Geometry and Static Flow Effects on Acoustic Radiation from Ducts

Abstract

An experimental study of the effects of geometry, flow, frequency, and source structure on the radiation and reflection of sound from duct inlets has been conducted. This work extends a previous study by comparing two inlet shapes typical of static and flight testing and by considering higher-order radial modes. Measurements of the internal and radiated acoustic fields are presented in terms of modal reflection coefficients and absolute radiated pressure directivity. Results indicate that geometry effects associated with these practical inlet shapes become less important for a fixed cutoff ratio as the mode order and frequency are increased. Also, static inflow, while reducing the reflected wave amplitude, is shown to have little effect on radiation directivity regardless of inlet geometry. Nomenclature complex modal amplitude coefficient, Eq. (3) radius of uniform duct sections, 0.15 m ambient speed of sound Bessel function of first kind and order m J k = free space wavenumber, w/c0