• 대한전자공학회논문지TC
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• 제46권2호
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• pp.134-140
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• 2009

본 논문에서는 마이크로스트립구조에서 $3.1{\times}10.6GHz$대역의 UWB 통신을 위한 사다리꼴 안테나모델에 대해 광대역화를 검토하였다. 두 사다리꼴의 마이크로스트립패치 치수를 서로 다르게 한 모노폴의 사다리꼴안테나 모델에 대해 컴퓨터 시뮬레이션을 통하여 UWB의 대역폭에서 반사손실 -10dB(VSWR<2)이하의 임피던스 정합특성을 보이는 안테나의 동작특성을 나타내 보였다. 또한, 본 논문에서 구한 최적화 안테나는 UWB의 전 대역에서 선형위상과 수평면내에서 대체로 무지향 방사특성을 나타내 보인다.

• 대한원격탐사학회지
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• 제17권2호
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• pp.141-153
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• 2001

A semi-empirical polarimetric backscattering model for bare soil surfaces is presented. Based on measurements by using polarimetric scatterometers and the JPL AirSAR, as well as the theoretical models, the backscattering coefficints $\sigma$$^0_w$, $\sigma$$^0_{hh}$ and $\sigma$$^0_{vh}$, and the parameters of the copolarized phase-difference probability density function, namely the degree of correlation $\alpha$ and the copolarized-phase-difference $\zeta$, are modeled empirically in terms of the volumetric soil moisture content m$_v$ and the surface roughness parameters $k_s$ and $k_l$, where k=2$\pi$f/c, s is the rms height and l is the correlation length.

• 한국광학회지
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• 제31권6호
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• pp.274-280
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• 2020

본 논문에서는 결맞음 빔결합 방식을 이용한 원거리 광학 에너지 전달 방법에 있어서 대기 외란의 영향을 위상판 모델을 사용하여 수치해석하고 분석한다. 결맞음 빔결합 방식은 3채널로 구성하고, 전송 거리는 1~2 km, 각 채널별 위상 및 조사 방향은 적절한 방식으로 보정된 것으로 가정하며, 각 채널 빔들이 자유공간을 진행할 때 발생하는 대기 외란 영향은 위상판 모델로 정량화한다. 위상판은 구조상수 Cn2 값의 변동 범위 10-15에서 10-13 [m-2/3] 내에서 통계적으로 생성하여 구성한다. 특별히, 본 논의에서는 대기 요동의 강도가 최종 빔결합 효율에 미치는 영향을 분석하기 위해 위상판 모델의 구조 상수를 변화시켜가며 해당 목표 지점에서 3채널 결맞음 빔결합 방식을 통해 전송된 빔의 결합 형태, 왜곡 정도 및 빔결합 효율을 계산한다. 본 수치 모델을 통해 분석한 결과, 상기 주어진 대기 요동 조건하에서도 원거리 광학 에너지 전송에 결맞음 빔결합 방식을 사용할 경우, 수신부 유효 도달 전력을 비결맞음 빔결합 방식 대비 최소 3배 이상 확보할 수 있음을 확인할 수 있다. 본 수치 모델은, 원거리 광학 에너지 전송을 전산모사함에 있어서 다양한 형태의 대기 외란의 영향 및 빔결합 방식을 분석하는데 효과적으로 활용될 수 있을 것으로 기대한다.

• 한국정보통신학회논문지
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• 제11권2호
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• pp.280-286
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• 2007

본 논문에서는 송신 및 수신 다이버시티(diversity) 기법을 적용하는 OFDM 통신 시스템에 서의 위상잡음에 의한 영향을 분석하였다. OFDM 시스템은 위상잡음에 매우 민감한 특성을 가지는데 반송파 간섭 및 시스템 잡음의 증가를 초래함으로서 전체적인 시스템의 성능열화 현상이 나타나게 된다. 따라서 이러한 위상잡음의 영향을 분석하기 위하여 위상잡음의 분포특성 및 그 크기 정도를 모델링하고 이러한 위상잡음의 특성에 따라 어느 정도까지 시스템 성능이 열화 되는지를 고찰하였다. 이러한 열화 현상은 송수신 다이버시티 구조에 따라 달라지는데 일반적으로 비교적 간단히 구현할 수 있는 2개의 송신 및 수신안테나를 쓰는 경우와 2개의 송신 안테나 다이버시티 기법만을 적용한 경우, 그리고 다이버시티 기법을 적용하지 않았을 때의 시스템 성능에 관하여 비교하고 분석하였다.

• Journal of Electrical Engineering and Technology
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• 제8권2호
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• pp.295-303
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• 2013

This paper reports an investigation of pulse width modulation (PWM) techniques for two-phase brushless DC (BLDC) motors fed by a two-phase eight-switch inverter in a fan application. The three-phase BLDC motor is widely applied in industry; however, a lower-cost two-phase BLDC motor and drive circuit has been greatly in demand in recent years. In this paper, we introduce a mathematical model of the two-phase BLDC motor with sinusoidal back electromotive forces (EMFs) based on traditional three-phase BLDC motors. To simplify the drive algorithm and speed up its application, we analyze the principle of block commutation for a two-phase BLDC motor drive in the 180-electrical-degree conduction mode, and we further propose five PWM schemes to improve the commutation performance of the two-phase BLDC drive. The effectiveness of the proposed PWM methods is verified through experiments.

