Ultrasound flow mapping of complex liquid metal flows with spatial self-calibration

Investigating the complex interaction of electrically conductive fluids and magnetic fields is relevant for a variety of applications from basic research in magnetohydrodynamics (MHD) to modeling of industrial processes involving metal melts, such as steel casting and crystal growth. However, experimental studies in this field are often limited by the performance of flow instrumentation for opaque liquids. Commercially available measurement systems usually lack the ability to provide a time-resolved imaging of transient flow structures. We present an ultrasound array Doppler velocimeter (UADV) for flow mapping in opaque liquids near room temperature. It is modular and flexible regarding its measurement configuration, for instance it allows capturing two velocity components in two planes (2d-2c) of 67×67 mm2 with a frame rate of 30 Hz. It uses up to 9 linear arrays with 25 ultrasound transducers each driven in a parallelized time division multiplex (TDM) scheme. A FPGA-based signal processing allows a continuous realtime operation of the measurement system. Combining the single-component velocity data of each linear array to a 2d-2c flow field demands precise knowledge of the relative geometric position of the transducer arrays. We present a novel method that performs a spatial self-calibration by a mutual time of flight measurement, significantly reducing alignment errors. A measurement example of a magnetically stirred flow of GaInSn in a rectangular container is given. The UADV is applied to experiments in the context of manufacturing crystalline silicon ingots for photovoltaics.