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The 24 April 2017 at 14h00

PhD Defense: Franchine NI – Accounting for complex flow-acoustic interactions in a 3D thermo-acoustic Helmholtz solver

Marie LABADENS |  Cerfacs, Salle de conférence Jean-Claude ANDRÉ |  


Environmental concerns have motivated turbine engine manufacturers to create new combustor designs with reduced fuel consumption and pollutant emissions. These designs are however more sensitive to a mechanism known as combustion instabilities, a coupling between flame and acoustics that can generate dangerous levels of heat release and pressure fluctuations. Combustion instabilities can be predicted at an attractive cost by Helmholtz solvers. These solvers describe the acoustic behavior of an inviscid fluid at rest with a thermoacoustic Helmholtz equation, that can be solved in the frequency domain as an eigenvalue problem. The flame/acoustics coupling is modeled, often with a first order transfer function relating heat release fluctuations to the acoustic velocity at a reference point.
One limitation of Helmholtz solvers is that they cannot account for the interaction between acoustics and vorticity at sharp edges. Indeed, this interaction relies on viscous processes at the tip of the edge and is suspected to play a strong damping role in a combustor. Neglecting it results in overly pessimistic stability predictions but can also affect the spatial structure of the unstable modes. In this thesis, a methodology was developed to include the effect of complex flow-acoustic interactions into a Helmholtz solver. It takes advantage of the compactness of these interactions and models them as 2-port matrices, introduced in the Helmholtz solver as a pair of coupled boundary conditions: the Matrix Boundary Conditions. This methodology correctly predicts the frequencies and mode shapes of a non-reactive academic configuration with either an orifice or a swirler, two elements where flow-acoustic interactions are important.
For industrial combustors, the matrix methodology must be extended for two reasons. First, industrial geometries are complex, and the Matrix Boundary Conditions must be applied to non-plane surfaces. This limitation is overcome thanks to an adjustment procedure. The matrix data on non-plane surfaces is obtained from the well-defined data on plane surfaces, by applying non-dissipative transformations determined either analytically or from an acoustics propagation solver. Second, the reference point of the flame/acoustics model is often chosen inside the injector and a new reference location must be defined if the injector is removed and replaced by its equivalent matrix. In this work, the reference point is replaced by a reference surface, chosen as the upstream matrix surface of the injector. The extended matrix methodology is successfully validated on academic configurations. It is then applied to study the stability of an annular combustor from Safran. Compared to standard Helmholtz computations, it is found that complex flow-acoustic features at dilution holes and injectors play an important role on the combustor stability and mode shapes. First encouraging results are obtained with surface-based flame models.


Yves AUREGAN       Research Director – Laboratoire d’Acoustique de l’Université du Maine                    Referee

Jonas MOECK          Associate Professor – TU Berlin ISTA                                                                        Referee

Aimee MORGANS    Associate Professor – Imperial College London                                                         Member

Yoann MERY             Research Engineer – Safran Aircraft Engine                                                              Member

Thierry POINSOT      Research Director –  Institut de Mécanique des Fluides Toulouse                             Advisor

Franck NICOUD        Professor – Université de Montpellier  IMAG                                                              co Advisor



Thierry Poinsot officially entered the French Academy of Sciences

CERFACS |  8 November 2021

Thierry Poinsot officially entered the French Academy of Sciences on October 12. See presentation here :Read more

The AVBP code from CERFACS at the heart of for PRACE projects from the 23rd call

CERFACS |  30 September 2021

Cerfacs is involved in three PRACE projects of the 23rd call for which hour allocation runs from 01/10/2021 to 30/09/2022. Researchers from ECL/LMFA UMR5509 (Ecole Centrale de Lyon) and IMFT (UMR 5502) laboratories have earned projects entirely based on the use of the LES solver developed by Cerfacs AVBP and involve the support of experts from the CFD and COOP teams underling the importance and effectiveness of collaborations between French labs and Cerfacs. Alexis Giauque from ECL/LMFA UMR5509 (Ecole Centrale de Lyon) has obtained not only one but two PRACE projects! The first project LESFAN (RA0101, 30 000 000 CPU hours on Irene/Rome TGCC) is based on the use of AVBP in the turbomachinery version to study the generation of noise by a fan of a real airplane engine. The second, PRACE-EDGES (RA0101, 40 000 000 CPU hours on Irene/Rome TGCC) focuses on LES modeling of dens gas in complex geometries. To do so, the LMFA Team has developed advanced thermodynamic closures in AVBP allowing the direct simulation of such flows. Laurent Selle from IMFT (UMR 5502) has received CPU hours for the GASTON project (RA0061, 30 000 000 CPU hours on Marenostrum BSC) which aims to study the structure of hydrogen flames in porous materials. For this, IMFT and Cerfacs will perform coupled simulations considering the reactive flow with AVBP as well as the conduction in the porous medium with AVTP which is known to play an central role in the flame stabilization process. Carlos Perez Arroyo from IMFT (UMR 5502) received 16 Mh CPU hours on Joliot-Curie Skylake partition to support his project WONDER.Read more