The present study investigates the effect of acoustic oscillations on swirl flow characteristics, using 2C and stereo PIV. The experiments were performed in a sudden expansion square geometry. A high amplitude acoustic field was established in the duct using external acoustic drivers (woofers) that were mounted on the test section downstream of the swirler. A constant acoustic pressure of 600 Pa was maintained at the exit of the swirler. The experiments were performed for different Strouhal numbers. It was observed that the fluctuations in axial velocity in the recirculation zone are much higher than that of the acoustic velocity. However, no significant change in the tangential velocity was observed. It was seen that the maximum fluctuation amplitude increases with Strouhal number in both low and high acoustic velocity fluctuations regimes. It was found that the use of vane passage size as the characteristic dimension leads to Strouhal numbers in the range of 0.2 to 0.5, where the axial velocity fluctuations in the recirculation zone are high. Copyright © 2009 by E. Arunraj, R. I. Sujith and Chakravarthy.