Order-of-magnitude enhancement of airborne acoustic streaming downstream of thin porous media.

Thin layers of porous materials are shown to produce an unexpected order-of-magnitude amplification of bulk-driven acoustic streaming generated by a high-powered airborne transducer, increasing the streaming velocity from 0.12 m/s to 2.5 m/s. Particle image velocimetry (PIV) measurements were used to characterise the streaming field produced by a Langevin horn operating at 27 kHz with a range of thin porous layers (various filter papers and a woven Kevlar fabric) placed in front of the source. We explain this effect as enhanced acoustic attenuation within the porous layer, which increases the acoustic body force on the fluid. The flow resistance of the porous layer counters this effect and reduces the streaming flow rate. Hence the streaming enhancement is thought to be governed by a subtle balance between the attenuation within the layer and the flow resistance of the layer. These experiments demonstrate that porous layers provide a passive mechanism for significantly enhancing acoustic streaming, enabling advances in haptics and levitation and broadening its applicability to a wide range of fluid systems.