Application of vortex identification to acoustic streaming flows.

Abstract

Acoustic streaming has an important role in a variety of industrial and medical ultrasound applications. Enhanced mixing and chemical processing can be achieved with the addition of ultrasound. Coherent structures in streaming flows, such as vortices, contribute to the overall transport and convective heat transfer. Their unsteady development is not well understood and thus the subject of on-going investigations. Previous studies measuring both acoustic heating and streaming have used thermocouples and been limited to specific spatial locations within the flow field. In this study, simultaneous velocity and temperature measurements are made using synchronized particle imaging velocimetry (PIV) and infrared thermography in a model sono-chemical reactor. This experimental system is water-filled acrylic tank with a 20 kHz Langevin acoustic horn mounted on the side. With overlapping fields of view for an IR camera and the PIV camera, the spatial-temporal evolution of the acoustic streaming and heating at the liquid surface is investigated. A vortex pair adjacent to the horn is identified with respect to its swirling strength and unsteady flow regimes are quantified in terms of the Lagrangian coherent structures (LCS) methodology. The swirling strength reveals the location of the vortex cores and LCS shows the evolving boundaries of the vortices.