THESE de DOCTORAT - cerfacs

1.2 Scope of the present work 17
flame and consequently influencing it by modifying the heat release. When an unstable
coupling of this nature takes place, one will talk about ‘thermoacoustics instabilities’
[78, 22]. This is a broad field of study within the combustion community [48]. Efforts
have been done over several decades to understand the influence of acoustics on flame
dynamics [78, 18, 19, 73, 87, 49, 86, 88, 63]
Understanding combustion noise in real engines means understanding all these coupled phenomena.
However, the most clear starting point to study combustion noise is to ‘un-couple’ all
these mechanisms, focusing on one at a time. As a consequence, it is assumed that the sources
of noise (flame, turbulence, etc) are independent of the acoustic field generated. In other words,
that no instabilities occur. Still, the following noise generation phenomena remain and must be
considered:
• flame/entropy/acoustics → ‘indirect combustion noise’. The sources of noise are the
entropy waves when crossing non-homogeneous regions (zones with mean flow gradients).
• Turbulence/acoustics → ‘aerodynamic noise’. The unsteady turbulent field is the source
of noise. The influence of walls as scattering/reflecting mechanism (ex: wing airfoils, cavities)
are also usually considered into the physical formulations.
• flame/acoustics → ‘direct combustion noise’. The unsteady heat release acts as a distribution
of acoustic monopoles, which generate pressure fluctuations.
1.2 Scope of the present work
Combustion noise has been widely studied in open flames. These predictions are based on
what is known as hybrid methods. In these approaches the sources of noise are computed separately
from the radiated acoustic field. Unsteady CFD methods such as LES or DNS are used
to compute the sources of noise whereas wave equations coming from acoustic analogies are
employed to compute the sound radiation produced by these sources. LES or DNS are rarely
used to estimate acoustic fluctuations directly, since these fluctuations are much smaller in
comparison to first order fluctuations, as velocity or temperature perturbations. The first objective
of this thesis regards this concern. It is seen how much the computation resolution of LES
influences the quality of the estimations of noise. A second objective is linked to the hybrid approach
for the estimation of combustion noise. No significative studies within the combustion
community have been performed to evaluate whether or not hybrid approaches are appropriate
to compute noise of confined flames. Confinement of flames leads to possible complexities:
one of them is that pressure fluctuations produced by the flame might contain an important
contribution of hydrodynamics, since in confined flames turbulence might be significant everywhere.
Another significant concern is the evaluation of acoustic-flow interactions. When