Experimental and Analytical Study of the Hydroacoustics of Propellers in Rigid Ducts

Abstract : This report describes the results of a detailed study of the fluid dynamics and acoustic signature of a rotor operating in a rigid duct. The study focused on the separation of the acoustic source functions and the system response, or transfer function. A new algorithm was developed and quantified to separate these functions from radiated sound measurements. Measurements of the approach flow turbulence and propeller wake turbulence provided insight into the nature of the acoustics sources. It was found that approach flow noise is only responsible for the sound produced at frequencies less than the first blade rate tone. Higher frequencies were dominated by complex rotor self noise that included broad humps in the source spectra at integer multiples of the blade passing frequency.

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