CAA Prediction of Turbofan Engine Combustion Noise Directivity

This paper discusses a high-fidelity modeling approach devised for predicting the far-field propagation of combustion noise using CAA (Computational Aeroacoustics) methods. The focus of the study is the far-field directivity of the combustion noise source. Using a generic engine CAA model, based on ACTRAN/DGM, far- field transfer functions relating input acoustic modes to far-field arc locations were calculated for three certification flight conditions. Based on the predicted transfer functions, combustion noise directivities were then calculated for a range of source mode distributions and compared against reference directivity curves from legacy semi-empirical methods. Combustion noise directivities were also calculated for a modern high-bypass ratio turbofan engine and the results were compared against far-field engine test data. The CAA predicted directivity peak angles agreed well with both the reference curves and engine data. The directivity shapes appear to agree better with data when the assumed source mode distributions skewed more toward higher order modes, potentially indicating participation of higher order source modes which were typically neglected in the past.