Ultrasound scattering by a swirling jet

Abstract

Recent analytical work has shown that when an acoustic plane wave propagates through a rotational flow field there is a linear relationship between the Fourier component of the scattered acoustic pressure and the Fourier transform in space and time of the vorticity component that is normal to the plane defined by the wave vectors of the incident and scattered acoustic waves. Hence, ultrasound scattering can be used as a non-intrusive spectral probe of vorticity and potentially as a tool for direct measurements of vorticity distributions. Some aspects of this technique have been tested in a swirling air jet emanating from a 2.54 cm diameter nozzle where the swirl is generated upstream of the jet nozzle by a rotating paddle. For a given exit volume flow rate, swirl numbers up to 0.4 are realized. Radial distributions of the streamwise and tangential velocity components downstream of the jet exit plane are measured using two-component hot-wire anemometry and the corresponding distributions of streamwise vorticity are computed. A nominally plane ultrasonic wave field is generated normal to the jet axis by a transmitter having a 16 cm square aperture. The scattered ultrasound in the radial direction is measured at a number of streamwise and azimuthal stations. In accord with the theory, the normalized amplitude of the scattered acoustic wave is a linear function of the magnitude of the centerline vorticity at the exit plane of the jet, and is independent of the intensity of the incident wave field. Fourier components of the vorticity distribution are directly measured by varying the scattering angle and are in good agreement with theoretical predictions.