Transition from Saltation to Collisional Regime in Windblown Sand.

We report experiments on windblown sand that highlight a transition from saltation to collisional regime above a critical dimensionless mass flux or Shields number. The transition is first seen through the mass flow rate Q, which deviates from a linear trend with the Shields number and seems to follow a quadratic law. Other physical evidences confirm the change of the transport properties. In particular, the particle velocity and the height of the transport layer increases with increasing Shields number in the collisional regime while the latter are invariant with the wind strength in the saltation regime. Discrete numerical simulations support the experimental findings and ascertain that mid-air collisions are responsible for the change of transport regime.