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A depth-weakening wall friction coefficient, µ  w  , has been reported from three-dimensional numerical simulations of steady and transient dense granular flows. To understand the degradation mechanisms, a scaling law for µ  w  /  f  and χ has been proposed where  f  is the intrinsic particle-wall friction and χ is the ratio of slip velocity to square root of granular temperature (Artoni &amp; Richard,  Phys. Rev. Lett. , vol. 115 (15), 2015, 158001). Independently, a friction degradation model has been derived which describes a monotonically diminishing friction depends on a ratio of grain angular and slip velocities, Ω (Yang &amp; Huang,  Granular Matter , vol. 18 (4), 2016, 77). In search of experimental evidence for how these two parameters degrade the µ  w  , an annular shear cell experiment was performed to estimate the bulk granular temperature, angular and slip velocities at sidewall through image-processing. Meanwhile, µ  w   was measured by a force sensor to confirm the weakening towards the creep zone. The measured µ  w  /  f  − χ and µ  w  /  f  − Ω were both well-fitted to the corresponding models showing that both granular temperature and angular velocity are significant mechanisms to degrade the µ  w   which broadens the research perspective on modeling the boundary condition of dense granular flows.

Influence of granular temperature and grain rotation on the wall friction coefficient in confined shear granular flows