THESE de DOCTORAT - cerfacs

5.3 Combustion noise Analysis 85
Pressure Fluctuation (Pa)
M5 M6 M7
1000
Premixer
Combustion chamber
500
0
−500
−1000
−0.2 −0.1 0 0.1 0.2
x (m)
0.3 0.4 0.5
(a) Quarter pressure Wave from Eq. (2.63)
Hybrid Computation
Pressure Fluctuation (Pa)
M5 M6 M7
1000
Premixer
Combustion chamber
500
0
−500
−1000
−0.2 −0.1 0 0.1 0.2
x (m)
0.3 0.4 0.5
(b) Quarter pressure Wave from LES
Direct Computation
Figure 5.15: Longitudinal pressure waves oscillating at 377 Hz
cillation to be triggered in this region.
Pressure Fluctuation (Pa)
M5 M6 M7
600
Premixer
Combustion chamber
400
200
0
−200
−400
−600
−0.2 −0.1 0 0.1 0.2
x (m)
0.3 0.4 0.5
(a) Pressure Wave from Eq. (2.63)
Hybrid Computation
Pressure Fluctuation (Pa)
M5 M6 M7
600
Premixer
Combustion chamber
400
200
0
−200
−400
−600
−0.2 −0.1 0 0.1 0.2
x (m)
0.3 0.4 0.5
(b) Pressure Wave from LES
Direct Computation
Figure 5.16: Longitudinal pressure waves oscillating at 251 Hz
In Fig. 5.17 two different types of pressure waves are observed at microphone 5 for the direct
and hybrid computations in the region around 1000 Hz. Whereas a pure acoustic standing
wave is obtained by the hybrid approach, a perturbed pressure wave is obtained in the direct
computation results. A pure standing acoustic wave can naturally have an acoustic pressure
node. If this pressure node is present close to the region of the measurement device a low
value of pressure fluctuation will be obtained. This is what happens for microphone 5 in the
zone close to 1000 Hz (Fig. 5.17a). Obviously, when the pressure fluctuations not only contain
acoustics but also hydrodynamic perturbations as in the direct computations (Fig. 5.17b),
no pressure node can be observed and the resulting SPL is much higher than in the hybrid
computation case. Similar conclusions can be drawn at frequencies around 1650 Hz where a
pressure node is found close to Microphone 6 (Fig. 5.18).
In order to not have any ambiguity measuring acoustic pressure fluctuations due to the presence
of pressure nodes, a more appropriate quantity to evaluate at the microphone positions is