6.7.1 Introduction to internal flows

Among the many aeroacoustics phenomena that may occur in internal flows, the experience of the industrial context leads us to consider the following first:

• Acoustic fluctuations due to turbulence-generated noise at low Mach number,

• Self-sustained oscillations and flow acoustic coupling at low Mach number, and

• Aeroacoustic instabilities at high Mach number.

These phenomena are similar to the ones observed in free flows, but in the context of confined flows, many features of these phenomena are enhanced; for example, flowacoustic interactions are stronger.

The numerical approaches based on LES or very large eddy simulation (VLES) we present in this chapter were developed in an industrial context. Thus, direct acoustic simulations by compressible DNS are out of our scope.

Section 6.7.2 presents a hybrid approach based on the computation of incompressible LES and LEE. ^[*] This approach is able to take into account the generation and the propagation of acoustics disturbances due to the presence of control flow devices (diaphragms, valves, etc.) in industrial ducts. In Section 6.7.3, a direct approach based on the computation of nonlinear Euler equations is able to model the flow-acoustics interactions at low Mach numbers when there are feedback phenomena and acoustic resonances. In Section 6.7.4, the same set of equations is solved in order to model transient aeroacoustic phenomena in high-Mach-number flows.

All these cases were defined and computed to obtain information about real problems occurring...