Application of theoretical acoustics to the reduction of jet engine noise

Abstract

Theoretical investigations of noise produced by jet engines should have the following objectives: (i) to understand the noise generation and propagation mechanisms; (ii) to suggest methods of reducing noise levels; (iii) to predict noise levels from various engine configurations.Because of the complex geometries associated with modern turbofan engines, idealized mathematical and simplified experimental models need to be studied and correlated with full-scale data.In the first part of this paper, a now classical example of the application of theory to rotor-stator interaction noise is discussed to illustrate how the first two objectives can be obtained essentially from a simple theoretical model. Some of the problems arising from predicting noise levels of interaction tone noise are considered, and a recent test is described that illuminates certain features of these problems.The second part of the paper describes some of the more recent work associated with the study of combination tone or multiple pure tone noise. This noise is produced in the inlets of turbofan engines whenever the fan blades have supersonic relative tip speeds. Results of large-scale fan tests are used to illustrate the physical characteristics of this noise. A mathematical model is introduced that determines the importance of blade shock-wave spacing in the noise generation process. Finally, a method of estimating the standard deviation of shock-wave spacing is presented and compared with full-scale data.These, and other similar models, are helping to achieve the three objectives mentioned. They are being incorporated in procedures for predicting noise of advanced engine designs.