Multipolar expansion for aircraft noise equivalent sources

Noise generated by aviation poses serious threats to its future development. Noise abatement is pursued by reducing noise at the source and using correct operation practices and farsighted land planning. Since noise evaluation is a computationally demanding process, developing fast techniques is crucial for that noise abatement effort. Widely used approaches separate near-field noise from far-field noise, using the optical analogy or other simplified techniques to radiate to the ground the noise evaluated in the near-field by high-fidelity models, including atmospheric and geographic effects. Here, we propose a multipolar expansion to define equivalent noise sources that can be quickly evaluated to simulate the perceived noise also in proximity of the source. This gives many advantages, such as introducing atmospheric effects closer to the source and using equivalent engine sources in fuselage scattering problems. The expansion is tested against numerical and experimental test cases of aeronautical interest. The results show that the expansion effectively reproduces noise at different distances than that used to train the equivalent source. When the emitted noise is dominated by rotary sources, the method correctly reproduces the amplitude but not the phase of the signals.