• 한국전자파학회논문지
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• 제20권1호
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• pp.67-74
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• 2009

본 논문에서는 광 이중 측파대역 변조 방식과 16 QAM(Quadrature Amplitude Modulation) 신호를 이용한 부반송파/파장 분할 다중화 시스템의 성능을 부반송파 채널 간격, 누화 전력 량(crosstalk power), BER(Bit-Error Rate) 특성을 통해 연구하고 있다. 광 전송 링크에서 분산 특성과 자기 위상 변조, 상호 위상 변조와 같은 비선형 현상을 고려하였을 때 BER특성을 살펴보았다. 컴퓨터 시뮬레이션 결과, 부반송파/파장 분할 다중화 시스템의 성능을 결정짓는 주도적인 인자는 마흐-젠더 변조기의 비선형 특성과 부반송파 채널 간격으로 관찰되었다. 광섬유의 분산에 의한 영향만 고려했을 때는 전송거리가 증가함에 따라 높은 주파수에 있는 부반송파 채널의 BER 성능이 나빠졌으나, 분산과 비선형 효과를 함께 고려되었을 경우 광섬유에 입사하는 전력이 클 때 높은 주파수에 있는 부반송파 채널에서 약간의 BER 성능 향상을 확인할 수 있었다.

• 조명전기설비학회논문지
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• 제23권1호
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• pp.92-99
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• 2009

심매설 접지봉의 경우 장마철에 빗물이나 지하수와 접촉하는 경우가 발생한다. 수중에 잠겨있는 접지극에 서지 전압이 인가되면 접지극 주변에서 이온화 현상이 발생하게 된다. 지중이나 수중에서의 이온화 현상은 접지 시스템의 에너지적인 특성에 의해 영향을 받는다. 이 논문의 목적은 축소된 전해 수조를 이용하여 임펄스 전압에 의한 접지시스템의 과도특성을 파악하는데 있다. 매틀랩 프로그램을 활용하여 이온화에 의한 전위저감과 방출된 에너지를 측정하여 정량적인 분석을 수행하였다. 접지극 끝단의 최대 전압은 물의 저항률과 Marx형 전압발생장치의 충전전압에 따라 다양하게 나타났다. 접지극 끝단의 전위는 절연파괴전압에 이르기 전 인가전압에서 대략 절반까지 감소하였다. 또한 절연파괴가 발생하기 전 인가된 에너지의 절반 이상의 에너지가 이온화에 의해 접지극을 통하여 방출되었다.

• 터널과지하공간
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• 제13권4호
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• pp.287-293
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• 2003

장대 수직 환기구는 지하공간에서 화재 발생시 매연을 지상으로 배출해 내기 위한 필수적 설치물이다. 현재 널리 사용되는 이층 존(zone) 모델은 기본 가정이 제한적임으로 인해 장대 수직 환기구에서의 매연유동을 해석하는데 부적합하다. 그러므로 그 대안으로서 전산유체역학에 기초한 필드 모델의 적용성이 검토되었다. 전산모델의 유효성을 조사하기 위해 이미 발표된 기존의 실험과 유사한 구조를 선택하여 모델링 하였고 그 결과를 서로 비교하였다. 모델링에서 일정한 기준의 질량분율을 일관되게 적용하여 얻어진 매연의 확산 상단면은 실험결과를 바탕으로 구한 매연확산의 경험적 상관관계식과 거의 유사한 속도로 환기구내에서 상승함이 관측되었다. 이것은 다른 연구자들이 수학적 이론으로부터 구한 상관관계식의 결과보다 더 우수한 결과이며 전산유체역학에 기초한 수치모델이 장대형 수직 환기구를 통한 매연확산 연구에 효과적인 도구가 될 수 있음을 보여준다.

