CERFACS CERFACS Scientific Activity Report Jan. 2010 – Dec. 2011

COMPUTATIONAL FLUID DYNAMICS
TU Berlin, TU Munich, EM2C Paris) has allowed to identify the paths to follow to combine experiments
and high fidelity simulations to study CIs.
FIG. 2.8: Large Eddy Simulation of azimuthal unstable modes in a full annular burner. Left : configuration
of a single sector. Center : instantaneous pressure field on combustor skin. Right : isosurfaces of velocity
colored by temperature. PhD of P. Wolf.
[CFD184]
Fig. 2.8 shows an LES of a self-excited CI in a gas turbine combustion chamber [CFD110] : this high
resolution LES (40 to 330 million cells) running on a BlueGene machine (16 000 proc.) captures the
instabilities which appeared for certain regimes in the first prototypes of the real engine. Acoustic waves
interact with combustion, leading to a strong unstable mode characterized by periodic flashback through
the swirling systems used to inject and mix fuel. For the fine grid (330 million cells), 1000 CPU years were
used.
2.2.3 Acoustic solvers (TA codes) (E. Gullaud, C. Silva, K. Wieczorek, P. Salas,
E. Motheau, JF. Parmentier, L. Giraud, T. Poinsot,F. Nicoud)
The efforts to build a powerful TA code have been intensified at CERFACS with the development of a
new branch dedicated to combustion noise. The TA and LES computations of the CESAM experiment
of EM2C [CFD173, CFD108] will be pursued during DISCERN ANR project started in october 2011.
Different models for the acoustic dissipation induced by perforated liners were also implemented [CFD170]
and were used for large scale computations of full engines. Such computations are now easier thanks
to the improvement of the numerics of AVSP made by L. Giraud (HIEPACS project) and his PhD
student (P. Salas) supported by the MYPLANET Marie Curie project lead by the CFD team (Dr. T.
Schönfeld). The required boundary conditions are also computed by a new dedicated tool (SNozzle which
took over Nozzle in 2011) which solves the full Linearized Euler Equations for nozzle flows under the
quasi-1D approximation. Thanks to SNozzle, the acoustic-entropy waves interactions in the accelerated
regions and the contribution from the rotor stages are now accounted for properly when computing the
effective impedance of compressors and turbines [CFD173]. A consistent way to use these equivalent
upstream/downstream boundary impedances in the framework of a Helmholtz solver such as AVSP was
also established by E. Motheau [CFD52]. At last, several theoretical works were conducted to support the
AVSP development : an original quasi-analytical model for instability modes in an annular combustion
chamber fed by N burners was proposed by J-F. Parmentier [CFD103]. This model was used to demonstrate
the ability of AVSP to perform the modal analysis of annular geometries where standing modes can combine
to produce rotating modes as observed in actual systems. The impact of the zero-Mach number assumption
on the modal analysis was also studied in details by K. Wieczorek [CFD175]. Theoretical assessments of
CERFACS ACTIVITY REPORT 139