• Transactions of the Korean Society of Mechanical Engineers B
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• v.39 no.4
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• pp.325-333
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• 2015

This study was devoted to elucidating the change in the flow characteristics of a laminar flow past a circular cylinder by modifying the cylinder shape with O-grooves. A numerical analysis was performed in a two-dimensional framework. The cylinder was represented using an immersed boundary method and marker particles on a Cartesian grid system. The number and locations of the O-grooves were the key parameters. An analysis of the flow pattern and flow-induced forces was performed at Re = 40 and 50. In addition, we calculated the critical Reynolds number depending on the number of O-grooves and their locations.

• Geophysics and Geophysical Exploration
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• v.23 no.1
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• pp.50-54
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• 2020

Herein, the closed-form expressions of the magnetic field due to an axially symmetric body such as a right cylinder, are derived. The magnetic field due to a right cylinder is converted from the gravity gradient tensor using Poisson's relation; the magnetic field induced by a constant magnetization can be obtained from the gravity gradient tensor with a constant density. Because of the axial symmetry of the cylinder, the expressions of gravity gradient tensor are derived in cylindrical coordinate and then transformed into Cartesian coordinates for the three components of the magnetic field using an arbitrary magnetization direction.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.1 no.1
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• pp.22-30
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• 1989

Wave propagation is changed by the effect of shoaling, current-depth refraction and shelter-ing etc. To solve these problems. numerous models have been developed. In the present study, a coordinate system is proposed based on the wave ray equation with the wave number equation including diffraction effects . The governing equation for the study was derived from the mild slope wave equation in non-steady state, including current effects (Kirby, 1986a) and trans-formed into an orthogonal curvilinear coordinate system on the basis of the wave ray equation. To obtain a numerical solution, an explicit finite difference scheme was used, and solved by the relaxation method. This model was tested for various cases: Firstly a submersed circular shoal and a constant unit depth. Secondly a submerged elliptic shoal on a slope, and finally a breakwater harbour with obliquely incident waves on a slope. The model was found to simulate the experimental results and other theoretical results in wave height and wave angle fairy well, and the applicability of the model around an arbitrary shaped coastal structure was also verified. To demonstrate the general usefullness of the present approach , the model is to be applied to a field situation with a complex bed topography.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.31 no.8
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• pp.19-26
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• 2003

A steady prediction method of dynamic stability derivatives is presented in the unified framework of the unsteady Euler equations. New approach does not require any modification of the governing equations except addition of non-inertial force terms. The present methods are applied to compute the pitch-damping coefficients using the lunar coning and the lunar helical motions in the Cartesian coordinate frame. The results for the ANSR and the Basic Finner are in good agreement with the PNS data, range data, and the results using the unsteady prediction method. The results show that the steady approach using the unified governing equations in the Cartesian coordinate frame can be successfully applied to predict the pitch-damping coefficients.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.36 no.11
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• pp.1072-1078
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• 2008

An adaptive control algorithm for spacecraft rendezvous and docking in a Keplerian orbit is presented. The equations of relative motion of two spacecrafts expressed in a local-vertical-local-horizontal rectangular frame are converted to a general Hamiltonian form, then an adaptive control method developed for the uncertain Hamiltonian system is applied to the rendezvous and docking problem. A smooth projection algorithm is applied to keep the parameter estimates inside a singularity-free region, and a numerical example shows that the developed controller successfully deals with the unknown mass of the chaser spacecraft.

• The Bulletin of The Korean Astronomical Society
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• v.37 no.2
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• pp.160.2-160.2
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• 2012

Unified State Model(이후 USM)은 Altman(1972)에 의해 처음 제안된 이후 Chodas(1981), Raol & Sinha(1985), Vittaldev et al.(2012) 등을 거치며 연구 발전되어 왔다. 이 모델은 공간상 6개 성분의 위치, 속도 벡터를 이용해 위성의 운동을 기술하는 기존 계산 방법과 달리 4개의 Quaternion 변수를 도입하여 위성의 위치를, 3개의 Hodograph 변수를 도입하여 위성의 속도를 각각 기술한다. USM의 장점은 직교좌표계로 표현된 위성의 위치, 속도 변수에 비해 USM 변수의 변화량이 상대적으로 작기 때문에 수치 계산 시 계산의 안정도가 높다. 또한 원궤도(${\omega}$ : undefined)와 적도면 궤도(i = 0, ${\Omega}$ : undefined) 계산 시에 나타나는 특이성(singularity) 문제가 발생하지 않는다. 본 연구에서는 USM 계산방법과 기존 방법에 의한 위성궤도 계산결과의 차이를 비교 분석하였다. 지구궤도 위성의 정밀계산을 위해 이체항 이외에 지구타원체 섭동항과 대기 항력에 의한 섭동항을 추가 적용하였다. 비구형 지구 중력 포텐셜에 의한 섭동은 J4항까지 고려하였으며, 대기 항력은 간단한 exponential 모델을 적용하였다. 또한 수치계산 시 적분 간격과 정밀도 차수를 조절하여 각 모델의 계산 안정성을 테스트하였다. 본 연구의 궤도계산 결과 USM 모델을 이용한 계산방법은 그 정밀성과 계산효율성이 매우 우수한 것으로 검증되었다.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.02a
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• pp.484-484
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• 2012

