Direct measurement of a green's function suitable for prediction of subsonic jet noise
Keywords:Anechoic chambers, Green's function, Mach number, Phase shift, Thermal gradients, Subsonic jet
AbstractThe possibility of direct measurement of a Green's function (GN) for the prediction of subsonic jet noise is discussed. The amplitude and phase of radiated signal are extracted, and this information leads to a semi-empirical model Green's function. The influence of flow and temperature gradients on sound was demonstrated by a rather elegant set of experiments. GN is measured for discrete frequencies, at it is possible to determine both amplitude and phase quite accurately using a lock-in amplifier technique. Measurements were performed in the anechoic room at the University of Toronto institute for Aerospace Studies, and the radiated sound was measured by a condenser microphone mounted on a boom which is centered on the jet exit. Measured values illustrated the phase shift induced by the flow, and it was found that the effects are quite small at low Mach numbers, but become more significant at moderate Mach numbers.
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