Simulations of granular bed erosion due to a mildly turbulent shear flow

Three-dimensional, time-dependent simulations of the erosion of a granular bed consisting of monosized, non-cohesive spherical particles are reported. The simulations fully resolve the turbulence and the flow around each of the spheres (particle-resolved, direct simulations). The erosion is due to the turbulent flow of a Newtonian liquid in a planar channel with a Reynolds number based on wall shear velocity and channel height of 168. The main control parameter in the simulations is the Shields number that was varied between 0.03 and 0.60. The simulations were performed by means of a lattice Boltzmann scheme supplemented with an immersed boundary method to impose no-slip conditions at the particle surfaces. The results show the impact of the Shields number on the mobility of the particles at the top and above the bed. They also show a strong coupling between solids motion and the strength of the turbulent fluctuations in the liquid. Specifically, directly above the bed the particles significantly enhance turbulence at the lower end of the Shields number range investigated.