The boundary conditions supported by the Granular Flow interface are controlled by the Wall, Inlet, Outlet, and Periodic Condition features.

The boundaries selected by the Wall feature are treated as rigid walls that can exert contact forces on the grains when they come into contact. The contact force models selected in the settings window of The Granular Flow Interface interface are used to model the grain–wall interactions. The corresponding contact parameters are specified in the Contact with Walls feature. Additionally, if the Compute Grain Temperature and the Compute Conductive Heat Transfer checkboxes are enabled, the boundary conditions for the wall temperature also need to be specified.

Many applications benefit from moving walls, and the motion of the walls is also controlled by the Wall Movement settings of this feature. The boundaries are treated as rigid walls, and no deformation is allowed. The allowed rigid body movements include translation, rotation, and a combination of the two.

The Inlet feature is used to release grains along the boundary such that the grain centers lie on the boundary at release time. This feature overrides any Wall features that are located above it in the Model Builder window, and therefore the selected boundaries are not considered for grain–wall interactions.

The Outlet feature allows the grains to pass through different adjoining domains or to remove the grains passing through it from the simulation domain. This feature naturally overrides the Wall features that are located above it in the Model Builder window, and therefore the selected boundaries are not considered for grain–wall interactions. This feature can be combined with the Inlet feature to affect the behavior of the grains passing through the boundaries selected in the Inlet feature.

The Periodic Condition feature is used to apply periodic boundary conditions in one of the x, y, or z directions. Grains near the periodic boundaries can interact with the periodic images of the grains and wall elements, and grains passing through the source boundaries will be replaced by their periodic images at the destination boundary.

This feature naturally overrides the Wall features that are located above it in the Model Builder window, and therefore the selected boundaries are not considered for grain–wall interactions. Additionally, this feature also overrides the Outlet feature but can be used along with the Inlet feature to release grains along the periodic boundaries.