Force and torque model sensitivity and coarse graining for bedload-dominated sediment transport

Abstract

We present results from Euler–Lagrange simulations of turbulent flow over an erodible monodisperse bed of particles at a shear Reynolds number of Reτ=180. The Galileo number Ga and the ratio of Shields to the critical Shields number Θ/θcr were varied in the range 11.4 to 29.8 and 1.32 to 5.98, respectively. Two drag force models were investigated along with the influence of lift, particle rotation, and tangential collision forces for each model. Both models give similar results as far as mean particle flux and Shields stress are concerned, however we find that excluding particle rotation, without ignoring the associated tangential collisional force, significantly reduces the particle flux with little influence on Shields stress. On the other hand, when both particle rotation and tangential collision forces are not taken into account, the particle flux is practically unchanged, but the excess Shields stress slightly increases compared to the case where both effects are considered. We also find the lift force to substantially influence particle flux. Additionally, we show the importance of spatial coarse graining and time averaging for Euler–Euler simulations and quantify the reduction in scatter for space and time dependent variables such as sediment flux, Shields stress, and bed surface fluid velocities.