Experimental study of a helical acoustic streaming flow

The acoustofluidic helix stirs fluid within a closed container efficiently with only one ultrasound source. The helical shape is obtained by reflecting the acoustic beam on the cavity walls. This acoustic forcing drives multiple descending jets, each impinging a vertical wall and wrapping around the central axis. Time-averaged and low-frequency unsteady flow structures have been obtained by three-dimensional particle tracking velocimetry and Eulerian field reconstructions. Both the velocity amplitudes of the overall time-averaged flow, and the vortex dynamics, depend on the dimensionless acoustic force magnitude called the acoustic Grashof number.