The balance of momentum and energy at an interface between colliding and freely flying grains in a rapid granular flow

A steady rapid flow of a granular material down an incline is considered and attention is focused on the relatively narrow region of transition between a concentrated regime of colliding grains and a more dilute regime of freely flying grains. Momentum and energy are balanced over a control volume with surfaces in each flow regime, the solid volume fraction is determined at the interface, and the boundary condition on the energy of the velocity fluctuations at the top of the collisional regime is obtained.