• 한국항공우주학회지
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• 제35권8호
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• pp.661-669
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• 2007

지면효과에 대한 수치해석을 위한 격자생성 과정에서 발생하는 어려움을 해소하기 위하여 중첩격자 기법을 이용한 수치해석 기법의 타당성에 대한 연구를 수행하고 그 신뢰성을 검증하였다. 지면효과가 있는 경우와 없는 경우 RAE 101 익형주위의 저속 난류 유동장에 대한 수치해석을 수행하였고 다중블록격자 기법과 중첩격자 기법을 적용한 수치해석 결과를 실험결과와 비교하였다. 이를 통하여 중첩격자 기법을 이용함으로써 지면효과에 대한 수치해석과정을 정확하고 격자생성 문제없이 효과적으로 수행할 수 있음을 보였다.

• International Journal of Naval Architecture and Ocean Engineering
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• 제11권1호
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• pp.294-306
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• 2019

On the basis of the Computational Fluid Dynamics technique (CFD) combined with the overlap grid method, this paper establishes a numerical simulation method to study the problem of ice-propeller interaction in viscous flow and carries out a simulation forecast of the hydrodynamic performance of an ice-class propeller and flow characteristics when in the proximity of milling-shape ice (i.e., an ice block with a groove cut by a high-speed revolving propeller). We use a trimmed mesh in the entire calculation domain and use the overlap grid method to transfer information between the domains of propeller rotation calculation and ice-surface computing. The grid is refined in the narrow gap between the ice and propeller to ensure the accuracy of the flow field. Comparison with the results of the experiment reveals that the error of the hydrodynamic performance is within 5%. This confirms the feasibility of the calculation method. In this paper, we calculate the exciting force of the propeller, analyze the time domain of the exciting force, and obtain the curve of the frequency domain using a Fourier transform of the time-domain curve of the exciting force. The existence of milling-shape ice before the propeller can greatly disturb the wake flow field. Unlike in open water, the propeller bearing capacity shows a downward trend in three stages, and fluctuating pressure is more disordered near the ice.

• Journal of Power Electronics
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• 제18권2호
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• pp.615-626
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• 2018

To ensure a DC current path and avoid large voltage overshoot of the DC-link inductor, alternating PWM pulses in the current-source rectifier should be supplemented by overlap time, which generates an overlap current and causes input current distortion. In this study, the influence of overlap time is illustrated by comparing the AC-side current before and after overlap time is added. The overlap current distribution caused by overlap time is discussed under different modulation carriers, including triangle carrier, positive-going carrier, and negative-going carrier. The quantitative relationship between the extra harmonics of the AC-side current and overlap time is based on the Fourier analysis. Based on the commutation analysis, a new carrier modulation scheme that can restrain overlap current is proposed. A 3 kW prototype is established to verify the effectiveness of the influence of overlap time and the proposed restraining modulation scheme.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2006년도 추계 학술대회논문집
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• pp.193-198
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• 2006

It takes a lot of time and effort to generate grids for numerical analysis of problems with ground effect because the relative attitude and height of airfoil should be maintained to the ground as well as the inflow. A low Mach number preconditioned turbulent flow solver using the overlap grid technique has been developed and applied to the ground effect simulation. It has been validated that the present method using the multi-block grid gives us highly accurate solutions comparing with the experimental data of the RAE 101 airfoil in an unbounded condition. Present numerical method has been extended to simulate ground effect problems by using the overlapped grid system to avoid tedious work in generating multi-block grid system. An extended method using the overlapped grid has been verified and validated by comparing with results of multi-block method and experimental data as well. Consequently, the overlapped grid method can provide not only sufficiently accurate solutions but also the efficiency to simulate ground effect problems. It is shown that the pressure and aerodynamic centers move backward by the ground effect as the airfoil approaches to the ground.

• Journal of applied mathematics & informatics
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• 제7권3호
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• pp.773-785
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• 2000

In this paper an algorithm is presented based on the additive Schwarz method for steady groundwater flow in a porous medium. The subproblems in the algorithm correspond to the problem on a coarse grid and some overlapping subdomains. It will be shown that the rate of convergence is independent of the mesh parameters and discontinuities of the coefficients, and depends on the overlap ratio.

• 한국기계가공학회지
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• 제13권1호
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• pp.30-38
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• 2014

The purpose of this study is to build and manage environment models with line segments from sonar range data on obstacles in unknown and varied environments. The proposed method therefore employs a two-stage data-transform process in order to extract environmental line segments from range data on obstacles. In the first stage, the occupancy grid extracted from the range data is accumulated to form a two-dimensional local histogram grid. In the second stage, a line histogram extracted from a local histogram grid is based on a Hough transform, and matching serves as a means of comparing each of the segments on a global line segments map against the line segments to detect the degree of similarity in the overlap, orientation, and arrangement. Each of these tests is formulated by comparing one of the parameters in the segment representation. After the tests, new line segments can be found at maximum-density cells in the line histogram, and they are composed onto the global line segment map. The proposed technique is demonstrated in experiments in an indoor environment.

