The physical processes of indirect combustion noise generation

Abstract

It is known that a uniform compressible flow supports three types of disturbances. One is entropy wave. These waves are made up of temperature and density fluctuations only. The other two are acoustic waves and vorticity waves. When entropy waves are convected into a nonuniform flow region, they are no longer independent waves. Their presence leads to pressure fluctuations, which result in noise radiation commonly referred to as indirect combustion noise. In the past, because entropy waves and acoustic waves are not independent waves in a nonuniform flow region, the mechanism responsible for the generation of indirect combustion noise had been attributed to mode coupling. That is, there is a coupling of entropy and acoustic modes. This leads to pressure fluctuations and hence noise radiation. The principal objective of the present investigation is to seek a more physical explanation of how indirect combustion noise is generated.