Experimental acoustic identification of flow noise sources in expansion chambers

For the aeroacoustic design of expansion chambers, commonly installed in e.g. HVAC ducts or automotive exhaust systems, the aerodynamic noise generation mechanisms as well as the transmission characteristics of acoustic waves propagating in a non-quiescent medium have to be taken into account. The attenuation of downstream propagating acoustic waves should be maximized with a minimum of additional flow noise generation and often a compromise between both performance parameters has to be made. In this paper, these two aeroacoustic properties are experimentally analyzed for a simple expansion chamber carrying a mean flow which is uniform in time. This is achieved using an active two-port representation which is validated for a straight duct with a loudspeaker placed at its center. The different experiments are carried out on an open circuit aeroacoustic wind tunnel where the test objects are located inside a semi-anechoic room. This allows to determine the noise emission and radiation in free-field environments and correlate the far-field pressure levels with the aerodynamic noise generation mechanisms described by the active two-port components.