• International Journal of Aeronautical and Space Sciences
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• 제8권1호
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• pp.95-104
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• 2007

A new design approach of complex geometries such as wing/body configuration is arranged by using overset mesh techniques under large scale computing environment. For an in-depth study of the flow physics and highly accurate design, several special overlapped structured blocks such as collar grid, tip-cap grid, and etc. which are commonly used in refined drag prediction are adopted to consider the applicability of the present design tools to practical problems. Various pre- and post-processing techniques for overset flow analysis and sensitivity analysis are devised or implemented to resolve overset mesh techniques into the design optimization problem based on Gradient Based Optimization Method (GBOM). In the pre-processing, the convergence characteristics of the flow solver and sensitivity analysis are improved by overlap optimization method. Moreover, a new post-processing method, Spline-Boundary Intersecting Grid (S-BIG) scheme, is proposed by considering the ratio of cell area for more refined prediction of aerodynamic coefficients and efficient evaluation of their sensitivities under parallel computing environment. With respect to the sensitivity analysis, discrete adjoint formulations for overset boundary conditions are derived by a full hand-differentiation. A smooth geometric modification on the overlapped surface boundaries and evaluation of grid sensitivities can be performed by mapping from planform coordinate to the surface meshes with Hicks-Henne function. Careful design works for the drag minimization problems of a transonic wing and a wing/body configuration are performed by using the newly-developed and -applied overset mesh techniques. The results from design applications demonstrate the capability of the present design approach successfully.

• 정보처리학회논문지D
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• 제14D권5호
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• pp.471-482
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• 2007

본 논문에서는 기존 그리드 기반의 질의 색인 기법을 변형하여 보다 적은 저장 공간을 사용하면서 보다 빠른 연산을 수행하는 연속 영역 질의 처리 기법을 제안한다. 제안하는 기법의 주요 특징은 두 가지 이다. 첫째, 각 질의에 비트 식별자를 부여하고 그리드의 각 셀은 이러한 비트 식별자의 조합으로 이루어진 비트 열을 이용하여 질의들의 겹침 정보를 반영한다. 이러한 비트 열을 통해 셀이 어떤 질의들에 포함되어져 있는지 빠르게 판단한 수 있으며, 두 셀 사이의 각 셀을 포함하는 질의 식별자들을 비교하지 않고 비트 열만을 비교하여 질의들의 포함관계를 알아내어 불필요한 연산을 줄일 수 있다. 둘째, 셀들을 그룹단위로 관리하여 불필요하게 비트 열의 길이가 증가하여 저장 공간을 낭비하고 비트 열의 비교 연산 시간이 증가하는 문제를 해결한다. 제안하는 기법이 기존 연속 영역 질의 처리 기법에 비해 우수함을 성능 평가를 통해 입증한다.

• 천문학회보
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• 제36권1호
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• pp.82.1-82.1
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• 2011

We present a new code for solving non-LTE radiative transfer problems in a general grid (RIG). RIG develops from RATRAN code (Hogerheijde & van der Tak 2000) using the Accelerated Monte-Carlo method, and it can cope with line overlap effect among multiple molecular and atomic species. In this algorithm we make grids in arbitrary coordinates adequate to the problem, but, on the other hand, photons propagate in the Cartesian coordinates. For spherical, cylindrical and other well defined coordinate, the problem of tracing photon's path reduces to solving simple quadratic equations. For example, the outflow in the star formation have high dynamic range in scales from a few AU to ~ 0.1 pc and have also cylindrical symmetry. So, we have used (r, ${\alpha}$) coordinate system, where r is the distance from the origin and ${\alpha}$ is z/ R2 in the cylindrical coordinate of (R,z). The (r, ${\alpha}$) coordinate realizes the density - power function of r - and temperature distributions of the problems with smaller numbers of grid than the cylindrical coordinate does, and the former consumes less time to solve the problems than the latter.

• 한국항공우주학회지
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• 제35권8호
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• pp.670-676
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• 2007

A comparative study on ground boundary conditions for the airfoil in ground effect has been carried out. The objective of the present study is to clarify effects of the ground boundary conditions so that it will be helpful to analyse results of wind tunnel tests using the fixed ground board or the image method. A low Mach number preconditioned Navier-Stokes solver using the overlap grid method has been applied. It has been turned out that results with the symmetric boundary condition are almost the same to those with the moving boundary condition. Results with the fixed ground boundary show discrepancy to those with the moving boundary condition when flow separation on the ground board takes place.