• International Journal of Aeronautical and Space Sciences
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• 제18권4호
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• pp.601-613
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• 2017

An adjoint-based error estimation and mesh adaptation study is conducted for two-dimensional viscous flows on unstructured hybrid meshes. The error in an integral output functional of interest is estimated by a dot product of the residual vector and adjoint variable vector. Regions for the mesh to be adapted are selected based on the amount of local error at each nodal point. Triangular cells in the adaptive regions are refined by regular refinement, and quadrangular cells near viscous walls are bisected accordingly. The present procedure is applied to single-element airfoils such as the RAE2822 at a transonic regime and a diamond-shaped airfoil at a supersonic regime. Then the 30P30N multi-element airfoil at a low subsonic regime with a high incidence angle (${\alpha}=21deg.$) is analyzed. The same level of prediction accuracy for lift and drag is achieved with much less mesh points than the uniform mesh refinement approach. The detailed procedure of the adjoint-based mesh refinement for the multi-element airfoil case show that the basic flow features around the airfoil should be resolved so that the adjoint method can accurately estimate an output error.

• 한국CDE학회논문집
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• 제5권4호
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• pp.347-358
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• 2000

The view dependency of typical spatial-partitioning based NC simulation methods is overcome by polygon rendering technique that generates polygons to represent the workpiece, thus enabling dynamic viewing transformations without reconstruction of the entire data structure. However, the polygon rendering technique still has difficulty in realizing real-time simulation due to unsatisfactory performance of current graphics devices. Therefore, it is necessary to develop a mesh decimation method that enables rapid rendering without loss of display quality. In this paper. we proposed a new mesh decimation algorithm thor a workpiece whose shape varies dynamically. In this algorithm, the 2-map data thor a given workpiece is divided into several regions, and a triangular mesh is constructed for each region first. Then, if any region it cut by the tool, its mesh is regenerated and decimated again. Since the range of mesh decimation is confined to a few regions, the reduced polygons for rendering can be obtained rapidly. Our method enables the polygon-rendering based NC simulation to be applied to the computers equipped with a wider range of graphics cards.

• 대한기계학회논문집A
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• 제25권6호
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• pp.1004-1014
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• 2001

The fully automatic algorithm from initial finite element mesh generation to remeshing in two dimensional geometry is introduced using bubble packing method (BPM) for finite element analysis. BPM determines the node placement by force-balancing configuration of bubbles and the triangular meshes are made by Delaunay triangulation with advancing front concept. In BPM, we suggest two node-search algorithms and the adaptive/recursive bubble controls to search the optimal nodal position. To use the automatically generated mesh information in FEA, the new enhanced bandwidth minimization scheme with high efficiency in CPU time is developed. In the remeshing stage, the mesh refinement is incorporated by the control of bubble size using two parameters. And Superconvergent Patch Recovery (SPR) technique is used for error estimation. To verify the capability of this algorithm, we consider two elasticity problems, one is the bending problem of short cantilever beam and the tension problem of infinite plate with hole. The numerical results indicate that the algorithm by BPM is able to refine the mesh based on a posteriori error and control the mesh size easily by two parameters.

• 국제초고층학회논문집
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• 제8권4호
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• pp.255-264
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• 2019

This paper describes large eddy simulation of wind pressures on a square cylinder in a uniform flow and a high-rise building immersed in an atmospheric turbulent boundary layer. For the atmospheric boundary layer case, the inflow turbulence is generated by a numerical wind tunnel. In the numerical simulation, particular attention is devoted to the performance of an auto hexahedral non-structural mesh. Both simulations are performed for three grid systems: an auto hexahedral non-structured grid, a structured Cartesian grid and a non-structured triangular prism grid, and for three grid numbers. The present study shows that the auto hexahedral unstructured mesh achieves the best simulation results for wind pressures on the square cylinder and the high-rise building. When the grid number is sufficiently large, the differences among the results obtained from the three investigated grid systems are not significant. However, the advantage of the auto hexahedral unstructured mesh becomes clear when the grid number decreases, because it enables a balanced distribution of orthogonal grids. The results described in this paper demonstrate that the auto hexahedral non-structured mesh has good potential applicability to simulation of urban flows.

• 한국항공우주학회지
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• 제33권8호
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• pp.1-7
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• 2005

본 연구에서는 삼각격자 균질화를 위하여, 확장된 위상학적 개선과정과 국소 최적화 기반 평활화를 결합한 국소 개선기법을 제안하였다. 먼저 격자의 연결 구조를 확장된 위상학적 개선과정을 적용하여 최적의 연결구조로 개선한다. 다음으로 격자의 질을 나타내는 비틀림척도를 최대화하기 위해 국소 최적화 기반 평활화를 수행한다. 이 국소 개선기법을 이용하여, 두 가지 격자 예제에 대하여 삼각격자 균질화를 수행하였다. 이 예들을 통하여, 본 연구에서 제안한 국소 개선알고리듬이 삼각격자의 질을 크게 향상시켜주는 경제적이며 효과적인 방법임을 보여준다. 또한, 이 기법은 적응격자 세분화의 격자 재생성과정에도 용이하게 적용될 수 있다.

