Experimental and analytical investigation of acoustic streaming generated by standing ultrasonic waves in an open boundaries

Acoustic streaming patterns, velocity fields, which is induced by a cylindrical ultrasonic exciter vibrating at 28.4 kHz in an open physical boundaries, is analytically and experimentally investigated using Particle Imaging Velocimetry (PIV). Induced acoustic streaming patterns and velocity fields for the gaps of 18 mm at which the irrotational tangential velocity becomes a maximum, resulting in a substantial increase in the acoustic streaming velocity and pronounced visualization of streaming patterns between the vibrator and quiescent glass plate are presented. The overall air flow patterns at the gaps of 24, 30, 36 mm are similar to the gap of 18 mm but as the gap increases the frequency of occurrence and irregularity of vortices in the gap appear to increase. The symmetric definite steady circular flow with local vortices is observed. The maximum streaming velocity measured stands at 0.16 cm/s with a vibration amplitude of 50 micrometers. Theoretical analysis indicates that the pattern of air flow in the gap is determined by the top and bottom limiting velocities induced by acoustic streaming within the Stokes boundary layer and that the streaming pattern is symmetrical with respect to the center axis of the vibrator by reason of symmetry. The comparison between the experimental data and the theoretical estimation based on Nyborg and Jackson is performed.