• 한국통신학회논문지
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• 제21권12호
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• pp.3205-3214
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• 1996

In this paper, the microstrip phased array antennas with coupling-slots for the base station of mobile communication is proposed and anlyzed with accurate analysis method which is based on both reciprocity principle and full-wave analysis. The basis functions used for the numerical analysis are determined depending upon the accuracy, convergence properties of the solution, and the computation time. The patch uses 3 EB mode and the slot uses IPWS mode. The designed phased array antenna has 8 slot-coupled microstrip patch array elements and the beam scanning capability is obtained by using the 4-bit PIN-diode phase shifters as switching devices which are consisted of the loaded line phase shifters for 30.deg. and 60.deg. and the reflection type phase shifters for 90.deg. and 180.deg. repectively. The 4-bits phase shifters which aremade by connecting each phase shifter have about 2.deg.-3.deg. phase errors and their insertion loss are about 3dB for each phase state. The fabricated 8-element phased array antenna with 4-bits phase shifters provides 12.deg.-14.deg. beamwidths depending on the scanning angle and is capable of scanning its beam to .+-.45.deg. with 9.deg. intervals, and the gain 12dBi. The overall results show that the slot-coupled phased array antenna has great advantages of wideband, high gain and reduced spurious radiation. Also, the antenna can be made small and thin. Furthermore, the scanning property of this antenna allows for its application in several areas, such as mobile communication system and PCS.

• 한국통신학회논문지
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• 제34권6A호
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• pp.484-493
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• 2009

최근 한정된 주파수의 사용 효율성을 높이기 위해 Cognitive Radio 시스템의 허가 주파수 대역 사용을 제한적으로 용인하는 CR 기술이 각광받고 있다. CR 기술에서 최우선적으로 고려되어야 할 사항은 주 사용자의 통신을 보호하는 것이다. 즉 CR 사용자는 주 사용자의 출현을 인지하는 즉시 현재 채널의 사용을 중지해야 하며, 이러한 상황을 사전에 고려하여, 운용이 가능한 주파수 대역을 폭넓게 확보해야 한다. 이를 위해 CR 사용자는 지속적인 스펙트럼 센싱을 수행한다. CR 사용자가 스펙트럼을 센싱하는 동안에는 이용 중인 서비스가 일시적으로 중단되는 현상이 발생한다. 본 논문에서는 주 사용자를 보호하기 위한 CR 센싱 요구조건을 만족하는 전제 하에, CR 사용자의 QoS를 최대한 보장하면서 스펙트럼 센싱을 수행하도록 하는 채널 센싱 스케쥴링 방법을 제안한다. 이를 위해, 센싱할 채널의 개수와 센싱 주기를 CR 센싱 요구조건의 주요 지표로 이용하였고, CR 사용자의 QoS 보장 요구사항은 패킷 지연과 손실 측면에서 분석하였다. 위의 센싱 스케쥴링 방법을 수치 분석을 통해 여러 파라미터들을 다양하게 변화시킴으로서, 다양한 환경에서 QoS를 만족시킬 수 있는 적합한 파라미터를 구할 수 있었다. 그리고 성능 평가를 통해 채널을 연속적으로 센싱하는 방법과 두 가지 측면, Delay와 성공적으로 전송한 패킷의 수를 비교하였을 때 우수한 성능을 나타냄을 확인하였다.

• 한국통신학회논문지
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• 제39B권12호
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• pp.856-863
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• 2014

지상 광무선 통신에서 대기의 교란 및 대기 입자에 의한 광의 감쇄가 있을 때, 대기교란의 강도 및 가시도가 수신 광 강도에 미치는 영향을 해석하였다. 광신호는 On-Off Keying(OOK), Pulse Position Modulation(PPM) 및 Digital Pulse Interval Modulation(DPIM)의 방식으로 변조하였다. 교란대기 하에서 각 변조방식에 대해 광 전송 거리에 따른 광수신 전력을 계산하고 이를 이용하여 전송거리에 따른 패킷 오류율(packet error rate: PER)을 계산하였다. 광신호를 전송하기 위한 광원의 파장은 850nm, 1310nm 및 1550nm을 선택하였다. 대기는 약 교란 상태로 가정하여 대기 굴절률 구조상수 $Cn2{\approx}10-14m-2/3$, 대기의 가시도 V=2km로 하였다. 약교란 대기 상태에서 세가지 변조방식 중 DPIM 방식이 우수하며, 광 신호의 전송을 위한 파장으로는 1550nm가 PER 성능이 우수함을 알 수 있었다.

