Stereoscopic imaging measurements of solid concentrations in dense granular flows

The paper outlines imaging methods developed for the measurement of volumetric granular concentrations in the vicinity of a transparent interface. The aim is to construct an imaging indicator which remains robust and sensitive across the whole range of concentrations, and in particular in the dense limit. As monocular (i.e. single camera) techniques have been found in past attempts to present inherent limitations, the present work resorts to a stereoscopic principle. Efficient techniques based on the 2D Voronoï diagram and the epipolar line concept are first developed to reconstruct the three-dimensional positions of visible grains from pairs of digital images. Concentration is then estimated from these three-dimensional positions, using the 3D Voronoï diagram to homogenise the discrete data and obtain a near-wall estimate which is not biased by occlusion effects. The approach is validated using fluidisation cell tests. The comparison between bulk measurements and stereoscopic imaging estimators shows the latter to be endowed with both robustness and sensitivity across the whole examined range