A universal dynamical critical condition controls cessation of sediment transport in fluid

Abstract:

One of the classical problems in sediment transport science is to predict the threshold flow strength below which a bed of loose sediment particles sheared by a homogeneous fluid flow ceases to move. Depending on the particle-fluid-density ratio (s), it has been believed for many decades that this threshold is a consequence of either fluid forces being just strong enough to dislodge particles resting on the bed (small s, e.g., water) or of particle-bed impacts being just strong enough to expel sufficient bed particles (large s, e.g., air). Here we challenge this believe. Through state-of-the-art numerical simulations of sediment transport, which are consistent with measurements in air and water (including the famous "Shields diagram"), we show that, regardless of s, the cessation of sediment transport is controlled by a dynamical critical condition which takes a form similar to a yield criterion. We show that this condition can be directly linked to the observation in our sediment transport simulations that the rate of collisional dissipation of granular temperature is usually much smaller than the rate of collisional transfer of horizontal into vertical granular temperature due to grazing particle-bed collisions.