Similarities of the free-field and in-duct formulations in rotor noise problems

This paper deals with the comparison between open rotor and ducted fan noise. Similar theoretical formulations for the freeeld and in-duct noise problems are derived from the original equation for noise generation given by Goldstein. The form of the nal equations enables to separate the terms related to noise propagation from those representing the acoustic source. Among the similarities shared by the freeeld and in-duct cases, the possibility to de ne a common cut-o ratio for the spinning modes provides some insight in the relation between the position of the sources and the excited modes in a way more general than that already proposed by the authors. With help of this formalism we hope to reach a better understanding of the di erences in noise levels to be expected on contrarotating open rotor (CROR) and contra-rotating turbofan (CRTF) concepts.

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