THESE de DOCTORAT - cerfacs

1.1 Noise in a combustion chamber 15
One can identify from Fig. (1.1) the airframe noise and power-plant noise. The scale of turbulence
induced by the airframe varies considerably and a multifrequency noise signal is then
produced. During the final landing approach, airframe noise rises significatively. The deployment
of high-lift devices (slats and flaps) and the landing gear not only create drag but also
considerable levels of noise. Of these two sources, the landing gear produces the most intense
noise, giving a spectrum-level increase of 5-10 dB and changing the directivity of the overall
source to near-spherical [94]. Aircraft propulsion systems, however, are without a doubt the
major sources of aircraft noise. Propulsion systems come in many forms: the jet, the turbofan,
piston and turbine driven propellers, the helicopter rotor and the developing ‘open rotor’ or
advanced, high-cruise-speed propeller. The level of noise produced by any of these engines is
related to the maximum thrust level and the thermodynamical cycle performed. As a consequence,
take-off is one of the noisiest phases of flight (75 dB-85 dB at about 3 miles) [58] since
at this stage, the highest levels of power are reached.
Three main sources of noise can be identified in a turbofan aeronautical engine: the fan-stator
interaction; the core noise; and the exhaust jet flow. Vortex shedding is created when the incoming
stream of air flows through the fan. These wakes slap against the stators like waves on
a beach generating in turn important acoustic waves. The design of blades (profile, thickness,
number) for both fan and stator, the angular velocity of the fan and the absorption treatment
for tones in the inlet nacelle/bypass air ducts are the main mechanisms to control fan noise.
Core noise is the noise produced inside the combustion chamber and is related to the interactions
that can arise between the flame and the surroundings. No significant progress has been
done to control core noise so far as it is little understood. Absorbing materials to damp acoustic
waves is out of the question in this extremely hot region; any fundamental advancement would
consist in controlling combustion dynamics. Finally, it remains the so-called jet noise. The pure
jet engines and the low-bypass-ratio engines have extremely high exhaust velocities, which in
turn cause the highest levels of noise when compared to fan and core noise. Strong turbulence
is created when a high velocity stream of hot gases is discharged into the atmosphere. The
shear layer generated accounts for strong fluctuations of the largest turbulent eddies, which in
turn generate big amplitude and low frequency pressure waves. Levels of jet noise have been
decreased mainly by velocity reduction of the jet flow that mixes with the ambient air. The design
of high by-pass ratio engines has been conclusive on this matter since reductions up to 10
dB have been reached [58]. As a conclusion, it can be stated that due to the different improvements
in the science of noise reduction, relevant to jet and fan noise, the relative importance of
combustion noise tends to increase. Those seeking advancement in the field of noise reduction
are now looking towards combustion.
1.1 Noise in a combustion chamber
Acoustics in a combustion chamber (CC) is generated by different physical mechanisms which,
coupled or not, occur either inside, outside or at the boundaries of the combustor: