Ultrasound Velocimeter with frequency modulated signals for 2d2c measurements of non-stationary flows with high temporal resolution

In conjunction with flow measurements in nontransparent fluids like liquid metals, there is an increasing demand for the measurement of non-stationary flows. Ultrasound Doppler Velocimeters (UDV) belong to the standard equipment of research. UDV emit short ultrasound pulses into the fluid which have a typical duration equivalent to 2-8 wavelengths. The ultrasound is partly reflected by small tracer particles inside the fluid and scattered back to the transducer. The tracer particles are assumed to move without slip. The echo signals are recorded and can be used to estimate the fluid velocity. The measurement principle of Ultrasound Doppler Velocimetry, however, suffers from inherent constraints: One problem is the limitation in time resolution. For non-stationary flows a high temporal resolution is essential, because the relevant flow information is partly contained in the high frequency part of the velocity signal. In practice, the problem of time resolution occurs because an average over several send pulses is required to get accurate velocity estimates. Another problem is the maximum velocity that can be detected unambiguously with Doppler systems and related narrowband methods.
The actual problem with time resolution is the fact that a sending pulse only contains little signal energy, which leads to a low signal to noise ratio (SNR). The method under examination uses sending signals with a length of ~20µs instead of typically 0.5µs to 2µs in UDV systems. Long sending signals contain more signal energy and result in a better signal quality in terms of SNR. With higher SNR, however, less averaging is needed, and thus time resolution can be increased.
We propose the use of a short (~20µs) sending signal with linear chirp. The coding is similar to that of Frequency Modulated Continuous Wave Radar (FMCW Radar). The chirp is needed to obtain spatial resolution. Similar to the crosscorrelation approach in Pulse Doppler Ultrasound, it finally leads to a time-of-flight evaluation. The main difference, however, is that more ultrasound energy can be sent into the fluid. Also, the maximum velocity limit of Doppler systems does not apply for this method, so that velocities above this limit can be detected unambiguously.