Comparisons of ANN prediction of collision energy and particle size with DEM simulations.
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•An ANN model was proposed to predict particle flow characteristics in rotating drums.•The ...model was based on the acoustic emission signals generated from DEM simulations.•The key features of the signals were obtained through principal component analysis.•Flow properties included filling level, particle size and energy distributions.•ANN predictions compared well with DEM simulations.
Rotating drums are widely used in industries for particle mixing, granulation and grinding. Linking internal particle flow condition with externally measured variables is crucial to online process monitoring and control. This work proposed a modelling framework to use an artificial neural network (ANN) model for quick prediction of particle flow based on the acoustic emission (AE) signals generated from the discrete element method (DEM) simulations. In total 131 DEM simulations were conducted under different conditions (i.e., different particle size distributions and filling levels). The AE signals on the drum surface were then obtained based on the simulated particle–wall collisions. Through FFT transformation and principal component analysis (PCA), 5 principal components (PCs) were obtained and, together with power draw, fed into the ANN model to predict to the unmeasurable internal flow conditions, including filling level and the distributions of particle size and internal collision energy. The back propagation neural network was adopted in the model. After being trained with 90 datasets, the ANN model was able to predict those internal variables with reasonable accuracy (R2 > 0.95). Finally, the potentials and limitations of the model to the optimal operation of drums were discussed.
The discrete element method (DEM) has been confirmed as an effective numerical method for modelling railway ballast, and successfully used to analyse a wide range of ballast-related applications ...(e.g. geomaterials). However, there still exists some aspects under development. Among them, the model calibration can be the most significant one (morphology, degradation and contact model). Because reliable and accurate results can be obtained only when the parameters are carefully selected. Therefore, diverse DEM applications and developments in railway ballast are critically reviewed. Furthermore, the model calibration methods are discussed. This is able to help future researchers improve the existing calibration methods, further, build more accurate, standardised and validated DEM models for ballast-related studies. Additionally, this paper can assist researchers to choose an appropriate model for specific applications.
One controversial issue associated with enhanced geothermal systems is induced seismicity, which limits their broader application. As induced seismicity has a strong correlation with the activation ...of pre-existing faults, understanding processes controlling the potential fault activation during geothermal heat extraction is of critical importance. In this study, a coupled thermal–hydraulic–mechanical (THM) scheme is formulated based on the combined finite–discrete-element method to investigate the triggering mechanisms of potential fault activation during geothermal heat extraction. The developed thermal and hydraulic solving frameworks account for the heat transfer of various kinds and fracture fluid migration in the rock mass, respectively. The fully coupled THM scheme is established by pairwise coupling between thermal, hydraulic, and mechanical solving frameworks, and is then progressively verified against analytical solutions by five validation examples. Finally, cold-fluid injection-induced potential activation of a fault during geothermal heat extraction is investigated. The results demonstrate that the gradual aseismic opening and slip of the fault at low injection pressure are caused by the thermal contraction-induced reduction of the imposed normal stress on the fault surface due to convective cooling. The thermally induced aseismic slip weakens the fault and contributes to triggering sudden (unstable) fault slips. The convective thermal transfer coefficient is found to greatly affect the onset time of the fault aseismic slip. The effects of the injection pressure and temperature as well as the in-situ stress field are also discussed. The findings justify thermal effects in controlling the potential fault slip behavior and illuminate the triggering mechanism of unexpected seismic activities during geothermal heat extraction even at injection pressure below the safety threshold value.
Highlights
A fully coupled thermal–hydraulic–mechanical (THM) scheme is formulated based on the combined finite–discrete-element method (FDEM) to investigate the potential fault activation during geothermal heat extraction.
Fault aseismic slip caused by thermal contraction due to convective cooling at low injection pressure contributes to triggering unstable fault slip.
The convective thermal transfer coefficient greatly affects the onset time of the fault aseismic slip.
The injection pressure affects the time at which the fault aseismic slip becomes stable and the in-situ stress heavily affects the aseismic slip amount.
Thermal effects are justified in triggering seismic activities during geothermal heat extraction at injection pressure below the safety threshold.
The angle of repose-i.e., the angle Formula: see text between the sloping side of a heap of particles and the horizontal-provides one of the most important observables characterizing the packing and ...flowability of a granular material. However, this angle is determined by still poorly understood particle-scale processes, as the interactions between particles in the heap cause resistance to roll and slide under the action of gravity. A theoretical expression that predicts Formula: see text as a function of particle size and gravity would have impact in the engineering, environmental, and planetary sciences. Here we present such an expression, which we have derived from particle-based numerical simulations that account for both sliding and rolling resistance, as well as for nonbonded attractive particle-particle interactions (van der Waals). Our expression is simple and reproduces the angle of repose of experimental conical heaps as a function of particle size, as well as Formula: see text obtained from our simulations with gravity from 0.06 to 100 times that of Earth. Furthermore, we find that heaps undergo a transition from conical to irregular shape when the cohesive to gravitational force ratio exceeds a critical value, thus providing a proxy for particle-scale interactions from heap morphology.
Voidage (porosity or void fraction) in packed particles (or pebbles) is of fundamental importance in calculating the pressure drop, obtaining the drag, predicting the bed permeability, estimating the ...neutron streaming, etc. For the case when particles are deformed, a method of voidage correction during the packing state is proposed using a Discrete Element Method (DEM) simulation of 3D pebble flow inside a bed of cycloidal base. A function to evaluate the remaining volume of a pebble intercepted by horizontal and vertical planes is proposed for voidage calculation. After that, the process of solving voidage distribution is provided in detail. Using this method, the voidage inside the cycloidal-base pebble bed is obtained to refer to reported similar data for validation. This method can be potentially used for dynamical voidage calculation in CFD-DEM simulation which can get suitable voidage distribution after the correction.
