Hybrid Particle Model Research Articles

A Hybrid Particle/Finite Element Model with Surface Roughness for Stone Masonry Analysis

Circular and spherical particle models are a class of discrete elements (DEM) that have been increasingly applied to fracture studies of quasi-brittle materials, such as rock and concrete, due to their proven ability to simulate fracture processes through random particle assemblies representing quasi-brittle materials at the grain scale. More recently, DEM models have been applied to old stone masonry fracture studies. In order to extend its applicability to structures of larger dimensions, an enhanced hybrid particle model is proposed here that allows finite elements with a given surface roughness, provided by the discretization of the element boundary with particles, to interact with the particulate media in which they are embedded. The performance of the hybrid model is compared with that of a traditional all-particle model under uniaxial testing. It is shown that similar results are obtained, namely, in the elastic phase, figures of rupture and pre-peak and post-peak behavior, while the hybrid model allows for a significant computational run time reduction of 20% to 25% in the coarse particle assemblies. Finally, the proposed hybrid model is applied in the simulation of shear tests of stone masonry walls and dry and mortared joints, providing reasonably good agreement with both the experimental results and predictions. For the rubble masonry tests, the hybrid model allows for a computation run time reduction of around 40% when compared with an all-particle model.

A hybrid particle model with advanced conversion parameters and dynamic drag model applied for the CFD modeling of an entrained-flow gasifier

The main focus of the current work is to establish a hybrid particle model to depict the char conversion process, taking into account the influence of pore diffusion on the carbon consumption rate. Compared to existing char conversion models, the new hybrid model uses advanced correlations for conversion parameters such as the density and diameter exponents α and β and the Random Pore Model correction factor γ, which are functions of char conversion and effectiveness factor. CFD simulation results for a Pressurized Entrained-Flow Reactor (PEFR) using the hybrid particle model are in good agreement with experimental data. In addition, the current work also introduces dynamic drag models based on the development of the shape of spherical and non-spherical particles during the char conversion process. The dynamic drag models, which up to now has been missed in calculation of particle trajectories, are examined to evaluate their influences on the total carbon conversion of the PEFR. Furthermore, the performance of ideal particle sub-models such as the Uniform Conversion Model and Shrinking Unreacted Particle Model are also studied and compared with that of the hybrid particle model. PEFR CFD simulation results illustrate considerable differences between the carbon conversion profiles obtained with these sub-models and those profiles obtained using the hybrid particle model.

Study of industrial titania synthesis using a hybrid particle-number and detailed particle model

We apply a hybrid particle model to study synthesis of particulate titania under representative industrial conditions. The hybrid particle model employs a particle-number description for small particles, and resolves complicated particle morphology where required using a detailed particle model. This enables resolution of particle property distributions under fast process dynamics. Robustness is demonstrated in a network of reactors used to simulate the industrial process. The detailed particle model resolves properties of the particles that determine end-product quality and post-processing efficiency, including primary particle size and degree of aggregate cohesion. Sensitivity of these properties to process design choices is quantified, showing that higher temperature injections produce more sintered particles; more frequent injections narrow the geometric standard deviation of primary particle diameter; and chlorine dilution reduces particle size and size variance. Structures of a typical industrial particle are compared visually with simulated particles, illustrating similar aggregate features with slightly larger primary particles.

Modeling rapidly growing cracks in planar materials with a view to micro structural effects

Dynamic fracture behavior in both fairly continuous materials and discontinuous cellular materials is analyzed using a hybrid particle model. It is illustrated that the model remarkably well captures the fracture behavior observed in experiments on fast growing cracks reported elsewhere. The material’s microstructure is described through the configuration and connectivity of the particles and the model’s sensitivity to a perturbation of the particle configuration is judged. In models describing a fairly homogeneous continuous material, the microstructure is represented by particles ordered in rectangular grids, while for models describing a discontinuous cellular material, the microstructure is represented by particles ordered in honeycomb grids having open cells. It is demonstrated that small random perturbations of the grid representing the microstructure results in scatter in the crack growth velocity. In materials with a continuous microstructure, the scatter in the global crack growth velocity is observable, but limited, and may explain the small scattering phenomenon observed in experiments on high-speed cracks in e.g. metals. A random perturbation of the initially ordered rectangular grid does however not change the average macroscopic crack growth velocity estimated from a set of models having different grid perturbations and imply that the microstructural discretization is of limited importance when predicting the global crack behavior in fairly continuous materials. On the other hand, it is shown that a similar perturbation of honeycomb grids, representing a material with a discontinuous cellular microstructure, result in a considerably larger scatter effect and there is also a clear shift towards higher crack growth velocities as the perturbation of the initially ordered grid become larger. Thus, capturing the discontinuous microstructure well is important when analyzing growing cracks in cellular or porous materials such as solid foams or wood.

Modeling Swarthmore spheromak reconnection experiment using hybrid code

A three-dimensional (3D) hybrid-particle model is developed for investigation of magnetic reconnection in the Swarthmore Spheromak Experiment (SSX). In this numerical model, ions are treated as fully kinetic particles, and electrons are treated as a massless fluid. The plasma responds to the electromagnetic fields in a self-consistent manner. The simulation is performed in a cylindrical domain. Initially, a pair of counter-helicity spheromaks are assumed, in which the magnetic field and plasma pressure are set up according to the MHD equilibrium. The ion particles are loaded with a Maxwellian distribution function. As the simulation proceeds, magnetic reconnection takes place at the current sheet between the pair of spheromak fields. Plasma is ejected away from the X line towards the central axis, where heating of the transmitted ion is found. Meanwhile, quadrupole out-of-plane magnetic field structure associated with the Hall effects is present around the reconnection site. The preliminary simulation shows consistency with the SSX experiment in several aspects.

The Plasma Environment of Comet 67P/Churyumov-Gerasimenko Throughout the Rosetta Main Mission

The plasma environment of comet 67P/Churyumov-Gerasimenko, the Rosetta mission target comet, is explored over a range of heliocentric distances throughout the mission: 3.25 AU (Rosetta instruments on), 2.7 AU (Lander down), 2.0 AU, and 1.3 AU (perihelion). Because of the large range of gas production rates, we have used both a fluid-based magnetohydrodynamic (MHD) model as well as a semi-kinetic hybrid particle model to study the plasma distribution. We describe the variation in plasma environs over the mission as well as the differences between the two modeling approaches under different conditions. In addition, we present results from a field aligned, two-stream transport electron model of the suprathermal electron flux when the comet is near perihelion.