• 조명전기설비학회논문지
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• 제27권7호
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• pp.52-58
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• 2013

The two-dimensional finite difference time domain algorithm is used to numerically reconstruct the electron density profile in O-mode ultrashort pulse reflectometry. A Gaussian pulse is employed as the source of a probing electromagnetic wave. The Gaussian pulse duration is chosen in such a manner as to have its frequency spectrum cover the whole range of the plasma frequency. By using a number of numerical band-pass filters, it is possible to compute the time delays of the frequency components of the reflected signal from the plasma. The electron density profile is reconstructed by substituting the time delays into the Abel integral equation. As a result of simulation, the reconstructed electron density profile agrees well with the assumed profile.

• Current Optics and Photonics
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• 제4권2호
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• pp.121-126
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• 2020

With the increasing demand for head-mounted display applications, the image quality provided by a near-eye display device is a key factor in satisfying the consumer. Among various techniques to realize a near-eye display that has a thinner volume than the working distance of a human eye, a lightfield image-generation method based on a pinhole array is attracting much attention, with its simple and thin structure. In this paper, we propose a numerical analysis of the visual parameters and verifications with computational reconstruction.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 1998년도 추계 학술대회논문집
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• pp.48-54
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• 1998

Three-dimensional incompressible Navier-Stokes equations have been solved by the node-centered finite volume method with unstructured hybrid grids. The pressure-velocity coupling is handled by the artificial compressibility algorithm and convective fluxes are obtained by Roe's flux difference splitting scheme with linear reconstruction of the solutions. Euler implicit method is used for time-integration. The viscous terms are discretised in a manner to handle any kind of grids such as tetrahedra, prisms, pyramids, hexahedra, or mixed-element grid. The numerical efficiency and accuracy of the present method is critically evaluated for several example problems.

• 대한기계학회논문집B
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• 제30권1호
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• pp.8-15
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• 2006

A Sparse Point Representation(SPR) based on interpolation wavelets is presented. The SPR is implemented for the purpose of CFD data compression. Unlike conventional wavelet transformation, the SPR relieves computing workload in the similar fashion of lifting scheme that includes splitting and prediction procedures in sequence. However, SPR skips update procedure that is major part of lifting scheme. Data compression can be achieved by proper thresholding method. The advantage of the SPR method is that, by keeping even point physical values, low frequency filtering procedure is omitted and its related unphysical thresholing mechanism can be avoided in reconstruction process. Extra singular feature detection algorithm is implemented for preserving singular features such as shock and vortices. Several numerical tests show the adequacy of SPR for the CFD data. It is also shown that it can be easily extended to nonlinear adaptive wavelets for enhanced feature capturing.

• 제어로봇시스템학회:학술대회논문집
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• 제어로봇시스템학회 1995년도 Proceedings of the Korea Automation Control Conference, 10th (KACC); Seoul, Korea; 23-25 Oct. 1995
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• pp.452-455
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• 1995

Various physical limitations which intrinsically exist in the manipulator control system, for example kinematic limits and torque limit, cause some undesirable effects. Specifically, when one or more actuators are saturated the expected control performance can not be anticipated and in some cases it induces instability of the system. The effect of torque limit, especially for redundant manipulators, is studied in this article, and an analytic method to reconstruct the control input using the redundancy is proposed based on the kinematically decomposed modeling of redundant manipulators. It results to no degradation of the output motion closed-loop dynamics at the cost of the least degradation of the null motion closed-loop dynamics. Numerical simulations help to verify the advantages of the proposed scheme.

• Journal of the Optical Society of Korea
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• 제14권4호
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• pp.409-414
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• 2010

This paper describes the surface form measurement of a spherical smooth surface by using single shot off-axis Fizeau interferometry. The demodulated phase map is obtained and unwrapped to remove the $2\pi$ ambiguity. The unwrapped phase map is converted to height and the 3D surface height of the surface object is reconstructed. The results extracted from the single shot off-axis geometry are compared with the results extracted from four-frame phase shifting in-line interferometry, and the results are in excellent agreement.

• 한국가시화정보학회지
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• 제16권1호
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• pp.48-53
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• 2018

In this study, the effect of injection pressure on the column diameter and droplet velocity of liquid jet with the weakly turbulent Rayleigh-like breakup mode is experimentally studied using digital microscopic holography (DMH). The injection nozzle has the diameter of $50{\mu}m$ and injection pressure is varied from 0.1 to 0.4 MPa. When the micro liquid jet is injected into still air, the double-pulsed holograms was recorded on a CCD sensor and numerically reconstructed in order to obtain well focused images. In this study, the liquid column diameter from $50{\mu}m$ orifice nozzle is shown to be changed slightly but the droplet velocity is increased proportionally as the injection pressure is increased.

• 천문학회지
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• 제34권4호
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• pp.237-241
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• 2001

Our goal is to present a simple volume-of-fluid type interface-tracking algorithm to compressible two-phase flow in two space dimensions. The algorithm uses a uniform underlying Cartesian grid with some cells cut by the tracked interfaces into two subcells. A volume-moving procedure that consists of two basic steps: (1) the update of volume fractions in each grid cell at the end of the time step, and (2) the reconstruction of interfaces from discrete set of volume fractions, is employed to follow the dynamical behavior of the interface motion. As in the previous work with a surface-tracking procedure for general front tracking (LeVeque & Shyue 1995, 1996), a high resolution finite volume method is then applied on the resulting slightly nonuniform grid to update all the cell values, while the stability of the method is maintained by using a large time step wave propagation approach even in the presence of small cells and the use of a time step with respect to the uniform grid cells. A sample preliminary numerical result for an underwater explosion problem is shown to demonstrate the feasibility of the algorithm for practical problems.

• 한국광학회지
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• 제25권3호
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• pp.142-145
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• 2014

본 논문에서는 광 스캐닝 홀로그래피 기술을 이용하여 실제 물체의 복소 홀로그램 정보를 스펙클 잡음 없이 촬영하였다. 촬영된 복소 홀로그램 정보를 수치적인 방법으로 복원한 뒤 결맞음 광원과 CCD 카메라를 이용하여 촬영한 실제 물체의 영상과 비교하여 스펙클 잡음에 의한 오염도를 평가하였다. 스펙클 잡음에 의한 오염도를 정량적으로 평가하기 위하여 두 영상의 스펙클 패턴의 대비 수치를 이용하였다.

• Journal of the Optical Society of Korea
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• 제15권2호
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• pp.146-151
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• 2011

We describe, throughout a Mach-Zehnder interferometric configuration, a new test sieves calibration method based on off-axis digital holography. The experiment is conducted on a test sieve of square openings. The nominal sieve opening is 1.00 mm with maximum individual opening of 1.14 mm in size. The recorded off-axis hologram is numerically processed using Fresnel transforms to obtain an object wave (amplitude and phase). From the reconstructed phase, the average size of the illuminated openings has been measured precisely. The proposed method can provide a real time solution for calibrating test sieves very precisely and with moderate accuracy.