Cross-sectional imaging of refractive-index-matched liquid-granular flows

In channelized liquid-granular flows, the liquid velocity, granular velocity and solid fraction all vary over transverse cross sections. A major experimental challenge is then to acquire internal measurements of these three fields. One useful first step, achievable using various materials, is to make the medium transparent by matching the refractive indexes of the liquid and granular phases.

Taking full advantage of this optical access, however, requires the development of new imaging methods. In this paper, we propose a new approach applicable to steady uniform flows and spherical immersed grains. The approach combines laser scans in the transverse and longitudinal directions. Using the transverse scans, liquid and granular motions in the laser plane can be captured by particle-tracking velocimetry.

The longitudinal scans, on the other hand, allow granular positions and velocities to be deduced from individual grain crossing events. These occur when flowing grains move across the laser plane and when the laser plane sweeps across stationary grains. The approach therefore applies to flows over erodible beds featuring both moving and stationary grains.

Using suitable algorithms, we show how to process these scans to map granular velocity, liquid velocity and granular concentration over the entire flow cross section, at resolutions finer than the grain diameter by a factor of 10.