Understanding and predicting acoustic instabilities in gas turbine combustion chambers requires the knowledge of the acoustic behaviour of all the elements feeding the combustion chamber (characterized by their impedance). Inlets and outlets of chambers are often represented as one-dimensional ducts and existing methods to evaluate impedances of choked and unchoked nozzles are described: (1) analytical formulae, 1-3 (2) numerical methods using the linearized Euler equations and a finite-difference solver in Fourier space and (3) full spacetime solver where the response of the nozzle is studied by forcing its inlet (or outlet) and measuring its response in the time domain. These three methods are compared in reference cases (a straight duct and a subsonic distributor). Practical implications for gas turbines * Post-doctoral fellow † Research Director, AIAA Associate Fellow Heat release amplitude(rad/s) φ Real-valued function ρ Volume mass (kg/m 3) ϕ Adimensionalized pressure fluctuation

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To cite this version:A. Fosso P., Hugues Deniau, Nicolas Lamarque, Thierry Poinsot. Comparison of outflow boundary conditions for subsonic aeroacoustic simulations. International Journal for Numerical Methods in Fluids, Wiley, 2011Wiley, , vol. 68, pp. 1207Wiley, -1233

Compressible Large Eddy Simulation (LES) of turbulent reacting flows potentially offers a realistic representation of the physical phenomena involved in thermoacoustic instabilities. Coupled with Helmholtz solvers, which provide a mean of identification of the acoustic eigenmodes in complex geometries, that advanced numerical tool may provide a very powerful environment to assess the stability of real engine combustion chambers. The common use of a fully unstructured LES solver and a Helmholtz eigenmode tool proves here to be very useful to diagnose two operating points of a real gas turbine chamber which are known to operate with self-sustained oscillations. If the geometrical complexity and proper computational domain are chosen, the two thermo-acoustic instabilities are very well reproduced. Predictibility of LES for such problems is thus demonstrated provided that issues pertaining to the inlet acoustic impedances can be properly answered.