THESE de DOCTORAT - cerfacs
THESE de DOCTORAT - cerfacs THESE de DOCTORAT - cerfacs
14 Chapter 1: General Introduction Noise Exposed population Produced highly annoyed (dB) (%) 45 1 50 5 55 10 60 17 65 26 70 37 75 49 Table 1.1: EU recommended relationships between annoyance and aircraft noise as interference with intended activities [104]. But humans are not the only ones that suffer from noise pollution; significant structural damage can occur to aircrafts when vibrations are generated at specific frequencies. But what is aircraft noise? Aircraft noise is the superposition of propagating pressure fluctuations that are generated by different mechanisms belonging to the aircraft. These pressure fluctuations propagate to both inwards into the cabine, affecting the passengers, and outwards to the environment, annoying the inhabitants of nearby regions. Within an aircraft, several sources of noise can be identified as shown in Fig. (1.1) Fuselage Fan Combus5on Chamber Jet Slats Flaps Landing gear Figure 1.1: Main sources of aircraft noise 1 Our hearing systems are not equally sensitive to all sound frequencies (ISO 1987a). Thus, not all frequencies are perceived as being equally loud at the same sound pressure level, and when calculating overall environmental noise ratings it is necessary to consider sounds at some frequencies as more important than those at other frequencies.
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:
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14 Chapter 1: General Introduction<br />
Noise Exposed population<br />
Produced highly annoyed<br />
(dB) (%)<br />
45 1<br />
50 5<br />
55 10<br />
60 17<br />
65 26<br />
70 37<br />
75 49<br />
Table 1.1: EU recommen<strong>de</strong>d relationships between annoyance and aircraft noise<br />
as interference with inten<strong>de</strong>d activities [104]. But humans are not the only ones that suffer<br />
from noise pollution; significant structural damage can occur to aircrafts when vibrations are<br />
generated at specific frequencies.<br />
But what is aircraft noise? Aircraft noise is the superposition of propagating pressure fluctuations<br />
that are generated by different mechanisms belonging to the aircraft. These pressure<br />
fluctuations propagate to both inwards into the cabine, affecting the passengers, and outwards<br />
to the environment, annoying the inhabitants of nearby regions. Within an aircraft, several<br />
sources of noise can be i<strong>de</strong>ntified as shown in Fig. (1.1)<br />
Fuselage<br />
Fan<br />
Combus5on<br />
Chamber<br />
Jet<br />
Slats<br />
Flaps<br />
Landing gear<br />
Figure 1.1: Main sources of aircraft noise<br />
1 Our hearing systems are not equally sensitive to all sound frequencies (ISO 1987a). Thus, not all frequencies are<br />
perceived as being equally loud at the same sound pressure level, and when calculating overall environmental noise<br />
ratings it is necessary to consi<strong>de</strong>r sounds at some frequencies as more important than those at other frequencies.
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