Measurements on a little known sound source - the Vortex Whistle
  Acoustic measurements on a vortex whistle corroborates earlier findings that the frequency increases linearly with the velocity of the air flowing into the whistle. Measurements in a reverberant chamber shows that the acoustic power generated by the whistle increases close to the sixth power of the frequency. PIV and hot-wire measurements give quantitative information on the flow. It is shown that the flow exits the pipe as a swirling annulus with a rotation corresponding to the frequency of the sound, and that the center of the swirl follows a nearly circular path in the exit plane. The PIV measurements also indicate a toroidal vortex with inflow along the center line close to the exit of the whistle. 
Ulf Kristiansen, Muriel Amielh
physics.flu-dyn physics.class-ph physics.ins-det