• Proceedings of the Computational Structural Engineering Institute Conference
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• 2011.04a
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• pp.756-759
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• 2011

일반적으로 컴퓨터를 이용한 수치 해석에는 격자 수치 해석 방법인 유한요소법 또는 유한차분법이 주로 사용되어 왔다. 그러나 이러한 방법들은 해석하고자 하는 영역을 요소나 격자 등으로 분할해야 하기 때문에 복잡한 현상들을 다루는 데 어려움을 갖게 된다. 이를 극복하기 위해 개발된 방법이 무요소법(Meshfree Method)이며 본 논문에서는 다양한 무요소법들 중 SPH(Smoothed Particle Hydrodynamics)가 고려되어진다. SPH는 라그랑지안 수치 근사 기법을 사용하는 입자법(Particle Method)으로 SPH를 정확하게 실행하기 위해서는 적절한 경계 처리법이 요구된다. 그러나 기존의 경계 처리법은 유체 입자의 침투현상 및 커널(Kernel) 끊김 현상이 발생하기 때문에 적합하지 않다. 따라서 지금까지 SPH의 경계 처리법을 향상시키기 위해 다양한 접근법들이 제안되었으며 본 논문에서는 이러한 접근법들 중 정반사(Specular Reflection), 재회복(Bounce-back), 재도입(Reintroduce) 방법 및 경계 반발력(Repulsive Force)과 가상 입자(Ghost Particle)의 적용이 분석되고 현상 접목을 통해 적절한 경계 처리법이 제안되어진다.

• Journal of Korean Tunnelling and Underground Space Association
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• v.14 no.4
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• pp.321-336
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• 2012

The shear behavior at the particle/surface interface such as rock joint can determine the mechanical behavior of whole structure. Therefore, a fundamental understanding of the mechanisms governing its behavior and accurately estimation of the interface strength is essential. In this paper, PFC, a numerical analysis program of discrete element method was used to investigate the effects of the surface roughness crushing on interface strength. The surface roughness was characterized by smooth, intermediate, and rough surface, respectively. Particle shape was classified into one ball model of circular shape and 3 ball model of triangular shape. The surface shape was modelled by wall model of non-crushing surface and ball model of crushing surface. The results showed that as the bonding strength of ball model decreases, lower interface strength is induced. After the surface roughness crushing was occurred, the interface strength tended to converge and higher bonding strength induced lower surface roughness crushing. Higher friction angle was induced in wall model and higher surface roughness induced the higher friction angle. From these findings, it is verified that the surface roughness and surface roughness crushing effect on the particle/surface interface shear behavior.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.27 no.1
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• pp.109-118
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• 2003

The recent trend towards miniaturization causes an increased demand for parts with very small dimensions. Milli-structure components are classified as a component group whose size is between macro- and micro-scale. The manufacturing process of these components of thin sheet metal forming has a microscopic properties in addition to a typical phenomenon of bulk deformation because of the forming size. Also, the material properties and the deformation behavior change with miniaturization, which means that, a coarse grained materials show a higher resistance against deformation, when the grain size is in the range of the sheet thickness. In this study, a new numerical approach is proposed to simulate intergranular milli-structure in forming by the finite element method. The grain element and grain boundary element are introduced to simulate the milli-structure in the bending. The grain element is used to analyze the deformation of individual grain while the grain boundary element is for the investigation on the movement of the grain boundary. Also, the result of the finite element analysis is confirmed by a series of milli-sized forming experiments.

