Validation using DEM simulations

Using MercuryDPM³³, we simulate the motion of 28,300 spherical particles with mean diameter 2.8 mm ± 10% inside a cylindrical shell of diameter 100 mm and height 100 mm. The sliding coefficient of friction between grains is taken to be 0.5, and the rolling resistance is 10⁻³. A rough base is generated by setting both of these coefficients to unity at the base, which is then sheared at a constant velocity 8.3 mm s⁻¹. The simulation is run with a time step of Δt = 0.08 s, until t = 76 s, although only the last 66 s are kept to allow the steady state to fully develop. Particle positions are output at the same frequency as the experimental radiographs (30 fps). These are then used to generate artificial radiographs from the three orthogonal directions, using a forward projection model described in ^{Supplementary Methods}. It is anticipated that such forward projections using DEM data can be used to further optimise the overall accuracy of the rheographic method in future.