THESE de DOCTORAT - cerfacs

7.3 Acoustic Evaluation 123
In the present helicopter system, this coefficient is close to η p = 0.87 for the first compressor
stage and η p = 0.9 for the second one. The mean field can be, as a result, be modeled properly
in the computational analysis. This is shown in Fig. 7.3.
12
10
8
SNozzle
Helicopter Engine Data
2.5
2
SNozzle
Helicopter Engine Data
6
1.5
4
2
1
0
0 0.2 0.4 0.6 0.8
Curvilinear Axis (m)
0.5
0 0.2 0.4 0.6 0.8
Curvilinear Axis (m)
a) Normalized Total Pressure b) Normalized Total Temperature
Figure 7.3: Total Pressure and Total Temperature profiles
7.3 Acoustic Evaluation
The reflection coefficient R = w + 2 /w− 2
is computed for the outlet section of the intake duct
which corresponds to the inlet of the helicopter combustion chamber. It is evaluated for two
different cases:
• CASE A → Airline with total pressure jump: The evaluation of acoustics is done for the
geometry including the compressors. It means that a total pressure jump is imposed to
the mean flow.
• CASE B → Airline without total pressure jump: In order to assess the actual influence
of the compressors on the acoustics of the system, the acoustic response of the intake is
done without any total pressure jump. In fact, this is how the reflection coefficient was
used to be computed [41] before the development of the Snozzle tool.
It is noticeable that the presence of the compressors increases the amplitude of the reflected
wave (Fig. 7.4 a). When no total pressure jump is imposed, the amplitude of the reflected wave
is similar to the forced one, whereas it is twice as large when the work done by the compressor
is accounted for. The phase of R (Fig. 7.4 b) also changes when the compressors are added to
the system.