• 한국대기환경학회지
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• 제16권3호
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• pp.225-236
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• 2000

Diurnal variations of wind field and pollutant dispersion over the Yosu area under the insolation conditions of summer and winter were investigated by using the Regional Atmospheric Modeling System (RAMS). Initially, horizontally homogeneous wind field were assumed on the basis of sounding data at the Kwangju upper-air station for days whose morning wind speeds were below 2m/s. In these days, the sea breeze prevailed in summer while the land breeze lasted for a few hours in the morning; the effect of synoptic winds was strong in winter with some inclusion of wind variations owing to the interaction between sea and land. The predicted wind direction at the location of the Yosu weather station captured an important change of the sea-land breeze of the observed one. The predicted wind speed and the air temperature agreed with observed ones in a reasonable range. In the morning, both in summer and winter, winds around the source location were diverged and became weak between the mountainous area to the southeast and the Kwangyang Bay to the north. Winds, however, accelerated while blowing to the east and south and blowing on the mountainous area. Complicated wind fields resulted in high pollutant concentrations at almost all receptors considered. These high concentrations in the morning were even comparable to the ISCST3 calculations with the worst-case and typical meteorological conditions designated by USEPA(1996). On the other hand, in the afternoon, the wind field was rather uniform even in the mountainous area with development of mixing layer and the concentration distributions being close to the Gaussian distributions.

• 한국재료학회지
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• 제27권2호
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• pp.100-106
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• 2017

Nanosized and aggregated $Y_2O_3:Eu$ Red phosphors were prepared by template method from metal salt impregnated into crystalline cellulose. The particle size and photoluminescent property of $Y_2O_3:Eu$ red phosphors were controlled by variation of the calcination temperature and time. Dispersed nanosol was also obtained from the aggregated $Y_2O_3:Eu$ Red phosphor under bead mill wet process. The dispersion property of the $Y_2O_3:Eu$ nanosol was optimized by controlling the bead size, bead content ratio and milling time. The median particle size ($D_{50}$) of $Y_2O_3:Eu$ nanosol was found to be around 100 nm, and to be below 90 nm after centrifuging. In spite of the low photoluminescent properties of $Y_2O_3:Eu$ nanosol, it was observed that the photoluminescent property recovered after re-calcination. The dispersion and photoluminescent properties of $Y_2O_3:Eu$ nanosol were investigated using a particle size analyzer, FE-SEM, and a fluorescence spectrometer.

• 대한원격탐사학회지
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• 제27권3호
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• pp.213-223
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• 2011

The purpose of this study is to estimate the spherical wave parameters that appears in synthetic aperture radar (SAR) image acquired over the coast of Chukk, Micronesia. The retrieval of ocean wave parameters consists of two main stages: the first is to determine the dominant wavelengths by Fast Fourier Transform (FFT) over 16 sub-image areas and the second is to estimate wave slopes and heights using dispersion relationship under various water wave conditions. It is assumed that the spherical waves are linear and progressive. These type of waves have the range and azimuth components traveling in radial directions. The azimuth travelling waves are more affected by the velocity bunching mechanism and it is difficult to estimate the wave parameters for these affected areas in SAR imagery. In order to compensate these effects, the velocity bunching ratio (VBR) based on modulation transfer function (MTF) was compared with the intensity ratio for neighbor area in the radial direction in order to assign the spherical wave properties for azimuthally travelling waves. Dispersion relation provides the good estimates for the wave heights for all the selected sub-image areas in the range of 1m to 2m. VBR based on MTF was found to be 0.78 at wave height of 1.36m, while the intensity-based VBR was 0.69 which corresponds to the height of 1.75m. It can be said that the velocity bunching accounts for azimuthally travelling spherical waves and the difference results from the sea-bottom effects.

• 한국재료학회지
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• 제25권4호
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• pp.202-208
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• 2015

Alumina dispersion strengthening copper(ADSC) alloy has great potential for use in many industrial applications such as contact supports, frictional break parts, electrode materials for lead wires, and spot welding with relatively high strength and good conductivity. In this study, we investigated the oxidation behavior of ADSC alloys. These alloys were fabricated in forms of plate and round type samples by surface oxidation reaction using Cu-0.8Al, Cu-0.4Al-0.4Ti, and Cu-0.6Al-0.4Ti(wt%) alloys. The alloys were oxidized at $980^{\circ}C$ for 1 h, 2 h, and 4 h in ambient atmosphere. The microstructure was observed with an optical microscope(OM) and a scanning electron microscope(SEM) equipped with energy-dispersive X-ray spectroscopy(EDS). Characterization of alumina was carried out using a 200 kV field-emission transmission electron microscope(TEM). As a result, various oxides including Ti were formed in the oxidation layer, in addition to ${\gamma}$-alumina. The thickness of the oxidation layer increased with Ti addition to the Cu-Al alloy and with the oxidation time. The corrected diffusion equation for the plate and round type samples showed different oxidation layer thickness under the same conditions. Diffusion length of the round type specimen had a value higher than that of its plate counterpart because the oxygen concentration per unit area of the round type specimen was higher than that of the plate type specimen at the same diffusion depth.

