2d-2c ultrasound Doppler array velocimeter for flow investigations in liquid metals

In magnetohydrodynamics ultrasound Doppler velocimetry (UDV) is applied for flow field investigations of liquid metal flows. Thereto, commercial UDV instruments are available. However, their application possibility for multi-dimensional, multi-componential flow mapping with high measurement rates is highly limited since these features cannot be applied simultaneously. Hence, we have developed an ultrasound Doppler velocimeter for measuring the two-dimensional two-componential (2d-2c) flow field of magnetically driven liquid metal. The velocimeter utilizes two identical linear ultrasound Doppler arrays arranged orthogonally to each other to facilitate the 2d-2c flow mapping in a plane of 70 x 70 mm². Each ultrasound array providing a centre frequency of 8 MHz comprises 25 transducer elements each with a piezo element size of 2.5 x 5 mm². In operation two adjacent elements are combined to a square transducer pair of 5 x 5 mm² allowing a small lateral scanning step width of 2.5 mm on the one hand and a low divergence of the ultrasound beam and accordingly a better lateral resolution up to 2 mm on the other hand. To achieve a high temporal resolution concurrently to the high spatial resolution, the measurement is parallelized as much as possible by simultaneous operation of four transducer pairs which are controlled by a special multiplex control unit. The operation principle, design considerations and the characterisation of the ultrasound Doppler array velocimeter will be reported. Flow mapping of a swirling flow in a cubic vessel with GaInSn generated by a rotating magnetic field were performed. Measurement results of a stationary as well as a periodically reversed flow will be presented.