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Aircraft-generated noise is nowadays an important, regulation-driven research topic. Many efforts have been devoted to understand the mechanisms responsible for noise generation and to design effective control strategies for noise reduction.
In the present work a system identification of an unforced, subsonic jet was carried out, in order to build a reduced-order model that reproduces the correct pressure dynamics in the near-field of the jet. This is achieved by analyzing experimentally obtained pressure measurements at different axial positions along the jet.
A linear auto-regressive ARMAX model, built through an optimization process, has been used to simulate the system behavior in the jet near-field. The goal is to reproduce the jet dynamics in this region in response to a given pressure dynamics in the near-nozzle region. Various combinations of the parameters involved in the definition of the model have been tested in order to evaluate its sensitivity. The identified model is found to provide satisfactory results, although its performance gets worse as the output signal position is moved downstream, far from the input location. By repeating the identification for different input reference positions, higher accuracy in the reconstruction of the reference signal has been obtained. This confirms recent observations showing that the jet behavior can be considered linear in a region comprised between the near-nozzle zone and the end of the potential core.
In a second step, a classical Proper Orthogonal Decomposition (POD) has been carried out in order to isolate the most energetic structures, and the model identification has been carried out again. This is motivated by recent studies where good agreement between the results predicted by linear Parabolised Stability Equations (PSE) and the pressure field reconstructed with the lowest POD modes has been found, in the region downstream the potential core. The identification performed on the signal reconstructed with only the first POD mode is able to reproduce the target output signal with a very high degree of accuracy, confirming the linear behavior of the coherent component of the turbulent field in the downstream region.