Experimental Measurements on the Coefficient of Restitution of Colliding Irregular Volcanic Particles

Dense volcanic granular flows are polydisperse in terms of grain size and density, and their flowcharacteristics are mainly governed by particle‐particle collisions and frictional forces acting at the boundaries.The parameter measuring the energy dissipation during the collisions is the coefficient of restitution (e), whichis proportional to the fraction of the original energy stored in the colliding particles that is restored to the sameones after the collision. e is fundamental in computational fluid dynamics (CFD) numerical models to simulatemultiphase granular flows because it is required to solve the particles motion and the particle‐particlemomentum exchange. The calculation of e for irregular volcanic particles is an unsolved challenging problem,which is here addressed by colliding particles through a pendulum‐type instrumental apparatus. e was calculatedfor volcanic particles with different density (ρ), diameter (d) and particles size ratio (dratio), and the data wereused to obtain linear relationships between e and the investigated parameters. Afterward, a multicollinearityanalysis and a multiple regression were applied to all data to adequately predict the value of e knowing thevalues of ρ, d, and dratio. The empirical law was finally validated against some large‐scale experiments by usingthe multiphase CFD simulation tool Multiphase Flow with Interphase eXchanges. The CFD simulationsinserting the predicted e showed a better agreement between simulated and experimental flow velocities, with anincrease of the simulation accuracy up to 20%. Hence, the current paper proposes a simple instrumentalapparatus to calculate e, demonstrating its importance in simulations of multiphase granular flows.