The propagation of ultrasound within a gas jet

A study is presented in which the directional characteristics of an ultrasonic signal have been modified due to propagation within an axial jet. The radiated ultrasonic field from a transducer positioned within the air jet has been studied at frequencies above 100 kHz for the first time. The effects of nozzle shape, nozzle diameter, and variations in air jet velocity and temperature have been investigated. At high air flow velocities, divergence of the ultrasonic beam was observed. This was attributed to the increased acoustic velocities in the direction of the flow. An effective waveguide was also demonstrated by cooling the air jet to below-ambient temperatures, so that the acoustic velocity in the air jet was lower than that in the surrounding ambient atmosphere. The result is likely to be of use in air-coupled ultrasonic materials inspection.

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