PIV applied to Eckart streaming produced by a medical ultrasound transducer

Particle image velocimetry (PIV) is applied for the first time to study Eckart streaming induced by a medical ultrasonic transducer operating at a frequency of 3.3 MHz and effective acoustic intensities of 0.25 and 3 W cm⁻². A temporal series of velocities in a two-dimensional plane were recorded resulting in an experimental set comprising over half a million velocity data points. These enabled average and fluctuating properties to be determined and clearly indicated the quasi-steady nature of the flow. The average large scale velocity fluctuations along the axis caused by this quasi-steady property were calculated to be 2 and 20 m ms⁻¹ at effective intensities of 0.25 and 3 W cm⁻² respectively corresponding to approximately 25% of the peak flow velocity in both cases. Furthermore averaged shear rates were calculated with peak values of 1 and 8 s⁻¹ for the low and high intensities respectively. The present investigation indicates the usefulness of PIV for such studies and serves as a prelude to investigations of streaming in biological type fluids.