• 대한전자공학회논문지SD
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• 제44권3호
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• pp.20-25
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• 2007

본 논문에서는 $LiNbO_3$의 선택적 영역을 도메인 반전을 수행하였으며, 이를 대역변조기 및 SSB 광변조기 제작에 응용하였다. 인가전압에 대한 회로적인 응답전류를 분석 및 고려함으로써 도메인 벽의 이동속도를 정확히 제어할 수 있었다. 과도한 도메인의 벽 이동속도에 의한 도메인 반전 형상을 확인하였고, 또한 도메인 벽의 진행방향에 따라 그 속도의 차이가 발생함을 알 수 있었다. 제작된 대역변조기는 30.3 GHz를 중심주파수로 하여 5.1GHz의 3dB 대역폭을 보였고. SSB 광변조기의 변조 스펙트럼으로부터 19dBm의 5.8GHz RF 입력신호에 대해 USB가 LSB에 비해 33dB정도 억제됨을 확인할 수 있었다.

• 한국연소학회지
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• 제1권1호
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• pp.41-49
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• 1996

Effects of ambient geometry on the liftoff characteristics are experimentally studied for nonpremixed turbulent jet flames. To clarify the inconsistency of the nozzle diameter effect on the liftoff height, the ambiences of finite and infinite domains are studied. For nonpremixed turbulent jet issuing from a straight nozzle to infinite domain, flame liftoff height increases linearly with nozzle exit mean velocity and is independent of nozzle diameter. With the circular plate installed on the upstream of nozzle exit, flame liftoff height is lower with plate at jet exit than without, but flame liftoff characteristics are similar to the case of infinite domain. For the confined jet having axisymmetric wall boundary, the ratio of the liftoff height and nozzle diameter is proportional to the nozzle exit mean velocity demonstrating the effect of the nozzle diameter on the liftoff height. The liftoff height increases with decreasing outer axisymmetric wall diameter. At blowout conditions, the blowout velocity decreases with decreasing outer axisymmetric wall diameter and liftoff heights at blowout are approximately 50 times of nozzle diameter.

• 한국통신학회논문지
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• 제19권10호
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• pp.1849-1860
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• 1994

본 논문은 비정질 회토류 천이 급속 박막내에서의 자벽 역학(Magnetic domain wall dynamics)을 란다우 리프쉬츠 길버트 (Landau Lifshitz Gilbert) 방정식을 이용한 수치적 해석을 수행하여 연구하였다. 박막을 이차원 정방형 격자($30\times30$)로 나누고, 각 격자 셀(Cell)에 쌍극자간 존재한다고 상정하여, 이들 쌍극자간의 상호 교환 작용과 자기 이방성, 외부 인가 자계, 그리고 감자계의 영향이 고려되었다. 단축 자기 이방성이 존재하고 역방향의 자화가 존재한다고 가정된 상태에서 자벽이 형성되는 시간과 자벽의 두께를 알아보았다. 또한 외부 자계의 인가에 따른 자벽 이동을 연구하였다. 시뮬레이션 결과, 감자계를 고려했을 때 자벽 형성 시간이 상당히 빨라졌고, 평균 자벽의 이동도(Mobility)는 약간 증가되었다.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2006년도 추계학술대회논문집
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• pp.156-162
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• 2006

Pipe thinning is one of the major issues for the structural fracture of pipes of nuclear power plants. Therefore a method to inspect a large area of piping systems quickly and accurately is needed. In this paper, we proposed the method for monitoring pipe thinning. Our basic idea come from that a group velocity of impact wave is different as wall thickness. If the group velocity is measured, wall thickness can be estimated. To obtain the group velocity, time-frequency analysis is used. This is because an arrival time difference can be measured easily in time-frequency domain rather than time domain. To test the performance of this technique, experiments have been performed for a plate and U type pipe. Results show that the proposed technique is quite powerful in the monitoring pipe thinning.

• 한국소음진동공학회논문집
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• 제16권12호
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• pp.1224-1230
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• 2006

Pipe thinning is one of the major issues for the structural fracture of pipes of nuclear power plants. Therefore a method to inspect a large area of piping systems quickly and accurately is needed. In this paper, we proposed the method for monitoring pipe thinning. Our basic idea come from that a group velocity of impact wave is different as wall thickness. If the group velocity is measured, wall thickness can be estimated. To obtain the group velocity, time -frequency analysis is used. This is because an arrival time difference can be measured easily in time-frequency domain rather than time domain. To test the performance of this technique, experiments have been performed for a plate and U type pipe. Results show that the proposed technique is quite powerful in the monitoring pipe thinning.

