• Geomechanics and Engineering
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• v.23 no.5
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• pp.455-466
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• 2020

This paper presents analysis of the interaction between tunnel and Qanat with a particular interest for the optimization of Qanat shape using the discrete element code, PFC2D, and the results will be compared with the FEM results of PLAXIS2D. For these concerns, using software PFC2D based on Discrete Element Method (DEM), a model with dimension of 100m ^⁎ 100 m was prepared. A circular tunnel with dimension of 9 m was situated 20 m below the ground surface. Also one Qanat was situated perpendicularly above the tunnel roof. Distance between Qanat center and ground surface was 8 m. Five different shapes for Qanat were selected i.e., square, semi-circular, vertical ellipse, circular and horizontal ellipse. Confining pressure of 5 MPa was applied to the model. The vertical displacement of balls situated in ground surface was picked up to measure the ground subsidence. Also two measuring circles were situated at the tunnel roof and at the Qanat roof to check the vertical displacements. The properties of the alluvial soil of Tehran city are: γ_dry=19 (KN/㎥), E= 750 (kg/㎠), ν=0.35, c=0.3(kg/㎠), φ=34°. In order to validate the DEM results, a comparison between the numerical results (obtained in this study) and analytical and field monitoring have been done. The PFC2D results are compared with the FEM results. The results shows that when Qanat has rectangular shape, the tensile stress concentration at the Qanat corners has maximum value while it has minimum value for vertical ellipse shape. The ground subsidence for Qanat rectangular shape has maximum value while it has minimum value for ellipse shape of Qanat. The vertical displacements at the tunnel roof for Qanat rectangular shape has maximum value while it has minimum value for ellipse shape of Qanat. Historical shape of Qante approved the finding of this research.

• Korean Journal of Materials Research
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• v.26 no.11
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• pp.611-622
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• 2016

Particle morphology change and different experimental condition analysis during composite fabrication process by traditional ball milling with discrete element method (DEM) simulation were investigated. A simulation of the three dimensional motion of balls in a traditional ball mill for research on the grinding mechanism was carried out by DEM simulation. We studied the motion of the balls, the ball behavior energy and velocity; the forces acting on the balls were calculated using traditional ball milling as simulated by DEM. The effect of the operational variables such as the rotational speed, ball material and size on the flow velocity, collision force and total impact energy were analyzed. The results showed that increased rotation speed with interaction impact energy between balls and balls, balls and pots and walls and balls. The rotation speed increases with an increase of the impact energy. Experiments were conducted to quantify the grinding performance under the same conditions. Furthermore, the results showed that ball motion affects the particle morphology, which changed from irregular type to plate type with increasing rotation speed. The evolution was also found to depend on the impact energy increase of the grinding media. These findings are useful to understand and optimize the particle motion and grinding behavior of traditional ball mills.

• Resources Recycling
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• v.27 no.3
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• pp.86-92
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• 2018

The amount of construction waste generated is steadily increasing every year, and the Law for Promotion of Recycling is enacted. However, it is difficult to use it as a recycled aggregate for concrete, which is presented in the quality standard of recycled aggregate with high water uptake and low density due to low separation of aggregate between concrete and cement paste. Therefore, in this study, a multi-stage impact crusher was used to remove mortar, which is essential for improving the quality of recycled aggregate. In analyzing the characteristics of the equipment, the spectrum of energy generated in each part between the particle and the equipment was calculated by using DEM. In order to generate an effective separation phenomenon, it was confirmed that the operation condition of 900 RPM was appropriate based on the ratio of the number of collisions (L/H) of the low energy group (L) to the number of collisions of the high energy group (H).

• International Journal of Highway Engineering
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• v.20 no.1
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• pp.9-18
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• 2018

PURPOSES : In this research, the initiation and development of corrugation on a gravel road with certain wheel and boundary conditions were evaluated using a coupled discrete-element method (DEM) with multibody dynamics (MBD). METHODS : In this study, 665,534 particles with a 4-mm diameter were generated and compacted to build a circular roadbed track, with a depth and width of 42 mm and 50 mm, respectively. A single wheel with a 100-mm diameter, 40-mm width, and 0.157-kg mass was considered for the track. The single wheel was set to run slowly on the track with a speed of 2.5 rad/s so that the corrugation was gradually initiated and developed without losing contact between the wheel and the roadbed. Then, the shape of the track surface was monitored, and the movement of the particles in the roadbed was tracked at certain wheel-pass numbers to evaluate the overall corrugation initiation and development mechanism. RESULTS : Two types of corrugation, long wave-length and short wave-length, were observed in the circular track. It seems that the long wave-length corrugation was developed by the longitudinal movement of surface particles in the entire track, while the short wave-length corrugation was developed by shear deformation in a local section. Properties such as particle coefficients, track bulk density, and wheel mass, have significant effects on the initiation and development of long-wave corrugation. CONCLUSIONS : It was concluded that the coupled numerical method applied in this research could be effectively used to simulate the corrugation of a gravel road and to understand the mechanism that initiates and develops corrugation. To derive a comprehensive conclusion for the corrugation development under various conditions, the driver's acceleration and deceleration with various particle gradations and wheel-configuration models should be considered in the simulation.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.38 no.1
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• pp.17-23
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• 2014

Graphics processing units (GPUs) are ideal for solving problems involving parallel data computations. In this study, the GPU is used for effectively carrying out a multi-body dynamic simulation with particle dynamics. The Hilber-Hushes-Taylor (HHT) implicit integration algorithm is used to solve the integral equations. For detecting collisions among particles, the spatial subdivision algorithm and discrete-element methods (DEM) are employed. The developed program is verified by comparing its results with those of ADAMS. The numerical efficiencies of the serial program using the CPU and the parallel program using the GPU are compared in terms of the number of particles, and it is observed that when the number of particles is greater, more computing time is saved by using the GPU. In the present example, when the number of particles is 1,300, the computational speed of the parallel analysis program is about 5 times faster than that of the serial analysis program.