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•A method of voidage correction for DEM-based simulation is proposed.•An analytic function to evaluate pebble volume intercepted by cells is proposed.•A method to correct the radius of particles with severe deformation is presented.•Voidage inside the cycloidal-base pebble bed is obtained with validation.•The method is potentially useful for dynamical voidage calculation in CFD-DEM.
•A novel joint element calibration procedure is proposed for FDEM.•A fracture energy calculation method with different element sizes is proposed to realize cross-scale simulation.•More accurate ...results can be obtained for tunnel excavation simulation using the proposed joint element calibration technique.•More accurate values of the rock fracture energies GI and GII can be calibrated by the modified Brazilian disc model and variable-angle shear model.
The combined finite–discrete element method (FDEM) has been widely used to simulate the fracture process of rock materials. The FDEM simulation requires many material parameters which should be calibrated through laboratory tests or directly cited from previous conclusions. However, it is difficult to obtain the joint element parameters, including the joint element penalty Pf and the type I and type II fracture energies GI and GII. Therefore, a novel calibration method for the above three parameters must be proposed. First, Pf can be obtained by uniaxial compression simulation, and Pf = 30E (E is Young’s modulus) is applicable for homogeneous and isotropic rock materials. Second, an accurate GI value in the static loading state can be obtained by Brazilian disc simulation with a central preprepared failure path. Third, an accurate GII value can be calibrated by the modified variable-angle shear model. Finally, the GI and GII values with different element sizes can be calculated by the proposed linear equations. In addition, normal uniaxial and triaxial compression simulations can be used to verify the reliability of the calibrated GI and GII values. Moreover, the tunnel excavation simulation results show that the joint element parameters calibrated by the modified method can achieve cross-scale simulation, and the simulation results are more accurate.
Optimizing particle gradations to improve the powder bed quality is of practical engineering interest for powder spreading in additive manufacturing (AM). The discrete element model of ceramic powder ...is introduced to simulate blade-spreading and roller-spreading processes. Based on the packing theory, the effect of particle gradations on the powder bed quality and particle microscopic behavior is analyzed. The results show that fine particle gradations weaken the wall effect. The powder shear dilation in blade-spreading strengthens the loosening effect, however, the compaction effect of roller-spreading weakens the loosening effect. As coarse particle gradations increase, contact force chains become loose, however, strong force arches lead to particle jamming, uneven distribution, and voids in the powder bed. Particle segregation occurs because fine particle gradations pass through the gap between the substrate and spreader, and are deposited at the front part of the powder bed, while coarse particle gradations due to jamming are deposited at the end part of the powder bed. When the particle gradation coefficient is in the range of 0.1 to 0.5, the powder bed quality and particle segregation phenomenon are significantly improved compared to Gaussian distribution. The results can provide valuable references for the selection of particle gradations in AM.
The discrete element method (DEM) micro parameter calibration has been a longstanding problem since the DEM was created. To date, the low-precision and time-consuming calibration procedures still ...pose difficulties for DEM applications. This study proposed an optimized differential evolution calibration method (OpDEC) to calibrate cohesive granular DEM material to the target macro mechanical properties. Macro parameter Young’s modulus, Poisson’s ratio, uniaxial compressive strength, and direct tensile strength can be calibrated to less than 5% weighted relative error within 5 h or less than 1% weighted relative error within 12.5 h. For this purpose, 180 calibrations were carried out to optimize the mutation strategy and control parameters of the differential evolution algorithm. A calibration evolutionary health monitoring scheme was devised to detect the possible ill calibrations in early time. The algorithm robustness was verified by 50 calibrations of 5 types of rock. Moreover, a laboratory-tested stress–strain curve of Äspö diorite was compared with 10 calibrated DEM models that showed a good agreement in terms of axial behaviour. The OpDEC has a great potential to serve as a fast and easy-to-implement method to calibrate the cohesive granular DEM material.
Tribocharger design optimisations presented in the literature are based typically on experimental investigations. While this approach is useful and necessary to evaluate the performance of a design, ...experimental investigations are limited to studying a finite matrix of parameters. Computational approaches, such as the discrete element method (DEM), offer greater flexibility, however they have not been used previously for tribocharger design optimisation. This work presents a novel approach using the DEM to study the effect of different tribocharger designs on the charging process using particle–wall and particle–particle contact areas as proxies for charge transfer. The bulk sample charge output from the model are compared with bulk charges measured experimentally, showing good agreement. Furthermore, a method to predict approximately the charging behaviour of complex mixtures from linear combinations of the simulation outputs of single species, single size particle samples is presented, demonstrating good agreement.
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•Computational approaches provide scope to investigate design optimisation problems.•The DEM is applied in a novel manner to optimise the design of a tribocharging device.•A 3D DEM model is used to predict the performance of an optimal design.•An approach to predict tribocharger performance from limited inputs is provided.
•New numerical model for the analysis of masonry structures is presented.•Numerical model is based on combined Finite–Discrete Element Method (FEM/DEM).•The model is aimed for simulating the entire ...failure mechanism of masonry structures.•The comparison of experimental and numerical results shows good agreement.
This paper presents a new numerical model based on the combined finite–discrete element method for the analysis and prediction of the collapse of masonry structures. The model consists of a new numerical model in a finite element and a new material model in an interface element which simulate the behaviour of the mortar joints and unit-mortar interface. The comparison between the numerical results obtained by this model and the available numerical and experimental results shows high accuracy in predicting the behaviour of the masonry structure through the entire failure mechanism from the continuum to the discontinuum.
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