• Bulletin of the Korean Chemical Society
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• 제10권1호
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• pp.19-22
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• 1989

The suitability of BMPI (2-bromo-N-methyl pyridinium iodide) for solid-phase peptide synthesis was investigated. The coupling rate of BMPI/HOBT procedure. BMPI/HOBT was superior to DCC/HOBT couplings using the solid-phase peptide bond formation proceeded to a greater degree of completion than DCC/HOBT method did. Double couplings with 2 equiv. of Bocamino acids and 1.5 equiv. of BMPI and $NEt_3$ and 2 equiv. of HOBT in DMF/MC (1:1 v/v) gave the best result for the preparation of a model compound. Stepwise solid phase peptide synthesis using BMPI/HOBT procedure was successfully utilized for the preparation of $(D-Ala)^2$-dynorphine A. BMPI/HOBT procedure for the synthesis of $(D-Ala)^2$-dynorphine gave better yield (20%) than DCC/HOBT procedure did.

• 한국전산유체공학회지
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• 제17권1호
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• pp.94-100
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• 2012

In this paper, the cavitating flows around a hydrofoil have been numerically investigated by using a 2-d multi-phase RANS flow solver based on pseudo-compressibility and a homogeneous mixture model on unstructured meshes. For this purpose, a vertex-centered finite-volume method was utilized in conjunction with 2nd-order Roe's FDS to discretize the inviscid fluxes. The viscous fluxes were computed based on central differencing. The Spalart-Allmaras one equation model was employed for the closure of turbulence. A dual-time stepping method and the Gauss-Seidel iteration were used for unsteady time integration. The phase change rate between the liquid and vapor phases was determined by Merkle's cavitation model based on the difference between local and vapor pressure. Steady state calculations were made for the modified NACA66 hydrofoil at several flow conditions. Good agreements were obtained between the present results and the experiment for the pressure coefficient on a hydrofoil surface. Additional calculation was made for cloud cavitation around the hydrofoil. The observation of the vapor structure, such as cavity size and shape, was made, and the flow characteristics around the cavity were analyzed. Good agreements were obtained between the present results and the experiment for the frequency and the Strouhal number of cavity oscillation.

• 경영과학
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• 제32권1호
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• pp.49-63
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• 2015

This paper investigates an aircraft-target assignment problem in consideration of deconfliction. The aircraft-target assignment problem is the problem to assign available aircrafts and weapons to targets that should be attacked, where the objective function is to minimize the total expected damage of aircrafts. Deconfliction is the way of dividing airspaces for aircraft flight to ensure the safety while performing the mission. In this paper, mixed integer programming model is suggested, where it considers deconfliction between aircrafts. However, the suggested MIP model is non-linear and limited to get solution for large size problem. The 2-phase decomposition model is suggested for efficiency and computation, where in the first phase target area is divided into sectors for deconfliction and in the second phase aircrafts and weapons are assigned to given targets for minimizing expected damage of aircraft. The proposed decomposition model shows outperforms the model developed for comparison in the computational experiment.

• 한국정밀공학회:학술대회논문집
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• 한국정밀공학회 2006년도 춘계학술대회 논문집
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• pp.657-658
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• 2006

Although there are many numerical models to simulate fs laser ablation of metals, no model can analyze the ablation phenomena over a wide range of fluence. In this work, a numerical code for simulating the fs laser ablation phenomena of metals has been developed. The two temperature model is employed to predict the ablation rate and the crater shape of metals using phase explosion mechanism in the relatively high fluence regime. Also, the ultrashort thermoelastic model is used for the low fluence regime to account for spallation of the sample by high strain rate. It has been demonstrated that the thermoelastic stress generated within the sample can exceed the yield stress of the material even near the threshold fluence. Numerical computation results are compared with the experiment for Cu and Ni and show good agreement. Discussions are made on the hydrodynamic model considering phase change and hydrodynamic flow.

• 한국전산유체공학회지
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• 제19권3호
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• pp.91-97
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• 2014

A high resolution numerical method aimed at solving cavitating flow was proposed and applied to gas-liquid two-phase shock tube problem with arbitrary void fraction. The present method with compressibility effects employs a finite-difference 4th-order Runge-Kutta method and Roe's flux difference splitting approximation with the MUSCL TVD scheme. The Jacobian matrix from the inviscid flux of constitute equation is diagonalized analytically and the speed of sound for the two-phase media is derived by eigenvalues. So that the present method is appropriate for the extension of high order upwind schemes based on the characteristic theory. By this method, a Riemann problem for Euler equations of one dimensional shock tube was computed. Numerical results of high speed flow phenomena such as detailed observations of shock and expansion wave propagations through the gas-liquid two-phase media and some data related to computational efficiency are made. Comparisons of predicted results and solutions at isothermal condition are provided and discussed.