COMPUTATION OF JET MIXING NOISE DUE TO COHERENT STRUCTURES - THE PLANE JET CASE

A computational approach to the prediction of jet mixing noise is desc ribed. It is based on Lighthill's analogy, used together with a semi-d eterministic modelling of turbulence (SDM), where only the large-scale coherent motion is evaluated. The features of SDM are briefly illustr ated in the case of shear layers, showing that suitable descriptions o f the mean flow and of the large-scale fluctuations are obtained. Aero dynamic calculations of two cold fully expanded plane jets at Mach num bers 0.50 and 1.33 are then carried out. The numerical implementation of Lighthill's analogy is described and different integral formulation s are compared for the two jets. It is shown that the one expressed in a space-time conjugate (kappa, omega)-plane is particularly convenien t and allows a simple geometrical interpretation of the computations. Acoustic results obtained with this formulation are compared to releva nt experimental data. It is concluded that the radiation of subsonic j ets cannot be explained only by the contribution of the turbulent cohe rent motion. In this case, directivity effects are well recovered but the acoustic spectra are too narrow and limited to the low-frequency r ange. In contrast at Mach number 1.33, especially in the forward quadr ant, results are satisfactory, showing that coherent structures indeed provide the main source of supersonic jet mixing noise.