Using large-eddy simulation technique for dense particle-fluid flows, the current-induced scour is predicted for both the mono- and bi-dispersed systems below a horizontal submarine pipeline exposed to unidirectional flow. The simulations are four-way coupled, which implies that both solid-liquid and solid-solid interactions are taken into account. Particles are assumed to behave as visco-elastic solids during interactions with their neighboring particles, and their motion are predicted by a Lagrangian method. The inter-particle normal and tangential contact forces between particles are calculated using a generalized Hertzian model. The other forces on a particle that are taken into account include gravitational, pressure gradient force accounting for the acceleration of the displaced liquid, the drag force resulting from velocity difference with the surrounding liquid, and the Magnus and Saffman lift forces. The predicted scour profiles for monodispersed system are found to compare favorably with the laboratory observations. For the bi-dispersed system, a seepage flow underneath the pipe (which is a major factor to cause the onset of scour below the pipeline) is found to be weakened using an appropriate size for the sand bed. This finding highlights the importance of the bed particle size distribution on the onset of scour below the pipelines.

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