A Coupled Computational Aero-Acoustics (CAA)/ Large-Eddy Simulation (LES) Approach for the Pressure Calculation in Internal Low-Mach Number Flows

The predictive unsteady simulation of pressure fluctuations in turbulent internal flows at low-Mach number is challenging because of the very different propagation speeds of the acoustic and entropy waves. In this paper, a hybrid Computational-Aero-Acoustics (CAA) / Large-Eddy-Simulation (LES) approach tailored for the numerical calculation of aero-acoustic noise and pressure in internal low-Mach number flow is developed to alleviate the acoustic time step restriction. The algorithm is based on the coupling of a fractional-step method used to solve the low-Mach number Navier-Stokes equations and a fully-implicit linear acoustics solver. The pressure field resulting from the Helmholtz equation computed by the acoustic solver is composed of both dynamic and acoustic contributions. The Newmark’s time integration method combined with implicit Non-Reflecting-Boundary-Conditions (NRBC) are implemented for solving implicitly the Helmholtz equation and then advancing the two solvers with the same convective time step. The properties of the linear acoustic solver are illustrated on simple test cases and the coupling method is then validated by performing the aero-acoustic simulation of the isothermal flow in a complex semi-industrial burner.