• Journal of Magnetics
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• 제11권2호
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• pp.74-76
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• 2006

Dynamic behaviors of transverse domain walls (TDWs) in rectangular shaped thin-film magnetic nanowires with different widths under applied magnetic fields less than the Walker field were studied by micromagnetic simulations. It was found that the velocity of stable TDWs in the viscous region increases from 147 to 419 m/s and their mass decreases from $6.24{\times}10^{-23}\;to\;2.70{\times}10^{-23}kg$ with increasing strength of the applied magnetic field ranging from 5 to 20 Oe for the nanowire with a dimension of 10 nm in thickness and $5{\mu}m$ in length, and 50 nm in width. With increasing the width of nanowires from 50 to 125 nm at a specific field strength of 5 Oe, the TDW's velocity also increases from 147 to 246 m/s and its mass decreases from $6.24{\times}10^{-23}\;to\;5.91{\times}10^{-23}kg$.

• Journal of applied mathematics & informatics
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• 제5권2호
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• pp.457-464
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• 1998

An alternative variational method for a steady tow-dimensional inviscid flow problem to determine the domain given the constant wall velocity and constant vorticity in the whole domain is presented. The uniqueness result is obtained for convex domains.

• Wind and Structures
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• 제30권5호
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• pp.485-497
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• 2020

A study, using potential water wave theory, is conducted on the oblique water wave motion over two fixed submerged rectangular blocks (breakwaters) placed over a finite step bottom. We have considered infinite and semi-infinite fluid domains. In both domains, the Fourier expansion method is employed to obtain the velocity potentials explicitly in terms of the infinite Fourier series. The unknown coefficients appearing in the velocity potentials are determined by the eigenfunction expansion matching method at the interfaces. The derived velocity potentials are used to compute the hydrodynamic horizontal and vertical forces acting on the submerged blocks for different values of block thickness, gap spacing between the two blocks, and submergence depth of the upper block from the mean free surface. In addition, the wave load on the vertical wall is computed in the case of the semi-infinite fluid domain for different values of blocks width and the incident wave angle. It is observed that the amplitudes of hydrodynamic forces are negligible for larger values of the wavenumber. Furthermore, the upper block experiences a higher hydrodynamic force than the lower block, regardless of the gap spacing, submergence depth, and block thickness.

• Journal of the Korean Society for Industrial and Applied Mathematics
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• 제20권1호
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• pp.37-50
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• 2016

A large eddy simulation (LES) of a turbulent channel flow is performed by using the third order low-storage Runge-Kutta method in time and second order finite difference formulation in space with staggered grid at a Reynolds number, $Re_{\tau}=590$ based on the channel half width, ${\delta}$ and wall shear velocity, $u_{\tau}$. To reduce the calculation cost of LES, algebraic wall model (AWM) is applied to approximate the near-wall region. The computation is performed in a domain of $2{\pi}{\delta}{\times}2{\delta}{\times}{\pi}{\delta}$ with $32{\times}20{\times}32$ grid points. Standard Smagorinsky model is used for subgrid-scale (SGS) modeling. Essential turbulence statistics of the flow field are computed and compared with Direct Numerical Simulation (DNS) data and LES data using no wall model. Agreements as well as discrepancies are discussed. The flow structures in the computed flow field have also been discussed and compared with LES data using no wall model.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2005년도 추계 학술대회논문집(수송기계편)
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• pp.65-68
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• 2005

In this paper, a theoretical study is carried out on the hydroelastic vibration of a rectangular tank wall. It is assumed that the tank wall is clamped along the plate edges. The fluid velocity potential is used for the simulation of fluid domain and to obtain the added mass due to wall vibration. In addition, the vibration characteristics of stiffened wall of the rectangular tank are investigated. Assumed mode method is utilized to the stiffened plate model and hydrodynamic force is obtained by the proposed approach. The coupled natural frequencies are obtained from the relationship between kinetic energies of a wall including fluid and the potential energy of the wall. The theoretical result is compared with the three-dimensional finite element method and then added mass effect is discussed due to tank length and potential mode.