THESE de DOCTORAT - cerfacs

148 Chapter B: Publications
LES succeed in predicting the central recirculation zone satisfactorily. The LES on the ‘refined’ mesh is
however more accurate for the outer region, particularly for the radial velocity. The fluctuating velocity
field is characterized by rms profiles. Figure 5 shows that on the ‘coarse’ mesh a high overprediction of
velocity fluctuations is obtained in both axial and radial components. On the fine grid however the LES
clearly recovers the experimental velocity fluctuating field.
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(a) Mean Axial Velocity (m/s)
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(b) Mean Radial Velocity (m/s)
Figure 4: Velocity Profiles: ◦ Experimental PIV measurements
– – – LES 3 million cells, —— LES 10 million cells
Acoustics and flame dynamics of the system represented by the heat release are, on the contrary, more
difficult to evaluate than the mean and fluctuating velocity field. The mean value of heat release is similar
in both LES and is close to the 40kW experimental thermal power, as can be observed in Fig. 6(a).
Different values in the variations of heat release are however obtained for each LES. Strong and more
regular fluctuations of heat release are obtained with the ‘coarse’ mesh while smaller and less periodic
fluctuations are given by the ‘refined’ mesh computation. It is likely that the coarser mesh does not
capture enough small turbulent scales and trigger too large turbulent eddies. These large coherent
structures might clearly have an influence on the flame dynamics and thus in the large fluctuations of
heat release. The value of the rate of change of heat release integrated over the whole volume of the
combustor has also been computed for the two different meshes and is shown in Fig. 6(b). On the
finer mesh a quieter flame is modeled, considering the smaller values of rate of change of heat release
compared to those obtained from the coarse mesh. As a consequence, smaller rms pressure values should
be expected on the finer mesh. Acoustics in the chamber is rather characterized by the Sound Pressure
Level (SPL) at a given point than rms values of the pressure. Figure 7 compares the SPL values at the
microphone 7 (see the location of M7 in Fig. 2) of the computations on the refined and coarse meshes
to the experimental measurements. Both LES clearly overestimate the sound levels with a significant
improve on the finer. It is then found that in order to correctly evaluate the dynamics of a flame and
the acoustics generated by this one it is not enough to satisfactorily model the fluctuating velocity field
as shown in Figs. 4 and 5. As stated before, computing acoustic pressure fluctuations is very challenging
since these values are very small compared to the aerodynamic fields. Several additional phenomena can
play an important role.
First, the performed LES assume a perfect premixed mixture of air and fuel in the reactive region. This
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