Dry and cohesive granular flows in a rotating drum: flow dynamics and scaling behaviours

Understanding the rheology of granular surface flows remains a significant challenge, particularly when inertial and cohesive interactions occur between particles to trigger complex regime transitions. This study investigates the steady flow dynamics of dry and cohesive granular materials through systematic rotating drum experiments, focusing on the effects of rotation speed, drum-to-particle size ratio and cohesion level. Scaling laws were further established to capture the combined effects of these factors and provide a unified description of flow behaviour across both dry and cohesive regimes. Results show that, for dry granular flows, both the normalised flowing layer thickness δ₀/d where δ₀ is flowing layer thickness and d represents the particle diameter and dynamic angle of repose tan β₀ increase with Froude number, with a critical transition angle of tan β₀ ≈ 0.58 marking the onset of cascading. The presence of interstitial liquid induces capillary cohesion, leading to a plug-like flow with a convex free surface and higher β₀ and δ₀. A new dimensionless governing parameter, derived from dimensional analysis incorporating a capillary time scale, successfully collapses nearly all experimental data onto single master curves that exhibit clear power-law behaviour, capturing the combined effects of inertia, size and cohesion. Furthermore, rheological interpretation within the μ(I) framework, where μ represents the effective friction coefficient and I denotes the inertial number reveals rate-dependent frictional strengthening for both dry and cohesive cases, with cohesive flows exhibiting consistently higher resistance induced by interparticle capillary cohesion.