• Aerospace Engineering and Technology
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• v.10 no.2
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• pp.20-28
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• 2011

The electro-optical payload of a low-earth orbit satellite is thermally decoupled with the bus, which supports a payload for a mission operation. The payload has a cooling unit of FPA(Focal Plane Assembly) which has a thermal behavior increasing its temperature instantly during an operation in order to dissipate a waste heat into the space. The FPA cooling unit should include a radiator and heatpipes with a sufficient performance in worst hot condition, and a heater design to maintain its temperature above a minimum allowable temperature in the worst cold condition. In this paper, we analyzed the thermal requirements and the heater design constraints from the thermal analysis results for the current thermal design of the FPA cooling unit and the design elements of the better heater design were found.

• Proceedings of the Korean Vacuum Society Conference
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• 2013.08a
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• pp.263.1-263.1
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• 2013

탄소나노튜브(CNT)를 이용한 초소형 X선 튜브는 근접 암치료, 비파괴 X선 영상 장치, 휴대용 X선 분광계 등에서 X선 발생소스로 많이 연구되고 있다. 2극형 CNT 에미터의 경우 구조가 단순하여 초소형 X선 튜브에 쉽게 장착할 수 있지만 아노드의 전압과 전류가 연동되기 때문에 튜브의 조작성이 제한적이다. 3극형은 상대적으로 복잡한 구조이고, CNT에서 방출된 전자가 게이트 전극으로 흐르는 누설 전류 그리고 절연체와 충돌하여 차징을 발생시킬 수 있기 때문에 직경이 좁은 초소형 X선 튜브에 구현하기가 쉽지 않다. 하지만 초소형 X선 튜브를 다양한 X선 장치에 응용하기 위해서는 아노드 전압과 전류의 독립된 조작이 가능한 3극형 CNT 에미터가 반드시 구현되어야 한다. 본 발표에서는 전자빔의 아노드 집속을 강화하고 절연체에서의 차징을 줄이는 포커싱 기능의 게이트(FFG) 구조를 제안하였고. 이를 적용하여 초소형 X선 튜브들을 제작하고, 분석하였다. FFG 구조가 성공적으로 적용된 초소형 X선 튜브는 게이트 누설 전류 없이 뛰어난 전류 및 X선 방출 특성을 보였다. 이와는 달리, 몇몇 초소형 X선 튜브들에서는 게이트 누설 전류가 나타났고, 아노드 전압에 의한 게이트 전압 상승이 발생하여 불안정한 구동 특성을 보였다. 초소형 X선 튜브를 밀봉하지 않고 진공 챔버에서 실험한 결과, 유도된 게이트 전압은 상당한 시간이 흐르거나 진공챔버에 공기를 주입하고 다시 진공상태로 만들면 유도전압이 제거되는 것을 볼 수 있었다. 결론적으로 CNT에서 방출된 전자빔이 정상궤도를 벗어나 게이트 누설전류와 차징에 의한 게이트 유도전압을 발생시키면 초소형 X선 튜브가 불안정한 구동을 하고, 결국 튜브의 심각한 결함으로 나타나게 된다. 즉, 게이트 누설 전류와 유도된 게이트 전압은 3극형 CNT 에미터가 장착된 초소형 X선 튜브의 디자인과 제작에 있어서 성공 기준이 될 수 있다.

• Asia-Pacific Journal of Business Venturing and Entrepreneurship
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• v.15 no.4
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• pp.83-96
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• 2020

Traditionally social problems had been largely left to be handled by non-profit organizations, not because of lack of interests in social causes but mostly because of lack of innovative ideas to solve them in commercially viable ways. This market failure has recently been fixed by social enterprises, which manage to provide both solutions for social issues and commercially viable business models. As a result, we have been witnessing a sudden spike of public interests in these social enterprises as well as a call for empirical investigation on social enterprises by prior studies. However, empirically investigating a social enterprise has been a challenging endeavor mostly due to the insufficient number of successful social enterprises. Answering to the call in this study, we empirically investigate Goyohan Taxi, a successful social enterprise that partners with visually impaired cabdrivers and offers a taxi service, by both longitudinally interviewing the founder and adopting in vivo observation throughout the entrepreneurial journey of Goyohan Taxi. Based on the single case study method, we find that Goyohan Taxi adopts two distinctive decision-making mechanisms, the de-escalation of commitment and the orthogonality of commercial and social goals. Although generalizability of the our finding is minimal due to the limitation of single case study method, our finding contributes to the research of social entrepreneurship by offering new avenue of research in decision-making process of social entrepreneurs.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.32 no.1
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• pp.55-68
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• 2020

