Critical assessment of two approaches for evaluating contacts between super-quadric shaped particles in DEM simulations

Study on Verification Approach and Multicontact Points Issue When Modeling Cyperus esculentus Seeds Based on DEM

In this paper, the Multisphere (MS) models of three varieties of Cyperus esculentus seeds are modeled based on DEM. In addition, for comparison, other particle models based on automatic filing in EDEM software are also introduced. Then, the direct shear test, piling test, bulk density test, and rotating hub test are used to verify the feasibility of particle models of Cyperus esculentus seeds that we proposed. By comparing the simulated results and experimental results, combined with the CPU computation time, the proposed particle models achieved better simulation accuracy with fewer filing spheres. According to simulation results, some limitation was present when using one single verification test; varieties of verification tests used could improve the verification reliability, and a more appropriate particle model could be selected. Additionally, the issue of multicontact points in the MS model was studied. The Hertz Mindlin (no slip) (HM) model and Hertz Mindlin new restitution (HMNR) model were both considered in simulations for comparison. The rotating hub test and particle–wall impact test were used, and the influences of multiple contact points on the motion behavior of individual particles and particle assemblies were analyzed. Simulation results showed that the multiple contact points affected the motion behavior of individual particles; in contrast, the influence of multiple contact points on the motion behavior of the particle assembly was insignificant. Moreover, the relationships between moisture content of seeds and Young’s modulus, Young’s modulus, and the number of contact points were also considered. Young’s modulus decreased with increasing moisture content. The number of contact points increased with a decreasing Young’s modulus.

A Universal Method for Modeling and Characterizing Non-Circular Packing Systems Based on n-Point Correlation Functions

A universal method for modeling and characterizing non-circular particles is developed. The n-point correlation functions (n = 1, 2 and 3) are efficiently computed with a GPU parallel computing procedure. An algorithm for dynamic packing of impenetrable non-circular particles is developed based on the fast estimation of overlap information using a one-point correlation function. The packing algorithm is independent of particle shape and proved to be reliable by examples of polygons and super-ellipses. In addition, penetrable packings are generated in an efficient and precise way. Using a two-point correlation function, these non-circular packs are accurately characterized and compared in terms of features such as penetrable and impenetrable, packing fraction and particle shape. In addition, three-point correlation functions are also illustrated and discussed.

A kinematics-based model for the settling of gravity-driven arbitrary-shaped particles on a surface

A discrete model is proposed for settling of an arbitrary-shaped particle onto a flat surface under the gravitational field. In this method, the particle dynamics is calculated such that (a) the particle does not create an overlap with the wall and (b) reaches a realistic equilibrium state, which are not guaranteed in the conventional discrete element methods that add a repulsive force (torque) based on the amount of overlap between the particle and the wall. Instead, upon the detection of collision, the particle’s kinematics is modified depending on the type of contact, i.e., point, line, and surface types, by assuming the contact point/line as the instantaneous center/line of rotation for calculating the rigid body dynamics. Two different stability conditions are implemented by comparing the location of the projection of the center of mass on the wall along gravity direction against the contact points to identify the equilibrium (stable) state on the wall for particles with multiple contact points. A variety of simulations are presented, including smooth surface particles (ellipsoids), regular particles with sharp edges (cylinders and pyramids) and irregular-shaped particles, to show that the method can provide the analytically-known equilibrium state.

Granular flow of cylinder-like particles in a cylindrical hopper under external pressure based on DEM simulations

Granular flow is widely found in nature or industrial production. Although the external driving force significantly affects the dynamic behavior of a granular system, a large number of numerical simulations have been conducted to study granular flows driven by gravity. In this study, a superquadric equation was used to construct spherical and cylindrical elements, and the flow processes of granular materials under external pressure were simulated by the discrete element method. To examine the validity of the DEM model, the Janssen effect of spherical particles, the static packing of cylindrical particles and the flow process of spherical particles under external pressure are simulated and compared with the previous experimental and theoretical results. Subsequently, the effects of blockiness, orifice diameter, and particle friction on the flow characteristics are investigated. Results show that the flow rate of spherical particles increases as the external pressure and opening diameter increase or the particle friction decreases. However, the flow rate of cylindrical particles decreases as the blockiness parameter increases, and the external pressure has little effect on the flow rate of the cylindrical particles when the blockiness parameter is greater than 4. Furthermore, the external pressure causes a change in the flow pattern of granular systems. In a gravity-driven granular flow, cylindrical particles appear in funnel flow, and spherical particles in both mass and funnel flows. In a pressure-driven granular flow, spherical particles appear in mass flow, and cylindrical particles in both mass and funnel flows. The critical height of the transition between mass and funnel flows decreases with increasing external pressure and eventually reaches a steady state. Meanwhile, the critical height increases with the blockiness parameter, which indicates that more cylindrical than spherical particles appear in funnel flow. Finally, the basic flow characteristics of granular materials under external pressure are further analyzed by the velocity uniformity index, the normal contact force between particles, and the bottom pressure. Overall, the numerical results are useful for understanding the changes in the flow characteristics of spherical and cylindrical granular materials under external pressure, and further provide guidance for the appropriate design and optimization of cylindrical hoppers.