Abstract
Many common techniques for flow measurement, such as Particle Image Velocimetry (PIV) or Ultrasonic Doppler Velocimetry (UDV), rely on the presence of reflectors in the fluid. These methods fail to operate when e.g centrifugal or gravitational acceleration leads to a rarefaction of scatterers in the fluid, as for instance in rapidly rotating experiments. In this article we present two low-cost implementations for flow measurement based on the transit time (or Time of Flight) of acoustic waves, that do not require the presence of scatterers in the fluid. We compare our two implementations against UDV in a well controlled experiment with a simple oscillating flow and show we can achieve measurements in the sub-centimeter per second velocity range with an accuracy of sim 5-10%. We also perform measurements in a rotating experiment with a complex flow structure from which we extract the mean zonal flow, which is in good agreement with theoretical predictions.
Highlights
The field of rapidly rotating experiments has been growing in the last 30 years, in particular in the context of planetary core and subsurface ocean dynamics
After a certain time that depends on the rotation rate and the density contrast, the bulk of the fluid will be depleted in reflectors and Ultrasonic Doppler Velocimetry (UDV) measurements are no longer possible
It is expected that the phase shift detection results in a much lower noise floor at ∼ 2 × 10−3cm s−1 than the travel time detection, which exhibits fluctuations on the order of ∼ 0.2 cm s−1 for the present configuration
Summary
The field of rapidly rotating experiments has been growing in the last 30 years, in particular in the context of planetary core and subsurface ocean dynamics. Ultrasonic Doppler Velocimetry (UDV) became the most common technique for measuring flow velocities in liquid. After a certain time that depends on the rotation rate and the density contrast, the bulk of the fluid will be depleted in reflectors and UDV measurements are no longer possible. In such cases, it is still possible to measure flow velocities with transit time or Time of Flight (ToF) methods, which rely on the travel time difference between up- and downstream traveling acoustic waves. From the travel time difference an averaged value for the flow velocity along the acoustic path is calculated
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