Radiation of Sound from the Open End of a Duct Simulating an Air Inlet under Static Conditions and in a Flow

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The paper presents the results of comparative experimental studies on the effect of a significant difference in the radiation patterns of individual acoustic modes from an open duct in steady-state and in-flight modes in the AC-2 anechoic chamber. When there is an incoming flow, the directivity changes and the amplitude of the maximum of individual modes may vary severalfold. This effect was originally discovered by numerical simulation, and the need to experimentally confirm this difference in the characteristics of individual modes set the task of developing a new technique for projection of the results of acoustic bench tests of aircraft engines to in-flight conditions within an airplane.