On the role of the shear term in jet noise

Abstract Increasing attention is being focused on Lilley's convected wave equation for the analysis of flow noise. It has the capability, not found in Lighthill's equation, of being able to deal with refraction. The other chief difference stems from the shift of a “source” term (shear noise) from the right-hand side of Lighthill's equation to the left-hand side to become a “propagation-amplification” term. It is argued herein, supported by comparative calculations according to theories of Mani and of Ri brier, that the alternative roles of the shear term have roughly equivalent effects on jet noise prediction outside the “refraction valley”. In either role certain mean flow “shrouding effects” are accounted for.