• 한국환경과학회지
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• 제26권5호
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• pp.621-638
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• 2017

Meteorological characteristics related to variations in ozone ($O_3$) concentrations in the Korean peninsula before, during, and after Typhoon Talas (1112) were analyzed using both observation data and numerical modeling. This case study takes into account a high $O_3$ episode (e.g., a daily maximum of ${\geq}90ppb$) without rainfall. Before the typhoon period, high $O_3$ concentrations in the study areas (e.g., Daejeon, Daegu, and Busan) resulted from the combined effects of stable atmospheric conditions with high temperature under a migratory anticyclone (including subsiding air), and wind convergence due to a change in direction caused by the typhoon. The $O_3$ concentrations during the typhoon period decreased around the study area due to very weak photochemical activity under increased cloud cover and active vertical dispersion under a low pressure system. However, the maximum $O_3$ concentrations during this period were somewhat high (similar to those in the normal period extraneous to the typhoon), possibly because of the relatively slow photochemical loss of $O_3$ by a $H_2O+O(^1D)$ reaction resulting from the low air temperature and low relative humidity. The lowest $O_3$ concentrations during the typhoon period were relatively high compared to the period before and after the typhoon, mainly due to the transport effect resulting from the strong nocturnal winds caused by the typhoon. In addition, the $O_3$ increase observed at night in Daegu and Busan was primarily caused by local wind conditions (e.g., mountain winds) and atmospheric stagnation in the wind convergence zone around inland mountains and valleys.

• International Journal of Fluid Machinery and Systems
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• 제6권1호
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• pp.11-17
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• 2013

Mini centrifugal pumps having a diameter smaller than 100mm are employed in many fields. But the design method for the mini centrifugal pump is not established because the internal flow condition for these small-sized fluid machines is not clarified and conventional theory is not suitable for small-sized pumps. Therefore, mini centrifugal pumps with simple structure were investigated by this research. Splitter blades were adopted in this research to improve the performance and the internal flow condition of mini centrifugal pump which had large blade outlet angle. The original impeller without the splitter blades and the impeller with the splitter blades were prepared for an experiment. The performance tests are conducted with these rotors in order to investigate the effect of the splitter blades on performance and internal flow condition of mini centrifugal pump. On the other hand, a three dimensional steady numerical flow analysis is conducted with the commercial code (ANSYS-CFX) to investigate the internal flow condition in detail. It is clarified from experimental results that the performance of the mini centrifugal pump is improved by the effect of the splitter blades. Blade-to-blade low velocity regions are suppressed in the case with the splitter blades and total pressure loss regions are decreased. The effects of the splitter blades on the performance and the internal flow condition are discussed in this paper.

• 한국항공우주학회지
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• 제48권3호
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• pp.167-174
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• 2020

내부에 공유피드백 유로를 갖는 공유형 초음속 유체진동기에서 나타나는 유동특성에 관한 연구가 수행되었다. 비정상 전산유체역학적 해석이 수행되었고 수치해석 결과는 동일한 운용조건에서 수행된 실험결과와 비교 검증되었다. 수치해석 결과, 공유피드백 유로가 해당 유체진동기의 진동 메커니즘에 큰 영향을 주어 진동기 출구 각 제트유동의 동조화에 큰 역할을 하고 있음이 확인되었다. 공유형 유체진동기는 동일 형상의 단일형 유체진동기와 비교하여 진동수가 증가하나 압력손실 또한 커짐이 확인되었다.

• 대한기계학회논문집B
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• 제20권9호
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• pp.2944-2952
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• 1996

In this paper the CSCM type upwind flux difference splitting Navier-Stokes method has been applied to study the ARL-SL19 transoni $c^ersonic compressor cascade flow. First, the general characteristics of baseline cascade flow were analyzed. At freestream Mach n.1.612 and exit/inlet pressure ratio 2.15, the results from current laminar flow were compared well in suction surface with the experiment; however, not well in pressure surface. Second, numerical study of the transoni $c^ersonic compressor cascade flow demonstrated the effectiveness of a passive control by the various size cavities. A cavity under the shock foot point at the suction surface of the blades was used as a passive control. The passive control of shock-boundary layer interaction by a cavity reduced total pressure losses. The effect of cavity length and depth was studied. The total pressure loss was reduced by about 10% and the isentropic efficiency was improved slightly. The effect of cavity depth in current study(d/l = 0.05, 0.02) was not found strong. Further adequate turbulence modeling and TVD schemes would help to capture the shock more accurately and increase the effectiveness of the current shock-boundary layer interaction study using upwind flux difference splitting computational methods.thods.

