Two Point Correlation Measurements in Liquid Metal Duct Flows using Ultrasound Phased Arrays

A key to improvements of industrial processes involving liquid metals like continuous steel casting is a good knowledge of the interaction between time-varying magnetic fields and conductive fluids. For investigations in this area, model experiments using low-melting liquid metals are conducted, which require suitable instrumentation for opaque media. Ultrasound Doppler velocimetry allows to measure the flow structures and thus to obtain turbulence statistics. For MHD experiments investigating small-scale and turbulent flows, a spatial resolution in the low millimeter range is required.
However, state of the art systems using arrays of unfocused single transducers are limited in their resolution by the divergence of the far field ultrasound beam. In addition, thus systems allow only one dimensional measurements along the propagation direction of the ultrasound beam. Thus interesting quantities in some experiments like turbulence statistics in the axial direction in duct flows can not be measured.
Using our novel phased array ultrasound Doppler velocimeter (PAUDV), the duct flow of an opaque liquid under the influence of an magnetic field can be studied. The measurement system combines the ultrasound Doppler velocimetry with the phased array technique, which allows to adaptively shape the sound field. Using its possibility to drive phased arrays with up to 256 individual channels simultaneously, the ultrasound beam is dynamically focussed in the measurement area, which increases the spatial resolution. In addition, the cross beam technique is used to enable two component measurements using a single acoustical access. This allows to measure turbulence statistics in the main flow direction of a fluid duct with only one ultrasound array on the duct wall.