OLED는 유기재료를 사용하는 특수성 때문에 선명한 고품질의 화질을 얻을 수 있다. 유기재료에 따른 수명이 정해져 있지만 열에 약한 재료를 사용하기 때문에 출력되는 색의 변화를 동반할 만큼 불안정 하다. OLED는 RGB를 순서에 따라 유리 기판 상에 나열한 구조이며 각 화소의 양극과 음극은 가로와 세로로 서로 직교하고 있다. 이러한 OLED의 구조 때문에 직, 간접적으로 전이되는 열과 OLED Device에서 발생하는 자체 열로 인하여 유기소자의 특성이 변형되는, 이른바 열화현상에 쉽게 노출되어 있다. OLED Device를 제작한 후 72시간동안 8V의 전압을 인가하여 열화현상을 촉진시킨 Aging샘플을 확보 하였다. Aging된 Device의 인가전압을 3V ~ 6V까지 변화를 주고 측정해본 결과 각각의 모든 전압에서 Aging Device의 Nomalized Intensity가 상대적으로 20% 감소했음을 확인 하였다. 또한 Aging 된 Device는 As is Device에 비해 단파장 쪽으로 Shift 되는 결과를 보여주었다. 이를 분석하기 위해 CIE 색 좌표계의 NTSC (National Television System(s) Committee)를 이용하였는데, 범위 안에 있던 As is Device의 색 좌표가 Aging 후에는 NTSC범위 밖으로 이동하였는데, 이는 열화현상이 발생하기 전에 비해 방출되는 빛의 파장이 변했다는 것을 의미하며 정확한 색 재현이 안 된다는 것을 보여준다. 또한 I-V특성을 보면 Aging Device의 구동전압 (3.3V) 이 As is Device (2.7V) 에 비해 더 커지는 것을 확인하였는데 이것은 스트레스로 인해서 발생한 트랩에 의하여 캐리어의 이동도가 떨어졌기 때문에 구동 전압이 증가한 것으로 확인할 수 있다. 연구 결과를 통하여 OLED Device의 사용시간 누적에 따른 열화현상은 색 재현성과 휘도의 저하 그리고 구동전압 증가에 영향을 미친다는 것을 확인 하였다.

• Journal of the Society of Naval Architects of Korea
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• v.28 no.2
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• pp.77-94
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• 1991

A finite difference method based on MAC method is used to simulate free-surface waves around a ship. Euler equations and continuity equation are differentiated using the forward time and central space, and solved by time marching scheme. By the employment of variable mesh system in horizontal and vertical direction, the numerical accuracy of wave simulation results is grossly improved. To verify the improvement of numerical accuracy, some numerical simulations are accomplished for Wigley, Series 60($C_{b}$=0.6) and a bulk carrier model. The computational results are compared to the various experimental data and show good agreements.

• Journal of Navigation and Port Research
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• v.27 no.1
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• pp.41-48
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• 2003

The most great GPS is working on CNS(Car navigation System) and Personal terminal, but ocean has not been. First studied GPS is sailing to ship but We have been connivance that The ship is not need detail navigation. Lately, harbor facilities and transportation service in harbors are complication. We have need accurate ship navigation system. In this paper, author developed algorithms of ellipsoid conversion between WGS84 and BESSEL and develope algorithms of map projection between ellipsoid coordinate system, and plane cartesian system. Author used 3-parameter in covering ellipsoids and used TM and UTM projection in converting between ellipsoid and plane cartesian coordinate. And author analyzed errors through static surveying and dynamic surveying of GPS for proving accuracy of GPS sensor, Furthermore author analyzed deflection error of received position. Finally author developed real time ship navigation system using cheep GPS sensor.

• Journal of the Society of Naval Architects of Korea
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• v.39 no.4
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• pp.17-23
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• 2002

The present paper describes the optimum inverse design of 2-D linear cascade airfoil. The pressure coefficient of an airfoil surface is taken as the objective function, and non-orthogonal incompressible Navier-Stokes equation is applied to calculate the pressure coefficient. Both of steepest descent and conjugate gradient method have been used to make the objective function go to zero. The 1st order finite differential method is applied to the searching direction and the golden section method is used to compute the searching distance. As a result of the present work, a good convergence to the target airfoil has been obtained.