• 대한기계학회논문집B
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• 제31권6호
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• pp.574-580
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• 2007

The objective of this article is to present an analysis of all heat transfer paths through the energy nose under closed door conditions when refrigeration system of household refrigerator-freezer is operating on. Both experimental and numerical methods are suggested as a means of determining the overall energy nose load amount as well as the load due to each pathway such as mullion section and F and R sides of the household refrigerator-freezer. In other words, all loads determined in this article are just energy nose and not the loads seen by the refrigeration system. We suggest good ideas for improving the heat transfer losses such as conduction and convection through the energy nose. As we can be known from the experimental test results, it is effective to prevent the heat loss of a mullion section. And energy efficiency is also decreased approximately 6% compared to that of a baseline sample test result. As we can be known from the Ansys 8.1 analysis, it is shown the steady state temperature distribution in figures from 6 to 8. And the direction of the heat flow through the energy nose section is also easily seen from that In conclusion, the article is focused on an energy nose section in household refrigerator-freezer for practical proposes which is the energy saving in a household refrigerator-freezer. And the method suggested may be applied to any make or model to aid in the search for high efficient energy nose section of household side by side refrigerator-freezer as well as top mounted refrigerator-freezer, commercial refrigerator and so on.

• Journal of Advanced Marine Engineering and Technology
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• 제10권2호
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• pp.156-164
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• 1986

In the case of boiling on high temperature wall, vapor film covers fully or parcially the surface. This phenomenon, film boiling or transition boiling, is very important in the surface heat treatment of metal, design of cryogenic heat exchanger and emergency cooling of nuclear reactor. Mainly supposed hydraulic-thermal accidents in nuclear reactor are LCCA (Loss of Coolant Accident) and PCM (Power-Cooling Mismatch). Recently, world-wide studies on reflooding of high temperature rod bundles after the occurrence of the above accidents focus attention on wall temperature history and required time in transient cooling process, wall superheat at rewet point, heat flux-wall superheat relationship beyond the transition boiling region, and two-phase flow state near the surface. It is considered that the further systematical study in this field will be in need in spite of the previous results in ref. (2), (3), (4). The paper is the study about the fast transient cooling process following the wall temperature excursion under the CHF (Critical Heat Flux) condition in a forced convective subcooled boiling system. The test section is a vertically arranged concentric annulus of 800 mm long and 10 mm hydraulic diameter. The inner tube, SUS 304 of 400 mm long, 8 mm I.D, and 7 mm O.D., is heated uniformly by the low voltage AC power. The wall temperature measurements were performed at the axial distance from the inlet of the heating tube, z=390 mm. 6 chromel- alumel thermocouples of 76 .mu.m were press fitted to the inner surface of the heating tube periphery. To investigate the heat transfer characteristics during the fast transient cooling process, the outer surface (fluid side) temperature and the surface heat flux are computed from the measured inner surface temperature history by means of a numerical method for inverse problems of transient heat conduction. Present cooling (boiling) curve is sufficiently compared with the previous results.

• 한국의학물리학회지:의학물리
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• 제15권3호
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• pp.121-127
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• 2004

본 연구에서는 수치해석을 이용하여 강자기장이 인가된 물팬텀 속에서 고에너지 전자의 경로를 근사적으로 계산하였다. 이를 위하여 자기장에서 전자의 방향변화에 관한 방정식을 세우고 오일러(Euler) 방법을 적용하여 전산코드로 구현하였다. 전산코드를 이용하여 입사전자와 수직 및 수평방향의 자기장이 인가된 물팬텀에 대하여 3, 5, 10, 15 MeV 전자의 궤적을 계산하였다. 본 결과는 전자의 다중산란이 고려되지 않아 실제 궤적과 차이가 있으나, 결과로부터 외부 자기장에 의한 물팬텀 속 선량분포의 변화를 설명할 수 있다.