• Journal of the Society of Naval Architects of Korea
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• v.39 no.4
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• pp.1-10
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• 2002

For the time-domain simulation of incipient breaking waves, usually the boundary integral method has been used so far, and it seems to be successful except a problem of too much computation time. The present paper shows a new computation technique for the simulation of breaking wave experiment. This technique uses the high-order spectral/boundary element method and the boundary integral method in sequence, and reduces the computation time remarkably. The wave generation and energy focusing process is efficiently simulated by the high-order spectral/boundary element method. Only the wave over-turning process is simulated by the boundary integral method. In the example calculation result, salient features of breaking waves such as high particle velocities and accelerations are shown.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.38 no.3
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• pp.275-282
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• 2014

Particle-reinforced metal matrix composites exhibit a strengthening effect due to the particle size-dependent length scale that arises from the strain gradient, and thus from the geometrically necessary dislocations between the particles and matrix that result from their CTE(Coefficient of Thermal Expansion) and elastic-plastic mismatches. In this study, the influence of the size-dependent length scale on the particle-matrix interface failure and ductile failure in the matrix was examined using finite-element punch zone modeling whereby an augmented strength was assigned around the particle. The failure behavior was observed by a parametric study, while varying the interface failure properties such as the interface strength and debonding energy with different particle sizes and volume fractions. It is shown that the two failure modes (interface failure and ductile failure in the matrix) interact with each other and are closely related to the particle size-dependent length scale; in other words, the composite with the smaller particles, which is surrounded by a denser dislocation than that with the larger particles, retards the initiation and growth of the interface and matrix failures, and also leads to a smaller amount of decrease in the flow stress during failure.

• Tunnel and Underground Space
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• v.31 no.6
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• pp.593-609
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• 2021

We proposed a numerical method for the thermo-mechanical behavior of rock fracture using a grain-based distinct element model (GBDEM) and simulated thermally induced fracture slip. The present study is the benchmark simulation performed as part of DECOVALEX-2023 Task G, which aims to develop a numerical method to estimate the coupled thermo-hydro-mechanical processes within the crystalline rock fracture network. We represented the rock sample as an assembly of Voronoi grains and calculated the interaction of the grains (blocks) and their interfaces (contacts) using a distinct element code, 3DEC. Based on an equivalent continuum approach, the micro-parameters of grains and contacts were determined to reproduce rock as an elastic material. Then, the behavior of the fracture embedded in the rock was characterized by the contacts with Coulomb shear strength and tensile strength. In the benchmark simulation, we quantitatively examined the effects of the boundary stress and thermal stress due to heat conduction on fracture behavior, focusing on the mechanism of thermally induced fracture slip. The simulation results showed that the developed numerical model reasonably reproduced the thermal expansion and thermal stress increment, the fracture stress and displacement and the effect of boundary condition. We expect the numerical model to be enhanced by continuing collaboration and interaction with other research teams of DECOVALEX-2023 Task G and validated in further study experiments.

• Journal of the Korean Geotechnical Society
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• v.24 no.2
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• pp.67-75
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• 2008

It is interesting to note that distinct element method has been used extensively to model the response of micro and discontinuous behavior in geomechanics. Impressive advances related to response of distinct particles have been conducted and there were difficulties in considering fluid effect simultaneously. Current distinct element methods are progressively developed to solve particle-fluid coupling focused on fluid flow through soil, rock or porous medium. In this research, numerical simulations of fluid injection into particulate materials were conducted to observe cavity initiation and propagation using distinct element method. After generation of initial particles and wall elements, confining stress was applied by servo-control method. The fluid scheme solves the continuity and Navior-Stokes equations numerically, then derives pressure and velocity vectors for fixed grid by considering the existence of particles within the fluid cell. Fluid was injected as 7-step into the assembly in the x-direction from the inlet located at the center of the left boundary under confining stress condition, $0.1MP{\alpha}\;and\;0.5MP{\alpha}$, respectively. For each simulation, movement of particles, flow rate, fluid velocity, pressure history, wall stress including cavity initiation and propagation by interaction of flulid-paricles were analyzed.

