Interparticle collision of natural sediment grains in water

Elastohydrodynamic theory and measurements of particle impacts on an inclin ed glass plane in water are used to investigate the mechanics of interparti cle collisions in sediment-transporting flows. A collision Stokes number is proposed as a measure of the momentum of an interparticle collision versus the viscous pressure force in the interstitial gap between colliding parti cles. The viscous pressure force opposes motion of the particles on approac h and rebound. A Stokes number of between 39 and 105 is estimated as the cr itical range below which particle impacts are completely viscously damped a nd above which impacts are partially elastic. The critical Stokes number is shown to roughly coincide with the Bagnold number transition between macro viscous and grain inertial debris flows and the transition between damped a nd partially elastic bed load transport saltation impacts. The nonspherical nature of natural particles significantly alters the motion of the center of mass after a partially elastic collision. The normal to the point of con tact between the particles does not necessarily go through the center of ma ss. Thus normal rebound of the center of mass may not occur. A model of par ticle motion after rebound for particles of arbitrary shape, conserving bot h linear and angular momentum, is proposed.