Granular Matter最新文献

The restitution coefficient is one of the microphysical properties that must be specified for a discrete element model simulation. The more accurate the input, the more accurate the simulation results are. Due to the differences in shape and surface roughness between the corn stalk particle model built-in EDEM software and the actual corn stalk, the simulation will be distorted if the measured recovery coefficients are directly introduced into the EDEM software for simulation. To address this issue, this paper presents experimental measurements of the collision behavior between the intermodal tissue, pith, and nodal tissue particles and the horizontal substrate based on kinematic principles and with the aid of high-speed camera technology to reconstruct the trajectory of the particles during free fall and bounce. Utilizing EDEM software to simulate the free fall and bounce processes, a quadratic polynomial prediction model of the recovery coefficient input values and calculated values was established. Combined with the simulation and physical tests, the coefficients of restitution between the pith and the iron plate and pith surface are 0.559, 0.767. The collision restitution coefficients were determined to be 0.767, 0.616, and 0.784 for the collision between the rind and the iron plate, rind surface, and pith surface, respectively. 0.549, 0.705, 0.687, and 0.723 for the collision restitution coefficients between the node and the rind surface, pith surface, and node surface, respectively. The calibration restitution coefficient was input to EDEM software for the simulation test, and the relative error between the simulation results and the physical examination was within 4.15%. The particle model created and the restitution coefficients calibrated can be used as a reference for designing a corn stalk processing machine and the discrete-element study on the motion of corn stalk particles inside such devices.

Graphic abstract

We studied experimentally and numerically the compaction and subsequent expansion dynamics of a granular bed composed of cylindrical repelling magnets contained in a two-dimensional cell. The particles are firstly compressed vertically with a piston at a given strain rate until a maximum force is reached. The piston is then removed at the same strain rate while the bed expands due to the magnetic repulsion of the particles. In the experiments, two different initial configurations were generated, a standard and a loose packing bed. The standard packing bed was simulated, and modelling the dry friction between the magnetic particles and the walls of the cell was crucial for the correct description of the compression and expansion dynamics. We found that the force acting on the piston increases continuously and exponentially with the piston stroke during compression, being very sensitive to the initial packing conditions of the bed. In contrast, a history-independent exponential decrease of this force was found during the expansion phase. The hysteresis in the system was quantified in terms of the average displacement of the particles. The continuous compression contrasts with the sudden force drops observed during the compaction of granular materials with direct particle-particle contacts, where stick-slip motion is induced by friction and force chain breakage. Moreover, we found that the short range of magnetic interaction induces density inversion and crystallization of the system. Our results can be useful to develop a new kind of magnetic granular dampers.

Graphical abstract

In this paper the effects of size-segregation on flow’s mobility and final morphology of axisymmetric collapses of dry, cohesionless, natural polydisperse grains, were experimentally investigated. Particular attention was paid to effects of initial particle organization of grains on runout properties. Several experiments were conducted on vertical columns of inner radius (r_{i}) and height (h_{i}), finding a key role of aspect ratio (a=h_{i}/r_{i}) on the structure of the dimensionless curves (r^{*}=(r_{f}-r_{i})/r_{i}) and (h^{*}=h_{f}/h_{i}) vs a. These curves can be characterized by power-law fitting functions, where (r_{f},h_{f}) are the final radius and height of deposits. The fitting parameters of these functions show two mobility regimes, but with significant departure from some results reported in literature. These differences emphasize the need of considering the effects of initial arrangement of grains into the column and the non-uniformity of grain-size distribution on these scaling curves. Measurements of the final profiles h(r) are also reported obtaining a persistent “Mexican-hat” shape, although showing a difficulty for defining an universal scaling curve for this data. Coincidentally, the dimensionless energy dissipation parameter ((epsilon (a))) follows a similar trend as that reported for uniform grains, suggesting that both polydisperse and uniform granular materials dissipate energy almost in the same way.

A micromechanical study of the relationship between contact force networks and energy dissipation is presented. A series of drained triaxial compression tests with different stress paths have been simulated using the discrete element method. Two existing contact force network partitioning methods have been used for analysing the energy dissipation, one based on the average contact force magnitude and the other based on the contribution of contact forces to the global deviator stress. For both methods, energy dissipation in neither the strong nor weak contact networks is negligible. When the average contact force partitioning method is used, over 70% of the energy dissipation occurs in the weak contact network, but the dissipation per sliding contact is higher in the strong contact network because the tangential contact force is higher. When the contact network is partitioned based on the contribution of forces to global deviator stress, more than 60% of the energy dissipation occurs in the strong contact network. A new normal contact force threshold for splitting energy dissipation is identified. Specifically, over 93% of energy dissipation occurs at contacts with a normal contact force below 2 times the average normal contact force. There is very small energy dissipation in contacts with higher normal contact force because there is little particle sliding.

Compressibility of Expanded Polystyrene (EPS) beads in mixtures of sand and EPS beads is considered by discrete element modeling of beads as clumps of particles bonded by a cohesive contact model. A calibration scheme is devised where input microparameters are calibrated stepwise, using experimental results and available data. The calibrated model is used to investigate the effect of parameters, namely normal pressure, EPS bead content and bead relative size on the compressibility and lateral stresses of the mixtures. Results showed an increase in mixture compressibility with the addition of EPS, which coincided with an increase in total coordination number. The increase in EPS bead size was observed to decrease compressibility. Microscale results showed a reduction in the overall, EPS-EPS and sand-EPS coordination numbers with the increase in bead relative size, resulting in the contact network being more dominated by sand-sand contacts which resulted in stiffer mixtures. However, this reduction in compressibility was negligible for larger EPS bead contents where coordination numbers showed that enough contacts are formed between EPS beads to create a continuous network across the sample, thus curtailing the effect of EPS bead size. Contact force networks showed that despite the increase in the number of sand-EPS and EPS-EPS contacts, the majority of forces are transferred through sand-sand contacts.