  We report numerical simulations on granular shear flows confined between two
flat but frictional sidewalls. Novel regimes differing by their strain
localization features are observed. They originate from the competition between
dissipation at the sidewalls and dissipation in the bulk of the flow. The
effective friction at sidewalls is characterized (effective friction
coefficient and orientation of the friction force) for each regime, and its
interdependence with slip and force fluctuations is pointed out. We propose a
simple scaling law linking the slip velocity to the granular temperature in the
main flow direction which leads naturally to another scaling law for the
effective friction.
