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Analysis of Particle Diffusion Due to Inter-Particle Collisions in Bed-Load Motion by Means of a 3D Numerical Model

Author(s): Patricio A. Moreno-Casas; Fabian A. Bombardelli

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Keywords: Particle-particle collision; Inter-particle collision; Saltation; Bed-load transport; Two-phase flow model

Abstract: Saltation is the predominant bed-load transport mechanism in rivers and streams. The main processes involved in saltation can be described as: i) Particle collision with the bed, ii) particle “free flight” between collisions with the bed, and iii) particle-particle collisions. These processes dictate the features of particle dispersion near the bed. Particle “free flight” and particle-bed collisions have been previously included in all numerical simulations. In turn, particle-particle collisions in bed-load transport have been mostly disregarded. Nikora and collaborators put forward ten years ago a set of mechanisms to elucidate the diffusion of particles in bed-load motion; there are associated issues on how particle collisions affect those diffusion processes. A three-dimensional (3D) Lagrangian particle-tracking model coupled with a logarithmic velocity profile is used herein to analyze the importance of particle-particle collisions when simulating the saltation motion of sand grains near the bed, and to address the diffusion process. The particle collision algorithm is based on the conservation of linear and angular momentum, considering only binary collisions through the application of a hard-sphere model. In order to isolate the contribution of particle-particle collisions to the particle dispersion process near the bed, the numerical algorithm was manipulated to disregard particle scattering in the transverse direction of the channel caused by collisions with the bed. In this paper, we validate the model with the experimental data of Lee and Hsu (1994). We analyze simulation results with particle sizes within the sand range, different flow intensities, and different particle concentrations, in terms of particle diffusion. To identify the importance of particle-particle collisions, simulations with and without inter-particle collisions were carried out. Our findings suggest that: i) At higher particle concentrations there are significant differences among simulation results with and without particle-particle collisions for high flow intensities; and ii) particle-particle collisions are an important source of transverse diffusion due to important changes in transverse particle velocity.


Year: 2013

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