Suppression of Refraction in Jet Noise by Cooling.

Noise generation in an air jet can be described in terms of a convection factor. The velocity gradients in the flow refract the sound, giving rise to a commonly observed final directivity. In the experiments outlined here the effects of convection and refraction on directivity are examined for a jet of very cold air. An attempt is made to determine whether any interdependence between convection and refraction can be detected experimentally in narrow band jet noise data. In the reported investigation liquid nitrogen was used to cool a jet of air (at nozzle speeds 0.21 to 0.28 of ambient sound speed) down to temperatures (between -110C and -155C) at which the refraction from the opposed velocity and temperature gradients produced cancellation effects.
Also pub. as ISSN-0082-5263.