A coupled thermal-hydro-mechanical (THM) model based on the combined finite-discrete element method (FDEM) is presented for simulating rock cracking driven by multi-physics. The THM model contains ...three parts: a fracture-pore mixed seepage model, a heat transfer model, and a fracture mechanics calculation model. By combining any two of the above three models, a coupled thermal-mechanical (TM) model, a coupled hydrothermal (TH) model, and a coupled hydromechanical (HM) model are constructed. Then, the TM model, TH model, and HM model are combined to build the THM model, which is implemented in a GPU parallel multiphysics finite-discrete element software, namely MultiFracs. Finally, we use this THM model to study the hydraulic fracturing process of hot dry rock. The simulation results indicate that in addition to the primary fracture perpendicular to the direction of the minimum in situ stress, branching fractures along the direction of the minimum in situ stress are also produced during the hydraulic fracturing process. The proposed THM model can simulate heat and fluid transfer in fractured reservoirs, crack initiation, propagation, and intersection.
Shaft furnaces are widely used in high‐temperature processes for granular materials due to their high energy efficiency. The modeling of these furnaces is challenging because of large domains and ...long process times. Small geometric details like the natural gas burner nozzles demand a fine grid on the computational fluid dynamics (CFD) side, resulting in a grid size smaller than the particle size. Resolving a discrete element particle over several cells is computationally expensive. Interpolation methods on non‐structured grids are complex. In order to provide a fast and simple solution, the volume fraction smoother was developed, and to shorten the calculation time, the time scale splitting method, which separates the time steps for CFD and the discrete‐element method (DEM), was introduced.
For the challenging task of modeling shaft furnaces, two techniques were developed: the volume fraction smoother (VFS), where the particle size can be independent of the cell size, and the time scale splitting method (TSSM), which allows separation of the fluid and discrete‐element method time scales to speed up the simulation.
•A multiphysics numerical model of the multi-layer L-PBF process is developed for the first time.•The CFD model consists of a multi-phase flow that includes evaporation, melting and solidification.•A ...DEM model based on the Lagrangian framework is implemented to simulate the powder-feeding.•Lack-of-fusion zones become smaller in the subsequent layers, due to heat accumulation from the sintered layers.•Porosity decreases in the next layers, due to better heat fluid flow conditions caused by elevated temperatures.•Elongated pores are formed parallel to the laser track and are vertically aligned in the cross-sections.
Laser-based powder bed fusion (L-PBF) is a branch of additive manufacturing technology which is considered to be a superior process due to its capability of producing complex designs with low material waste. Despite L-PBFs various unique characteristics, manufactured parts still suffer from a wide variety of defects, among which porosity is one of the most important. In this paper, a multiphysics numerical model for the multi-track/multi-layer L-PBF is developed and used for analysing the formation and evolution of voids caused by lack of fusion and improper melting. The multiphysics model is in meso-scale and is used to track and observe the formation of porosities, and considers phenomena such as multi-phase flow, melting/solidification, radiation heat transfer, capillary and thermo-capillary (Marangoni effect) forces, recoil pressure, geometry dependant absorptivity and finally evaporation and evaporative cooling. A novel methodology has been introduced to model the two subsequent powder-laying and fusion processes, for each layer, by means of a discrete element method (DEM) in a Lagrangian framework and a computational fluid dynamics (CFD) model, both implemented in Flow-3D. The results for the investigated process parameters indicate that the porosities (voids) are mainly formed in between the tracks, largely due to improper fusion of the particles. Moreover, it is observed that the pores are mostly elongated in the direction parallel to the laser scanning paths, as expected. The probability of the presence of pores is also observed to be higher in the first layer, where the average layer temperature is lower as well. Furthermore, the lack of fusion zones are seen to become smaller in the subsequent layers, largely due to better fluid flow and higher temperatures, because of heat accumulation in those layers.
In this study, a coupled contact heat transfer and thermal cracking model is proposed for discontinuous/granular media based on the computational framework of Finite-Discrete Element Method (FDEM). ...The model considers heat conduction within the continuum and heat transfer through contacts between discontinuous or granular materials, as well as the combined effects of contact heat transfer and thermal cracking. The numerical model is validated against theoretical solutions through several examples. Moreover, the effects of the thermal contact conductance, the influence of thermal cracks on heat transfer are investigated. Two examples of heat transfer and thermal cracking in granular/block materials are provided. A sensitivity analysis indicates that the thermal cracking model can provide statistically converged results for simulating thermal cracks. All these numerical examples demonstrate the excellent capability of the model, which can be applied to a wide range of continuous and discontinuous media, such as granular materials, ceramics, concrete, rock mass, and so on.
•A coupled contact heat transfer and thermal cracking model is proposed based on Finite-Discrete Element Method.•Heat conduction and contact heat transfer models are developed for discontinuous and granular materials.•Numerical examples demonstrated excellent capability of the model.
Summary
This paper focuses on the efficiency of finite discrete element method (FDEM) algorithmic procedures in massive computers and analyzes the time‐consuming part of contact detection and ...interaction computations in the numerical solution. A detailed operable GPU parallel procedure was designed for the element node force calculation, contact detection, and contact interaction with thread allocation and data access based on the CUDA computing. The emphasis is on the parallel optimization of time‐consuming contact detection based on load balance and GPU architecture. A CUDA FDEM parallel program was developed with the overall speedup ratio over 53 times after the fracture from the efficiency and fidelity performance test of models of in situ stress, UCS, and BD simulations in Intel i7‐7700K CPU and the NVIDIA TITAN Z GPU. The CUDA FDEM parallel computing improves the computational efficiency significantly compared with the CPU‐based ones with the same reliability, providing conditions for achieving larger‐scale simulations of fracture.
