model is used to derive expressions for the acoustic spectra of the component quadrupoles, the inclusion of whose time scales allows the temporal manifestation of the flow anisotropy to be modelled. Model predictions are compared with acoustic measurements performed on the same jet and good agreement is found for emission angles between 30° and 100°.