On the Noise Radiated from a Turbulent High Speed Jet

Abstract

The study of noise radiated from aero dynamically unsteady fluid flows was studied by Lighthill, who showed there exists an “acoustic analogy” whereby the exact fluid flow can be replaced by an equivalent distribution of acoustic quadrupole sources in which the sources may move but not the fluid. The problem posed by Lighthill was that although this “acoustic analogy” represented an exact solution of the Navier-Stokes equations for the density or pressure, in applications of engineering interest the flow field is rarely known to the exactness required to provide an accurate prediction of the radiated sound field, its acoustic power output, its intensity distribution with distance and direction, and its spectrum. Approximate models of the flow field for subsonic and supersonic jets at rest or moving at a uniform speed through a stationary atmosphere have been introduced into Lighthill’s “acoustic analogy” for the prediction of the radiated sound and to provide a comparison with experimental results. The difficulties in the applications of these models and the limitations in the success achieved with such models has been the subject of continuing study.

This paper gives a brief status report of the current position of the prediction methods used in the noise from jets, and comments on the available experimental data used to verify the accuracy of these models and procedures. Also discussed are the possible computational methods which could exploit current advances made in supercomputer technology in solving the Euler and Navier-Stokes equations for certain classes of flows and which would lead to the evaluation of the radiated sound field from such flows. The problems of computational domains containing embedded flows and their boundary conditions together with the resolution requirements are considered. Also considered are the advantages of the deployment of the “acoustic analogy” over the direct computation of the fluctating pressure and density fields within and external to the flow.