Travelling standing waves: A feasibility study

Lately, there has been significant interest in the noninvasive manipulation of particles and liquids. The reported acoustic methods rely on either the acoustic radiation force or acoustic streaming. The latter can be used in developed flows to induce fluid velocities angled to the liquid flow direction. These methods often use standing wave fields induced locally through continuous compressional waves. This has a drawback for industrial applications, i.e. continuous flow reactors, the standing wave field is excited locally, whereas it should be excited along the entire pipe length. Solving this by using numerous actuators along the pipe length is impractical and cost prohibitive. In this work the feasibility is investigated of using guided waves to induce a standing wave field over a pipe´s radius, but traveling along a pipe´s length. Also, it is investigated whether this standing wave field significantly affects the flow velocity field and fluid mixing. The method relies on exciting Lamb waves in the wall of a liquid filled pipe, which partially refracted into the liquid. The frequency was chosen such that: 1) a radial resonance was excited in the liquid, and 2) the reflected waves in the liquid interfere constructively with the refracted wave energy. The experimental geometry consisted of a copper pipe with an attached transmitting piezo of such dimensions that L(0,1) waves were excited. A 2^nd piezo was mounted to detect and optimize the guided wave intensity. The pressure in the liquid was measured using a hydrophone. The effect of the pressure waves on the mixing of salt injections was measured using conduction probes at the inlet and outlet of the pipe. The 7^th and 8^th order standing wave fields were measured in the liquid at 650 and 770 kHz with transmit efficiencies of 20 and 22 kPa/V, respectively. This demonstrates the feasibility of inducing radial standing wave fields traveling along a pipe. Also, the salt concentration curves were altered in shape and surface area, if the ultrasound was switched on. This suggests that the convective mixing was increased by the induced radial fluid velocity components.