Beichuan Yan

Three‐dimensional ellipsoidal discrete element modeling of granular materials and its coupling with finite element facets

Purpose – The purpose of this paper is to develop a discrete element (DE) and multiscale modeling methodology to represent granular media at their particle scale as they interface solid deformable bodies, such as soil‐tool, tire, penetrometer, pile, etc., interfaces.Design/methodology/approach – A three‐dimensional ellipsoidal discrete element method (DEM) is developed to more physically represent particle shape in granular media while retaining the efficiency of smooth contact interface conditions for computation. DE coupling to finite element (FE) facets is presented to demonstrate initially the development of overlapping bridging scale methods for concurrent multiscale modeling of granular media.Findings – A closed‐form solution of ellipsoidal particle contact resolution and stiffness is presented and demonstrated for two particle, and many particle contact simulations, during gravity deposition, and quasi‐static oedometer, triaxial compression, and pile penetration. The DE‐FE facet coupling demonstrat...

Concurrent Multiscale Computational Modeling for Dense Dry Granular Materials Interfacing Deformable Solid Bodies

A method for concurrent multiscale computational modeling of interfacial mechanics between granular materials and deformable solid bodies is presented. It involves two main features: (1) coupling discrete element and higher order continuum finite element regions via an overlapping region; and (2) implementation of a finite strain micromorphic pressure sensitive plasticity model as the higher order continuum model in the overlap region. The third main feature, adaptivity, is not currently addressed, but is considered for future work. Single phase (solid grains) and dense conditions are limitations of the current modeling. Extensions to multiple phases (solid grains, pore liquid and gas) are part of future work. Applications include fundamental grain-scale modeling of interfacial mechanics between granular soil and tire, tool, or penetrometer, while properly representing far field boundary conditions for quasi-static and dynamic simulation.

ONR MURI project on soil blast modeling and simulation

Current computational modeling methods for simulating blast and ejecta in soils resulting from the detonation of buried explosives rely heavily on continuum approaches such as Arbitrary Lagrangian-Eulerian (ALE) and pure Eulerian shock-physics techniques. These methods approximate the soil as a Lagrangian solid continuum when deforming (but not flowing) or an Eulerian non-Newtonian fluid continuum when deforming and flowing at high strain rates. These two extremes do not properly account for the transition from solid to fluid-like behavior and vice versa in soil, nor properly address advection of internal state variables and fabric tensors in the Eulerian approaches. To address these deficiencies on the modeling side, we are developing a multiscale multiphase hybrid Lagrangian particle-continuum computational approach, in conjunction with coordinated laboratory experiments for parameter calibration and model validation. This paper provides an overview of the research approach and current progress for this Office of Naval Research (ONR) Multidisciplinary University Research Initiative (MURI) project.

Overlapped-coupling between spherical discrete elements and micropolar finite elements in one dimension using a bridging-scale decomposition for statics

Purpose – The purpose of this paper is to develop a concurrent multiscale computational method for granular materials in the quasi-static loading regime. Design/methodology/approach – Overlapped-coupling between a micropolar linear elastic one-dimensional (1D) mixed finite element (FE) model and a 1D chain of Hertzian nonlinear elastic, glued, discrete element (DE) spheres is presented. The 1D micropolar FEs and 1D chain of DEs are coupled using a bridging-scale decomposition for static analysis. Findings – It was found that an open-window DE domain may be coupled to a micropolar continuum FE domain via an overlapping region within the bridging-scale decomposition formulation for statics. Allowing the micropolar continuum FE energy in the overlapping region to contribute to the DE energy has a smoothing effect on the DE response, especially for the rotational degrees of freedom (dofs). Research limitations/implications – The paper focusses on 1D examples, with elastic, glued, DE spheres, and a linear elas...

Superlinear speedup phenomenon in parallel 3D Discrete Element Method (DEM) simulations of complex-shaped particles

Abstract Strong superlinear speedup has been discovered in large scale simulations of parallel 3D DEM for complex-shaped particles, which is based on an algorithm of spatial domain decomposition, and exhibits the “high-CPU-low-memory” characteristics. The interpretation of this phenomenon requires a careful examination of the speedup theory and practice in the field of parallel computing. The superlinear speedup is investigated from three perspectives: (i) memory footprint per process, (ii) cache miss rates of L1, L2 and L3 level caches, and (iii) uniprocessor performance, using a wide range of problem size (across five orders of magnitude of simulation scale regarding number of particles) and number of compute nodes (1–2048 nodes) on DoD supercomputers. The Performance-API (PAPI) is employed in the source code to measure cache miss rate and FLOPS. The strong scaling measurements show that cache miss rate is sensitive to the memory consumption shrinkage per processor, and the last level cache (LLC) contributes most significantly to the strong superlinear speedup among all of the three cache levels, and this is also revealed in the weak scaling measurements. The findings are associated with the inherently perfect scalability of 3D DEM: its memory scalability function is a nonlinearly decreasing function of the number of processors. In addition, a constant (non-increasing) uniprocessor FLOPS performance w.r.t problem size can also contribute to the superlinear speedup. The superlinear speedup is a common phenomenon for large scale 3D DEM simulations of complex-shaped particles, and the larger the scale, the stronger is the superlinear speedup. DEM researchers should take advantage of this effect to speedup their parallel simulations.

Large-scale dynamic and static simulations of complex-shaped granular materials using parallel three-dimensional discrete element method (DEM) on DoD supercomputers

Purpose The purpose of this paper is to extend complex-shaped DEM simulations from a few thousand particles to millions of particles using parallel computing on DoD supercomputers, and study the mechanical response of particle assemblies composed of a large number of particles in engineering practice and laboratory tests. Design/methodology/approach Parallel algorithm is designed and implemented with advanced features such as link-block (LB), border layer and migration layer, adaptive compute gridding technique, MPI transmission of C++ objects and pointers, etc, for high performance optimization; performance analyses are conducted across five orders of magnitude of simulation scale on multiple DoD supercomputers; and three full-scale simulations of sand pluviation, constrained collapse and particle shape effect are carried out to study mechanical response of particle assemblies. Findings The parallel algorithm and implementation exhibit high speedup and excellent scalability, communication time is a decre...