A hybrid LES-CAA approach for the simulation and analysis of thermoacoustics in aero-engines

The collective global initiative to combat climate change has prompted aircraft manufacturers to adopt measures to reduce carbon emissions. One such measure involves the development of innovative combustor systems that use lean combustion modes to lower the emissions of pollutants such as NOx and CO2. However, this can result in fluctuations in mixing and heat-release rate, which can increase thermoacoustic activity, potentially affecting engine operation. Therefore, a comprehensive understanding of thermoacoustic phenomena is imperative for the effective and reliable design of large-scale combustion systems. Numerical simulations are often a helpful complement to experimental acoustic characterizations of industrial configurations. These methods usually use either fully compressible and resource-intensive Computational Fluid Dynamics (CFD) simulations, which naturally contain acoustic fluctuations in their computations, or two-step approaches that rely on source terms extracted from CFD simulations to supply subsequent Computational Aeroacoustics (CAA) simulations. However, compressible simulations can be costly depending on the configuration, and two-step approaches mostly do not include any two-way coupling between acoustics and the flow field. Hybrid methods simultaneously considering acoustics and fluid dynamics offer an elegant third option for numerically predicting combustion instabilities. In this thesis, a hybrid CFD-CAA framework for combustion noise investigations is extended to additionally enable an acoustic feedback on the flow, which allows for the description of thermoacoustic instabilities. The approach involves coupling acoustic and convective phenomena by deriving mean flow field quantities and acoustic sources from the low-Mach CFD solution and input them into the acoustic governing equations in the CAA solver. The resulting fluctuating acoustic quantities, namely pressure and velocity, are subsequently integrated into the solution of the low Mach number Navier-Stokes equations, where they ...