• Proceedings of the Korea Water Resources Association Conference
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• 2006.05a
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• pp.588-593
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• 2006

자연하천은 일반적으로 만곡수로나 사행수로 형태를 보이고 있으며, 직선수로에서와 달리 원심력에 기인한 이차류 영향을 받게 된다. 이차류에 의해서 수면에서는 만곡부 바깥쪽으로, 하상에서는 만곡부 안쪽으로의 흐름특성을 보이게 된다. 만곡부 안쪽으로 가해지는 하상 전단응력에 기인하여 하상에서의 입자가 만곡부 안쪽으로 이송되며, 결과적으로 만곡부 안쪽에는 점사주가, 바깥쪽에는 소(pool)가 생성된다. 또한 지형경사의 생성으로 입자에 가해지는 중력효과도 변화된다. 따라서 이와 같은 자연하천의 흐름과 하상변동을 수치모의 하기 위해서는 만곡부 이차류 특성을 고려한 모형이 필요하다. 본 연구에서는 수심 적분된 흐름방정식과 하상토 보존방정식 (Exner equation)을 이용한 하상변동을 위한 비연계 수치모형을 위해서 하상토 보존방정식의 유한요소 알고리즘을 개발하였다. 하상토 보존방정식은 흐름 특성에 따른 평형 유사량의 공간변화율을 이용하여 일정 기간 동안의 하상 변화량을 계산한다. 이 때 이차류에 의한 하상 전단응력의 편각 및 지형경사 변화에 따른 실제 입자의 이송방향을 보정하여 평형 유사량이 계산된다. 이러한 보정식을 적용시키기 위해서는 유속성분의 공간변화량 및 지형경사의 공간성분이 필요하다. 유한요소법은 연속성 변수를 이산화시켜 근사해를 구하는 수치기법의 일종이기 때문에, 요소망이 불규칙적으로 구성되었을 경우 임의의 절점에서 연속성을 지닌 변수의 공간변화율을 계산하는데 어려움이 있다. 따라서 본 연구에서는 평형 유사량 계산 시에 절점이 아닌 요소 내부에서 평형 유사량을 계산하는, 하상토 보존방정식의 새로운 유한요소 알고리즘을 개발하고, 새로운 알고리즘을 적용시킨 수치모형의 검증을 행하였다. 경계조건 알고리즘의 검증으로 위해서 Soni 등 (1980)이 행한 상류 유입 유사량에 따른 하상변동을 수치 모의하고 실험치와 비교하였으며, Sutmuller와 Glerum (1980)이 수행한 만곡수로에서의 하상변동을 모의하고 실험과 비교하였다. 새로운 알고리즘을 적용시킨 하상토 보존방정식의 유한요소 수치모형의 결과는 매우 안정적이며, 실험과 매우 유사한 결과를 얻을 수 있었다. 본 수치모델은 현재 균일한 입자의 하상토만을 고려하므로, 입자분급이나 하상 장갑화 현상 등은 무시한다.

• The Journal of Engineering Geology
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• v.19 no.2
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• pp.205-215
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• 2009

The conventional numerical models to analyze flow in subsurface porous media under the transient state usually generate numerical oscillation and unstability due to local flux domain for critical cases such as infiltration into initially dry soil during rainfall period. In this case, it is required refined mesh and small time step, but it decrease efficiency of computation. In this study, numerical unstability in discontinuity domain is removed by applying particle tracking algorithm to simulate unsteady subsurface flow with inflow boundary condition. Finally the hybrid LE FEM improving numerical stability is proposed. The hypothetical domains with unsteady uniform and nonuniform flow field were used to demonstrated algorithm verification. In comparison with analytic solution, we obtained reasonable results and conducted simulation of hypothetical 3-D recharge/pumping area. The proposed algorithm can simulate saturated/unsaturated porous media with more practical problems and will greatly contribute to accuracy and stability of numerical computation.

• Journal of the Korean Vacuum Society
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• v.8 no.1
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• pp.1-8
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• 1999

A boundary condition reflecting automatically, regardless of the direction of the gas flow, the pressure drop due to the entrance and exit effect occurred at a connection of two elements with different cross-sections in calculating the pressure profile of a vacuum network composed of many chambers and pipes which have different shape and vacuum properties was developed. The method of correcting the pressure drop in an element of varying cross-section like a cone was also introduced to keep the system free from directionality. The developed boundary conditions are applied as an example to a linear composed vacuum system to show how to make a set of simultaneous equations based on the particle balance equation and how to obtain its solution.