# Collapse dynamics and runout of dense granular materials in a fluid

## V. Topin, Y. Monerie, F. Perales, F. Radjai

We investigate the effect of an ambient fluid on the dynamics of collapse and spread of a granular column simulated by means of the contact dynamics method interfaced with computational fluid dynamics. The runout distance is found to increase as a power law with the aspect ratio of the column and, surprisingly, for a given aspect ratio and packing fraction, it may be similar in the grain-inertial and fluid-inertial regimes but with considerably longer duration in the latter case. We show that the effect of fluid in viscous and fluid-inertial regimes is to both reduce the kinetic energy during collapse and enhance the flow by lubrication during spread. Hence, the runout distance in a fluid may be below or equal to that in the absence of fluid due to compensation between those effects.

### Videos:

###### Collapse of a column of grains of aspect ratio a=1 in a fluid in the viscous regime. The grains and fluid are colored according to the amplitude of their velocities.

###### Collapse of a column of grains of aspect ratio a=7 in a fluid in the viscous regime. The grains and fluid are colored according to the amplitude of their velocities.

###### Collapse of a column of grains of aspect ratio a=10 in a fluid in the viscous regime. The grains and fluid are colored according to the amplitude of their velocities.

###### Collapse of a column of grains of aspect ratio a=1 in a fluid in the fluid-inertial regime. The grains and fluid are colored according to the amplitude of their velocities.

###### Collapse of a column of grains of aspect ratio a=7 in a fluid in the fluid-inertial regime. The grains and fluid are colored according to the amplitude of their velocities.

###### Collapse of a column of grains of aspect ratio a=10 in a fluid in the fluid-inertial regime. The grains and fluid are colored according to the amplitude of their velocities.

Collapse of a column of grains of aspect ratio a=1 in the grain-inertial regime (dry case). The grains are colored according to the amplitude of their velocities.

Collapse of a column of grains of aspect ratio a=7 in the grain-inertial regime (dry case). The grains are colored according to the amplitude of their velocities.

Collapse of a column of grains of aspect ratio a=10 in the grain-inertial regime (dry case). The grains are colored according to the amplitude of their velocities.