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Modeling the initiation of underwater granular avalanches: an Eulerian approach

In this work, the two-phase flow model SedFoam is developed to reproduce the initiation of underwater granular avalanches. In agreement with previous results, the behavior of a layer of granular material fully immersed in a fluid flowing down an inclined slope is strongly influenced by the initial volume fraction. Loose granular packings accelerate rapidly, whereas dense granular packings exhibit a time delay before flowing. Indeed, shear stresses exerted by the fluid flow on the granular bed not only induce a motion on the tangent component but also a motion in the wall-normal direction. Such behavior is explained by a combination of the dilatancy angle and pore pressure feedback on the grains. Dilation mechanism increases the volume fraction triggering significant pore pressures inside the granular layer. Additionally, fluid is sucked into the pore space pressing the solid particles together, leading to increased normal stress and the frictional forces. Likewise, contraction results in an increment of the pore pressure and a reduction of the shear strength. The dynamics of underwater avalanches captured by the numerical model accounting for the dilatant-contractant behavior are qualitatively validated with available experimental data. This work is a first step toward incorporating these mechanisms in the Eulerian framework. This a major step in the development of the model that will allow to tackle more complex problems in the future, such as piping, granular collapse or mine burial.