• Journal of the Korea institute for structural maintenance and inspection
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• v.15 no.5
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• pp.169-177
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• 2011

In this study, an impact resistance of concrete segmented composites adopted shell's structures which have the excellent impact resistance was assessed. In order to enhance the performances of concrete segmented composite, the bond strength of mortar between the concrete blocks should be improved. Hence, in this study polymer mortars were applied to increase the bond strength of mortar. From the results of bond tests, the 15% latex mortar was selected and static and low-velocity impact tests were carried out for the specimens applied the plain and latex mortar. The concrete segmented composites, of which the bond strength of mortar was enhanced, showed improved low-velocity impact resistances. A Nonlinear finite element analysis using the discrete crack model showed similar energy dissipating capacities to the impact test's results. Consequently, by improving the analysis models for segmented composites, the impact resistances for manifold variables can be predicted and assessed.

• Nuclear Engineering and Technology
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• v.46 no.6
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• pp.783-796
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• 2014

The coarse mesh finite difference (CMFD) method is applied to the discontinuous finite element method based discrete ordinate calculation for source convergence acceleration. The three-dimensional (3-D) DFEM-Sn code FEDONA is developed for general geometry applications as a framework for the CMFD implementation. Detailed methods for applying the CMFD acceleration are established, such as the method to acquire the coarse mesh flux and current by combining unstructured tetrahedron elements to rectangular coarse mesh geometry, and the alternating calculation method to exchange the updated flux information between the CMFD and DFEM-Sn. The partial current based CMFD (p-CMFD) is also implemented for comparison of the acceleration performance. The modified p-CMFD method is proposed to correct the weakness of the original p-CMFD formulation. The performance of CMFD acceleration is examined first for simple two-dimensional multigroup problems to investigate the effect of the problem and coarse mesh sizes. It is shown that smaller coarse meshes are more effective in the CMFD acceleration and the modified p-CMFD has similar effectiveness as the standard CMFD. The effectiveness of CMFD acceleration is then assessed for three-dimensional benchmark problems such as the IAEA (International Atomic Energy Agency) and C5G7MOX problems. It is demonstrated that a sufficiently converged solution is obtained within 7 outer iterations which would require 175 iterations with the normal DFEM-Sn calculations for the IAEA problem. It is claimed that the CMFD accelerated DFEM-Sn method can be effectively used in the practical eigenvalue calculations involving general geometries.

• Journal of the Earthquake Engineering Society of Korea
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• v.9 no.2 s.42
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• pp.47-58
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• 2005

Dynamic response analysis in time dimain is conducted for floating bridges with discrete pontoons subject to spatial variation of ground motions. The Spatial variation of ground motions is considered with the coherency function model which represents wave passage, incoherence and local site effects. The superstructure of the bridge is represented by space frame and elastic catenary cable elements, the abutment us modelde with the spring element of FHWA guideline for considering soil structure interaction and the concept of retardation function is utilized to consider the frequency dependency of the hydrodynamic coefficients which are obtainde by boundary element method. multiple support excitations considering the spatial variation. The noticeable amplification of the response can be shown when the spatial variation of ground motions is incorporated in the anallysis of floating bridges.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.30 no.4
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• pp.363-370
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• 2017

This paper is concerned with the evaluation of mixing performance of a particle mixer, which consists of a vertical cylindrical vessel and a rotating impeller with several blades. We consider four design variables for the mixer blades, such as the angle, length, and number of blades, and the gap between the blades and the vessel bottom. The particle mixing process due to the impeller rotation is simulated using the discrete element method, and the mixing performance is quantitatively evaluated by introducing a mixing index. Analyzing the main effects and interactions of the four design variables through the design-of-experiments approach, it is concluded that the blade angle has the most dominant influence on the mixing performance whereas the gap has no significant influence. In addition, we determine the best combination of design parameters to maximize the mixing performance.

• Structural Engineering and Mechanics
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• v.66 no.3
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• pp.343-351
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• 2018

Brazilian disc test is one of the most widely used experiments in the literature of geo-mechanics. In this work, the pre-holed concrete Brazilian disc specimens are numerically modelled by a two-dimensional discrete element approach. The cracks initiations, propagations and coalescences in the numerically simulated Brazilian discs (each containing a single cylindrical hole and or multiple holes) are studied. The pre-holed Brazilian discs are numerically tested under Brazilian test conditions. The single-holed Brazilian discs with different ratios of the diameter of the holes to that of the disc radius are modelled first. The breakage load in the ring type disc specimens containing an internal hole with varying diameters is measured and the crack propagation mechanism around the wall of the ring is investigated. The crack propagation and coalescence mechanisms are also studied for the case of multi-holes' concrete Brazilian discs. The numerical and experimental results show that the breaking mechanism of the pre-holed disc specimens is mainly due to the initiation of the radially induced tensile cracks which are growth from the surface of the central hole. Radially cracks propagated toward the direction of diametrical loading. It has been observed that for the case of disc specimens with multiple holes under diametrical compressive loading, the breaking process of the modelled specimens may occur due to the simultaneous cracks propagation and cracks coalescence phenomena. These results also show that as the hole diameter and the number of the holes increases both the failure stress and the crack initiation stress decreases. The experimental results already exist in the literature are quit agree with the proposed numerical simulation results which validates this simulation procedure.