  The rheology of 2D steady surface flow of cohesionless cylinders in a
rotating drum is investigated through {\em Non Smooth Contact Dynamics}
simulations. Profile of volume fraction, translational and angular velocity,
rms velocity, strain rate and stress tensor were measured at the midpoint along
the length of the surface flowing layer where the flow is generally considered
as steady and homogeneous. Analysis of these data and their inter-relations
suggest the local inertial number - defined as the ratio between local inertial
forces and local confinement forces - to be the relevant dimensionless
parameter to describe the transition from the quasi-static part of the packing
to the flowing part at the surface of the heap. Variations of the components of
the stress tensor as well as the ones of rms velocity as a function of the
inertial number are analysed within both the quasi-static and the flowing
phases. Their implications are discussed.
