Flow Field Imaging With Ultrasonic Guided Waves for Exploring Metallic Melts.

Ultrasonic guided waves enable flow measurements under harsh conditions, which are important, for instance, to monitor and optimize industrial solidification processes. The usage of single mode waveguides overcomes the problem of overheating the transducers, but requires a mechanical scanning for imaging. A multimode waveguide can carry the information of an image, but a scrambling of the signals occurs due to multiple reflections at the waveguide's boundaries. We propose a new approach to overcome the scrambling and enable flow imaging through a short waveguide: the time-reversal virtual array (TRVA) method. The time invariance of the wave equation in a linear medium allows the refocusing on a limited set of calibrated points, which form the virtual array. This virtual array can conceptually be treated as a phased array. In this paper, the TRVA has been characterized theoretically, numerically, and experimentally. For the first time, a planar velocity measurement of a rotating flow in liquid gallium-indium-tin is demonstrated through a borosilicate waveguide at room temperature. A comparison with reference measurements showed good agreement.