Fluid forces or impacts: What governs the entrainment of soil particles in sediment transport mediated by a Newtonian fluid?
AbstractIn 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” Vb, 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 Vb for a wide range of the particle-fluid-density ratio s, Galileo number Ga, and Shields number Θ using direct sediment transport simulations with the numerical model of Durán et al. [Phys. Fluids 24, 103306 (2012)PHFLE61070-663110.1063/1.4757662], 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., Vb is constant in natural units) when the “impact number” Im=Gas+0.5≳20 or Θ≳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., Vb scales with the near-bed horizontal fluid velocity) only for sufficiently viscous bedload transport at grain scale (i.e., for Im≲20 and Θ≲1/Im). Finally, we do not find a strong correlation between Vb, or the classical slip velocity, and the transport-layer-averaged horizontal particle velocity vx¯, which challenges the long-standing consensus that predominant impact entrainment is responsible for a linear scaling of the transport rate with Θ. For turbulent bedload in particular, vx¯ increases with Θ despite Vb remaining constant, which we propose is linked to the formation of a liquidlike bed on top of the static-bed surface.