Fig. 8: Clustering of critical Stokes number particles at the peripheries of coherent structures.

a–e Snapshots from a numerical multiphase Eulerian-Eulerian fluid particle four-way coupled simulation of the physical experiment at approximately mid-flow runout distance. The contour plots show the spatial variation across the simulated PDC in the particle concentration of five modelled grainsizes of 2 mm (a), 500 µm (b), 125 µm (c), 32 µm (d) and 8 µm (e). Technical aspects of the numerical simulation are detailed in the Methods section. Particles with Stokes numbers St > 1 (a), which are only supported by turbulence at the scales of the largest coherent structures, have sedimented into the basal flow and do highlight any coherent structures at mid-flow runout. Particles with critical Stokes numbers St = 1 (b and c) concentrate at the peripheries of coherent structures. The effect is stronger for particle sizes of 125 µm than for particle sizes of 500 µm because of the significantly higher abundance of the 125 µm particles over 500 µm particles. Particles with low Stokes numbers St < 1 (d and e) tend to be homogeneously suspended or slightly concentrate in the central parts of coherent structures.