ULTRASOUND SCATTERING BY A SWIRLING JET

Recent analytical work has shown that when an acoustic plane wave prop agates through a rotational flow field there is a linear relationship between the Fourier component of the scattered acoustic pressure and t he Fourier transform in space and time of the vorticity component that is normal to the plane defined by the wave vectors of the incident an d scattered acoustic waves. Hence, ultrasound scattering can be used a s a non-intrusive spectral probe of vorticity and potentially as a too l 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, sw irl numbers up to 0.4 are realized. Radial distributions of the stream wise and tangential velocity components downstream of the jet exit pla ne are measured using two-component hot-wire anemometry and the corres ponding distributions of streamwise vorticity are computed. A nominall y 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 azim uthal 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 independ ent of the intensity of the incident wave field. Fourier components of the vorticity distribution are directly measured by varying the scatt ering angle and are in good agreement with theoretical predictions. (C ) 1998 American Institute of Physics.