On the generation of indirect combustion noise

It is known that a uniform compressible flow supports three types of disturbances. They are the entropy waves, which consist of only temperature and density fluctuations, the acoustic waves, and the vorticity waves. When entropy waves are convected into a non-uniform flow region, they are no longer independent waves. Their presence leads to pressure fluctuations, which results in the 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 has been attributed to mode coupling, that is, the coupling of entropy and acoustic modes. The principal objective of the present investigation is to seek a more physical explanation of how indirect combustion noise is generated. In addition, a numerical simulation is carried out to demonstrate the generation of indirect combustion noise arising from the passage of a line entropy wave pulse through the non-uniform flow field created by a turbine blade. This simulation serves as a simple example illustrating the complex processes involved in sound generation and radiation that take place when entropy waves from a combustor pass through a turbine stage.