In order to investigate the hydraulic characteristics of a boundary layer streaming over the beach cusps appeared in swells prevailing mild seas, we numerically simulated the shoaling process of Edge waves over the beach cusp. Synchronous Edge waves known to sustain the beach cusps could successfully be duplicated by generating two obliquely colliding Edge waves in front of beach cusps. The amplitude A_B and length L_B of Beach Cusp were elected to be 1.25 m and 18 m, respectively based on the measured data along the Mang-Bang beach. Numerical results show that boundary layer streaming was formed at every phase of shoaling process without exception, and the maximum boundary layer streaming was observed to occur at the crest of sand bar. In RUN 1 where the shortest waves were deployed, the maximum boundary layer streaming was observed to be around 0.32 m/s, which far exceeds the amplitude of free stream by two times. It is also noted that the maximum boundary layer streaming mentioned above greatly differs from the analytical solution by Longuet-Higgins (1957) based on wave Reynolds stress. In doing so, we also identify the recovery procedure of natural beaches in swells prevailing mild seas, which can be summarized such as: as the infra-gravity waves formed in swells by the resonance wave-wave interaction arrives near the breaking line, the sediments ascending near the free surface by the Phase II waves orbital motion were carried toward the pinnacle of foreshore by the shoreward flow commenced at the steep front of breaking waves, and were deposited near the pinnacle of foreshore due to the infiltration.

• Journal of Korean Society of Transportation
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• v.22 no.6
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• pp.35-47
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• 2004

Using data envelopment analysis, this paper annually estimates Malmquist total factor productivity indices and decomposes them into productive efficiency change, technical change, and scale change components for three publicly-owned rail transit properties: the Seoul Subway Corporation(SSC), the Seoul Metropolitan Electrified Railways Sector of Korea National Railroad (SMESRS) and the Busan Urban Transit Authority (BUTA). The paper then conceptualizes that the property produces a single output(car-kilometers) using four inputs(labor, electricity, car and maintenance, and track) and uses unbalanced panel data consisted of annual observations on SSC, SMESRS and BUTA. The results show that the average annual growth rate of productivity of the three properties is 6.6 percent, which is 0.5 percent less than the average annual increasing rate of their labor price. They also show that the greatest part of the growth in productivity is explained by technical change and to a lesser degree by scale change and changes in productive efficiency though each of the three components contributes more than 20 percent to the growth in productivity, These results suggest that the three properties should base the increasing rate of their wages on the growth rate of their productivity and utilize existing technologies more efficiently prior to introducing new ones to raise their productivity, and that all the three components should be considered to evaluate their productivity more correctly.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.20 no.10
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• pp.26-32
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• 2019

The purpose of this study is to improve the running speed on a small-radius curved section on an existing train line. When the transition curve was extended, and the amount of movement in the horizontal direction tended to increase as the curve radius increased. The amount of increase in the transition curve extension was lowest when extending the curve radius, and the amount of horizontal movement was the lowest when changing the curve radius to a cosinusoidal curve. As a result of applying the pass rate after improvement to the Kyeongbu line, there was a time-shortening effect of 9.4 to 11.6% and a facial expression speed increasing effect when the curve radius was fixed and the transition curve was changed to a sinusoidal curve. In conclusion, the most effective way to improve the running speed on an existing train route is to change the image to concrete and to change the relaxation curve to a cosinusoidal curve. The amount of horizontal movement of the track is small, and the speed improvement effect is excellent.

• Korean Journal of Remote Sensing
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• v.39 no.6_1
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• pp.1477-1482
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• 2023

This study examines marine heat wave (MHW) in the Northeast Asia region from 2012 to 2021, utilizing geostationary satellite Communication, Ocean, and Meteorological Satellite (COMS)/Meteorological Imager sensor (MI) and GEO-KOMPSAT-2A (GK-2A)/Advanced Meteorological Imager sensor (AMI) Sea Surface Temperature (SST) data. Our analysis has identified an increasing trend in the frequency and intensity of MHW events, especially post-2018, with the year 2020 marked by significantly prolonged and intense events. The statistical validation using Optimal Interpolation (OI) SST data and satellite SST data through T-test assessment confirmed a significant rise in sea surface temperatures, suggesting that these changes are a direct consequence of climate change, rather than random variations. The findings revealed in this study serve the necessity for ongoing monitoring and more granular analysis to inform long-term responses to climate change. As the region is characterized by complex topography and diverse climatic conditions, the insights provided by this research are critical for understanding the localized impacts of global climate dynamics.