Is Lighthill’s acoustic analogy still relevant?

Abstract

More than fifty years after Lighthill’s seminal paper, the acoustic analogy appears to be out of favor, even though the theory has been able to cope with all aspects of jet noise. This review examines the principal features of Lighthill’s acoustic analogy through the lens of the self-and shear noise theory (one of several possible formulations), introduced by Proudman and extended by Ribner and others. One to infer a great deal about the sound field of a jet with a minimum of information about the details of the flow. For example, the well-known U8 scaling law is readily extended to predict the variation of the far-field power spectra of jet noise. The validity has been demonstrated by applying the prediction algorithm to measured jet noise data. Correlations of the source terms and the far-field sound have been measured and shown to exhibit closure. Subtle features such as two-point correlations of far-field sound pressures are accurately described. Recently, Ribner has proposed a means by which the elusive refraction due to flow and temperature gradients can be dealt with in a consistent manner. Predictions based thereon agree quite well with measurement, even for small angle from the jet axis.