Abstract
Exploration of the transient evolution of the rheological properties of dense inertial flow can reveal the equilibrium mechanism of granular materials maintaining their own stability under shear. Here, discrete element method simulations are performed to study the transient flow characteristics of a dense granular system under plane shear in the inertial regime. We quantitatively analyze the changes in the system’s flow state, interfacial friction coefficient, effective friction coefficient, microstructure anisotropy, and internal shear strength. Simulation results show that the evolution of the horizontal flow experiences three typical stages, namely transmission, adjustment, and stabilization. Moreover, the shear dilatancy caused by the vertical movement of particles, gradually weakens the spatial geometric constraint and reduces the system’s tangential load-bearing capacity, thereby decreasing the interfacial friction coefficient \({\mu }^{^{\prime}}\), which represents the boundary driving strength. On the other hand, the shear flow induces variations in the anisotropies of both contact orientation and contact forces, thus improving the system’s internal shear strength \({\mu }^{*}\) and increasing the effective friction coefficient \({\mu }_\text{e}\). By comparison, \({\mu }^{^{\prime}}\) is greater than \({\mu }_\text{e}\) until approximately equal in the steady flow state. Therefore, during the evolution of the flow state, the boundary driving strength is reduced while the system’s shear resistance is enhanced, and eventually a balance between them is achieved. Distinguished from the micromechanical behaviors, the internal shear strength always mainly originates from the anisotropies in contact orientation and in normal contact force. Moreover, the contribution of the anisotropy in contact orientation becomes more predominant with increasing shear velocity.Graphical abstract
Highlights
Under shear, granular materials exhibit fascinating properties capable of sustaining stress as a solid, or flowing as a fluid [1-3]
By virtue of the above-mentioned studies, the steady rheological properties of dense granular materials have been well studied under the equilibrium condition, where the boundary drive strength is balanced with the internal shear strength
Since the internal shear strength of granular system mainly comes from the anisotropies in the contact orientation and in the contact forces, it is necessary to get a picture of their spatial distributions first
Summary
Granular materials exhibit fascinating properties capable of sustaining stress as a solid, or flowing as a fluid [1-3]. I.e., the interfacial friction coefficient and the shear strength, are most commonly employed to characterize the rheological properties of dense granular flow [19-23]. By virtue of the above-mentioned studies, the steady rheological properties of dense granular materials have been well studied under the equilibrium condition, where the boundary drive strength is balanced with the internal shear strength. It is still not clear regarding how the balance between the two strength quantities is gradually established in the continuous shear process, which requires a dedicate investigation on the transient evolution of the rheological properties. The bottom plate is fixed, and the top plate slides tangentially at a constant velocity Vx and applies a constant normal stress P to the granular system
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