Calendering is an essential process step to manufacture electrodes for lithium-ion batteries. The relationship between the various component material properties and calendering conditions has a large ...impact on the battery performance. In this work, Discrete Element Method (DEM) was used to investigate the electrode structure evolution under different calendering conditions. The initial positions of active material (AM) particles were obtained from an uncalendered electrode microstructure characterised experimentally by X-ray tomography and then imported to DEM simulations. Simulated structures under different processing conditions were obtained by compression tests in DEM. The Edinburgh elasto-plastic adhesive (EEPA) model and bond model were used to describe the mechanical response of AM particles and binder phase during compression. Detailed stress and structural evolutions at microscopic scale were further analysed. For the first time, the results demonstrate a promising way to predict and design battery electrode structures by combining X-ray tomography and DEM analysis.
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•DEM with bond model is developed to understand the electrode calendering process.•For the first time, electrode structure from X-ray tomography was used in DEM.•Structural evolutions, mechanical stress and transport properties were analysed.•This work provides more confidence for modelling of electrode structural evolution.
•A new calculation equation for normal contact stiffness is proposed.•The value of the coefficient in the new equation is proposed and the robustness of this value is verified.•The influence of ...different normal contact stiffness on uniaxial compression and tunnel excavation simulations are studied.
The combined finite-discrete element method (FDEM) has been widely used in numerical studies in the fields of rock mechanics and geotechnical engineering. The normal contact stiffness between triangular elements is an important influencing parameter, but there is currently no effective method of measuring it. First, an equation for normal contact stiffness is proposed (Pb = αPf, where Pb is the basic stiffness, α is the coefficient, and Pf is the joint penalty; the specific stiffness of each contact couple is determined by Pb and the geometric sizes of the triangular elements together); then, the compression-shear failure of a single joint element is used to study the value range and robustness of α; finally, uniaxial compression and tunnel excavation simulations are used to study the influence of different α values on rock crack propagation. The study results show that (1) an α value of 0.1448 is optimal and robust for a single joint element simulation and is therefore suitable for all simulation conditions; in other words, the value of α should not be too low to avoid decreasing the rock mass stiffness of existing natural fractures and deviating from the actual situation; in addition, the value of α should also not be too large to avoid the development of additional cracks, especially for hard rock simulation, in which the value of α is more sensitive; and (2) the crack topologies of the surrounding rock obtained by tunnel excavation simulation with a large α value deviate from the actual rock core (i.e., when α = 0.1448, the simulation results coincide well with the actual fractures). Through this study, the reliability of FDEM numerical simulation results is improved.
The discrete element method (DEM) is used across a wide range of applications. However, accurate predictions can only be made if the input parameters are carefully selected. In this paper a ...calibration process is proposed to calibrate the parameter values for crushed rock particles up to 40 mm in size. A large shear box with a diameter of 590 mm was designed and built for this purpose. Confined compression tests were used to determine the particle stiffness and direct shear tests to determine the particle–particle friction coefficient. Two methods were used to create the clump particles: a manual process and an optimised process to create clumps comprising of 2, 4 or 8 spheres. The clump types were individually calibrated to obtain a unique set of parameter values for each. The angle of repose is often used for the calibration of the particle–particle friction coefficient and was included in the calibration process for comparison with the direct shear test results. The calibration process was validated by modelling anchor pull-out tests and hopper discharge using the different clump types. The results showed that care should be taken when the angle of repose is used to calibrate the particle–particle friction coefficient. It can result in a friction value which is too low for use in other applications, although the angle of repose is accurately predicted.
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•A calibration procedure using a large shear box is proposed.•All clump shapes (number of spheres per clump) performed equally well.•Direct shear test is more reliable than the angle of repose to calibrate friction.•Anchor pull-out and hopper discharge can be successfully modelled.
Summary
In this work, we propose a novel hydraulic solver in order to simulate key mechanisms that control fluid‐driven cracks in the framework of the combined finite‐discrete element method (FDEM). ...The main innovative aspect of the present work is the independence of the fluid's critical time step size with the fracture opening. This advantage is extremely important because it means that very fine meshes can be used around areas of interest, such as boreholes, without penalizing the computational cost as fractures propagate (ie, open) and the fluid flows through them. This is a great advantage over other recently introduced approaches that exhibit a dependency of the time step in the form of Δtcrit ∝ (l/a)2 where l is the element size and a is the fracture aperture. This paper presents a series of benchmark cases for the proposed solver. The rationale adopted by the authors was to benchmark and validate the implementation of the hydraulic solver in an incremental fashion, starting from the simplest cases and building in complexity. The results shown in this work clearly demonstrate that the proposed approach is able to reproduce analytical results for fluid flow through a single crack. The results presented in this paper also demonstrate that the new approach is robust enough to deal with complex fracture patterns and complex geometries; the obtained fluid‐driven fracture patterns in the vicinity of a borehole certainly stand to the scrutiny of human visual perception.
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