This chapter discusses acoustic wave propagation in combustor environments. As noted in Chapter 2, acoustic waves propagate energy and information through the medium without requiring bulk advection of the actual flow particles. For this reason, and as detailed further in this chapter, the details of the time-averaged flow has relatively minor influences on the acoustic wave field (except in higher Mach number flows). In contrast, vortical disturbances, which propagate with the local flow field, are highly sensitive to the flow details. For these reasons, there is no analogue in the acoustic problem to the myriad different ways in which vorticity can organize and reorganize itself as in the hydrodynamic stability problem. Rather, the acoustic field is insensitive to these details and is largely controlled by the boundaries and sound speed field.

The acoustic problem, however, has its own unique, distinctive set of rich physics. In particular, sound waves reflect off of boundaries and refract around bends or other obstacles. In contrast, vortical and entropy disturbances advect out of the domain in which they are excited – the only way in which they can further influence the disturbance field in the system is if they excite backward-propagating sound waves, a topic discussed in Section 6.4.2. The wave propagation nature of sound waves also implies that an acoustic disturbance in any part of the system will make itself felt in every other region of the flow. For example, an acoustic disturbance in the combustor propagates upstream and causes oscillations throughout the air flow passages, into fuel supply systems, and all other locations downstream of a sonic point.