Modular Ultrasound Array Doppler Velocimeter with FPGA-based Signal Processing for Flow Mapping in Liquid Metals

Investigating the complex interaction of conductive fluids and magnetic fields is relevant for a variety of applications from basic research in magnetohydrodynamics (MHD) to modeling industrial processes involving metal melts, such as the crystal growth process in the photovoltaic industry. This enables targeted optimizations of the melt flow and allows to significantly increase the yield and energy efficiency of an industrial process. However, experimental studies in this field are often limited by the performance of flow instrumentation for opaque liquids. We present an ultrasound array Doppler velocimeter (UADV) for flow mapping in opaque liquids at room temperature. It is modular and flexible regarding its measurement configuration, for instance it allows capturing two velocity components in two planes (2d-2c). It uses up to 9 linear arrays with a total element count of 225, driven in a parallelized time division multiplex (TDM) scheme. A FPGA-based signal pre-processing allows to handle the massive data bandwidth of typ. 1.2 GB/s and enables a continuous and near-realtime operation of the measurement system. Combining the velocity information of multiple arrays necessitates precise knowledge of their relative geometric position. We present a novel method for spatial self-calibration by a mutual time of flight measurement that significantly reduces the alignment errors.
The capabilities of the UADV system are demonstrated in an experiment for basic MHD research. A cubic plexiglas container (67mm3) is filled with a metal melt (GaInSn, melting point 10°C). The flow induced by time-varying rotating magnetic fields is captured with a temporal resolution of 250ms and an uncertainty of approx. 1% for the horizontal and vertical central cross-section of the cube (2d- 2c).