• 한국전산유체공학회지
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• 제12권4호
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• pp.68-73
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• 2007

Computational Fluid Dynamics (CFD) approach is now playing an important role in the engineering process in these days. Generating proper grid system in time for the region of interest is prerequisite for the efficient numerical calculation of flow physics using CFD approach. Grid generation is, however, usually considered as a major obstacle for a routine and successful application of numerical approaches in the engineering process. CFD approach based on the unstructured grid system is gaining popularity due to its simplicity and efficiency for generating grid system compared to the structured grid approaches, especially for complex geometries. In this paper an automated triangular surface grid generation using CAD(Computer Aided Design) surface data is proposed. According to the present method, the CAD surface data imported in the STL(Stereo-lithography) format is processed to identify feature edges defining the topology and geometry of the surface shape first. When the feature edges are identified, node points along the edges are distributed. The initial fronts which connect those feature edge nodes are constructed and then they are advanced along the CAD surface data inward until the surface is fully covered by triangular surface grid cells using Advancing Front Method. It is found that this approach can be implemented in an automated way successfully saving man-hours and reducing human-errors in generating triangular surface grid system.

• 한국전자파학회논문지
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• 제14권1호
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• pp.27-32
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• 2003

본 논문에서는 수 만개 이상의 미지수를 필요로 하는 복잡한 3차원 구조에서의 정전용량 추출을 위한 고속화 알고리즘(Fast mutilpole method)과 결합한 효과적인 적응 삼각요소 분할법(Adaptive triangular mesh refinement algorithm)을 제안하였다. 적응 삼각요소 분할법은 3차원 물체의 표면을 초기요소로 분할하여 전하의 분포를 구하고, 전하밀도가 높은 영역에서의 요소세분화를 수행하여 이루어진다. 제안된 방법을 이용하여 많은 미지수를 필요로 하는 68-pin cerquad package구조에서의 정전용량을 추출하였다.

• 한국컴퓨터그래픽스학회논문지
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• 제21권1호
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• pp.13-21
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• 2015

본 논문에서는 삼각형 메시의 스웹 볼륨 표면을 정밀하고 안정적으로 계산하는 GPU 알고리즘을 제안한다. 삼각형 메시의 기하 요소들을 스웹하여 근사적으로 삼각형 집합을 생성하고, 이 집합의 엔벨롭을 계산하면 스웹 볼륨의 최외곽 경계 표면을 얻을 수 있다. 엔벨롭을 찾기 위하여 우리는 삼각형 집합의 공간 분할을 계산하고 그 분할의 최외곽 경계를 추출하였다. 알고리즘의 안정성을 확보하기 위하여 우리는 스웹 정점들을 초기에 랜덤 섭동하는 방법과 다중 정밀도 구간 연산 기법을 적용하였다. 전체 알고리즘은 대부분의 계산을 GPU에서 처리하도록 구현되었고, 결과적으로 기존 알고리즘에 비해 수십~수백 배의 성능을 보여준다.

• 한국전자파학회논문지
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• 제13권1호
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• pp.80-86
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• 2002

본 논문에서는 유전체층을 가진 3차원 도체의 정전용량 계산을 위한 적응요소 MLFMA을 제안하였다. 고속화 알고리즘인 MLFMA와 더불어 높은 전하밀도를 가지는 영역에서 삼자요소를 세분화함으로써 효율적으로 요소를 분할하게 된다. 제안된 기법은 유전체 층위의 3차원 도체의 정전용량을 계산하는데 적용되어 균일한 요소에 의한 계산결과의 수렴보다 빠른 결과를 얻을 수 있었다. 또한, 3차원의 복잡한 구조를 가진 문제의 해석에 있어서도 과도한 추가계산시간을 요구하지 않으므로, 제안된 방법을 MLFMA의 장점에 부합되는 적절한 기법이라 할 수 있다.

• Nuclear Engineering and Technology
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• 제55권6호
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• pp.2172-2194
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• 2023

A variational nodal method (VNM) with unstructured-mesh is presented for solving steady-state and dynamic neutron diffusion equations. Orthogonal polynomials are employed for spatial discretization, and the stiffness confinement method (SCM) is implemented for temporal discretization. Coordinate transformation relations are derived to map unstructured triangular nodes to a standard node. Methods for constructing triangular prism space trial functions and identifying unique nodes are elaborated. Additionally, the partitioned matrix (PM) and generalized partitioned matrix (GPM) methods are proposed to accelerate the within-group and power iterations. Neutron diffusion problems with different fuel assembly geometries validate the method. With less than 5 pcm eigenvalue (k_eff) error and 1% relative power error, the accuracy is comparable to reference methods. In addition, a test case based on the kilowatt heat pipe reactor, KRUSTY, is created, simulated, and evaluated to illustrate the method's precision and geometrical flexibility. The Dodds problem with a step transient perturbation proves that the SCM allows for sufficiently accurate power predictions even with a large time-step of approximately 0.1 s. In addition, combining the PM and GPM results in a speedup ratio of 2-3.