Full-waveform acoustic tomography for fluid temperature and flow

Using the travel time of sound waves advected by a moving carrier medium, acoustic tomography allows to reconstruct temperature and flow fields in opaque fluids without tracers or scattering particles. Reconstruction algorithms are conventionally based on the ray approximation and pose difficulties, especially in enclosed domains: Interferences of early reflections can prevent the assignment of each arrival to the correct ray path. We develop a full-waveform inversion for acoustic tomography in laboratory-scale experiments, perform synthetic tests, and benchmark these with a straight-ray algorithm. Multiple late arrivals of reflected waves are considered in order to increase the quality of the reconstructions when restricted to a sparse transducer array. In addition, the full-waveform algorithm allows to invert simultaneously emitted signals from all sources, decreasing the acquisition time in which a flow must be assumed stationary. These findings make the new method especially interesting for researchers experimenting with enclosed, opaque fluids where no optical imaging is feasible. Furthermore, we envision a potential application of the newly developed method to map flows around objects or complex wall geometries and even multiphase flows.