Fluid forces or impacts: What governs the entrainment of soil particles in sediment transport mediated by a Newtonian fluid?

In steady sediment transport, the deposition of transported particles is balanced by the entrainment of soil bed particles by the action of fluid forces or particle-bed impacts. Here we propose a proxy to determine the role of impact entrainment relative to entrainment by the mean turbulent flow: the bed velocity V-b, which is an effective near-bed-surface value of the average horizontal particle velocity that generalizes the classical slip velocity, used in studies of aeolian saltation transport, to sediment transport in an arbitrary Newtonian fluid. We study V-b for a wide range of the particle-fluid-density ratio s, Galileo number Ga, and Shields number Theta using direct sediment transport simulations with the numerical model of Duran et al. [Phys. Fluids 24, 103306 (2012)], which couples the discrete element method for the particle motion with a continuum Reynolds-averaged description of hydrodynamics. We find that transport is fully sustained through impact entrainment (i.e., V-b1 is constant in natural units) when the impact number Im = Gav root s + 0.5 greater than or similar to 20 or Theta greater than or similar to 5/Im. These conditions are obeyed for the vast majority of transport regimes, including steady turbulent bedload, which has long been thought to be sustained solely through fluid entrainment. In fact, we find that transport is fully sustained through fluid entrainment (i.e., V-b scales with the near-bed horizontal fluid velocity) only for sufficiently viscous bedload transport at grain scale (i.e., for Im less than or similar to 20 and Theta less than or similar to 1/Im). Finally, we do not find a strong correlation between V-b, or the classical slip velocity, and the transport-layer-averaged horizontal particle velocity (v(x)) over bar, which challenges the long-standing consensus that predominant impact entrainment is responsible for a linear scaling of the transport rate with Theta. For turbulent bedload in particular, (v(x)) over bar increases with Theta despite V-b remaining constant, which we propose is linked to the formation of a liquidlike bed on top of the static-bed surface.