Two-dimensional ultrasound Doppler velocimeter for investigations of liquid metal flows

In the research area of magnetohydrodynamics (MHD) the pulsed-wave ultrasound Doppler velocimetry (UDV) is a worthwhile measurement technique for flow field investigations of liquid metal flows. Thereto, several UDV instruments are available on the market. However, their capability for two-dimensional mapping of flows is highly limited due to restricted number of scanning elements (max. 20 transducers) and low temporal resolution. Hence, an ultrasound Doppler velocimeter based on ultrasound arrays is developed which allows the measurement of both flow field vectors in the measuring plane of 67 x 67 mm². Thereby, it provides high measurement rates and a high spatial resolution with a significant increased number of scanning lines simultaneously.
The linear ultrasound transducer arrays are arranged orthogonally to each other. There each array measures one flow field component by electronic traversing.
The ultrasound arrays comprise 2x 25 transducer elements each with a centre frequency of 8 MHz and 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 according to this a high lateral resolution up to 2 mm on the other hand.
The requirements for a high temporal resolution and a high number of scanning lines are opponent. To achieve a high measurement rate nevertheless, the measurement is parallelized as much as possible by simultaneous operation of four transducer pairs. Thereby a specific distance between the active transducer pairs avoids spatial crosstalk. For the same reason both arrays are actuated alternatively. This excitation scheme is implemented by a fast multiplex electronics which is a proprietary development. Depending on the averaging time frame rates up to 33 Hz are achieved at pulse repetition rates of 1.5 kHz.