• Elastomers and Composites
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• 제46권1호
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• pp.60-69
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• 2011

[ $2.1\;{\mu}m$ ]입자경의 단분산 PBA seed를 이용한 2단계 분산중합을 seeded batch process 와 seeded semi-continuous process를 이용하여 다양한 형태의 PBA/PMMA composite 미립자를 제조하였다. Batch process의 경우 PBA/PMMA composite 입자의 모폴로지는 중합 분산매인 methanol/water 비와 중합 2단계에서의 PBA seed/MMA단량체의 비에 따라 크게 영향을 받으며, methanol/water 중합분산매중 물의 량이 증가함에 따라, 그리고 중합 2단계에서 MMA단량체량이 증가함에 따라 PBA와 PMMA간의 상분리현상이 심화되어 구형의 입자로부터 egg, snowman, confetti, peanut와 같은 비구형의 PBA/PMMA composite 입자들이 제조되었다. Semi-continuous process의 경우 단량체 부가시간을 조절함으로서 PBA/PMMA composite 입자의 모폴로지가 변함을 발견하였고 특히, 단량체의 결핍된 조건하에서 구형의 PBA(코어)/PMMA(쉘) composite 라텍스가 제조되었다.

• 펄프종이기술
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• 제30권3호
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• pp.74-83
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• 1998

The phenomenon of decrease in sizing efficiency when the stock temperature is increased is well recognized as summer sizing, and this is believed to be caused by uneven distribution of sizing agents on paper surface most often incurred by coagulation of sizing agents. When unevenly sized paper is used as coating base stock, nonuniform consolidation of the coating layer may result, which, in turn, causes uneven distribution of binder on coating surface. This causes nonuniform ink absorption to produce print mottle. In this study the effects of simple or polymeric electrolytes, storage temperature and time on the coagulation of cationic dispersed rosin size were investigated using a turbidity measurement method which was verified to correlate well with the particle size of rosin emulsion or its coagulates. Handsheets sized with rosin dispersions coagulated under various conditions were prepared and their sizing degree and coated paper properties including gloss and ink density were examined. The relationship between the sizing nonuniformity of coated papers and its ink absorption property was evaluated. Turbidity of rosin emulsion increased as the storage temperature and time were increased. Addition of simple or polymeric electrolytes caused reduction in $zeta$ -potential of the rosin dispersion and accelerated the coagulation tendency substantially. Reversion of the $zeta$ -potential of rosin dispersion, however, did not occur when coagulation was induced with simple electrolytes. On the other hand, addition of an anionic polyelectrolyte reversed the $zeta$ -potential of the flocculated rosin dispersion. This indicated that electrical double layer compaction and bridging flocculation were coagulation mechanisms for simple and polymeric electrolytes, respectively. Sizing degree decreased as coagulation of rosin was increased. Paper gloss, ink gloss and ink density were increased when sizing degree of base stock was increased most probably due to prevention of base paper swelling and increased binder migration to coating surface. This suggested that uneven printing ink density occurred when uneven sizing development was induced by coagulation of rosin particles.

• Nuclear Engineering and Technology
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• 제54권5호
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• pp.1549-1559
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• 2022

As an accident tolerant fuel (ATF) concept, oxide dispersion strengthened Zircaloy-4 (ODS Zry-4) cladding has been developed to enhance the mechanical properties of cladding using laser processing technology. In this study, a simulation technique was established to investigate the mechanical properties and effects of Y₂O₃ particles for the ODS Zry-4. A 3D representative volume element (RVE) model was developed considering the parameters of the size, shape, distribution and volume fraction (VF) of the Y₂O₃ particles. From the 3D RVE model, the Young's modulus, coefficient of thermal expansion (CTE) and creep strain rate of the ODS Zry-4 were effectively calculated. It was observed that the VF of Y₂O₃ particles had a significant effect on the aforementioned mechanical properties. In addition, the predicted properties of ODS Zry-4 were applied to a simulation model to investigate cladding deformation under a transient condition. The ODS Zry-4 cladding showed better performance, such as a delay in large deformation compared to Zry-4 cladding, which was also found experimentally. Accordingly, it is expected that the simulation approach developed here can be efficiently employed to predict more properties and to provide useful information with which to improve ODS Zry-4.