• 한국주조공학회지
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• 제30권6호
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• pp.231-234
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• 2010

The effect of $C_2Cl_6$ for preventing to the surface oxidation and ignition of molten Mg alloy was studied with metallographic analysis, X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. The alloy used for this study was AM100A Mg casting alloy with high strength. In order to investigate the surface protective characteristic of this molten alloy by $C_2Cl_6$ addition, we added them into molten AM100A alloy at $700^{\circ}C$ and then the melts were slowly cooled under a protective atmosphere of air containing Ar gas and $C_2Cl_6$ flux addition. The result found that the surface oxidation and ignition reaction of molten AM100A Mg alloy by adding $C_2Cl_6$ flux was more slowly occurred than that of the only a protective atmosphere of containing Ar gas with increasing time. This result was due to a dense protective film formed containing $MgCl_2$ on surface of molten Mg alloy during casting and solidification. The $MgCl_2$ was formed by a reaction of $C_2Cl_6$ with molten Mg.

• 한국광학회지
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• 제28권4호
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• pp.146-152
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• 2017

우주용 인공위성 카메라를 구성하는 반사경의 정렬은 광학계의 고분해능, 고성능을 얻기 위한 중요한 과정 중 하나이다. 반사경들의 상호정렬에는 큐브미러(cube mirror)가 대신 사용되기 때문에, 각 반사경과 해당 큐브미러 간의 상호위치관계 정보가 우선 필요하다. 따라서 우주용 카메라 반사경들의 정렬을 위해 각 반사경의 정점과 해당 큐브미러의 상대 좌표값을 정확하게 측정해야하며, 본 논문에서는 컴퓨터 제작 홀로그램(computer-generated hologram, CGH)의 정렬용 세그먼트와 광섬유를 이용하는 새로운 측정 시스템을 제안함으로써 우주용 카메라를 구성하는 반사경의 정점을 요구조건 이내로 측정할 수 있었다. 측정 시스템은 광학계 평가용 간섭계, CGH, 광섬유, 반사경으로 구성되어 있으며, 최종적으로 데오도라이트를 이용해 큐브미러를 기준으로 주 반사경의 정점에 위치한 광섬유 끝단의 3차원 상대 좌표값을 $25{\mu}m$ 이하의 정밀도로 측정할 수 있었다.

• 한국재료학회지
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• 제21권6호
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• pp.314-319
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• 2011

Melt foaming method is one of cost-effective methods to make metal foam and it has been successfully applied to fabricate Mg foams. In this research, AZ31 Mg alloy ingot was used as a metal matrix, using AlCa granular as thickening agent and $CaCO_3$ powder as foaming agent, AZ31 Mg alloy foams were fabricated by melt-foaming method at different foaming temperatures. The porosity was above 41.2%~73.3%, pore size was between 0.38~1.52 mm, and homogenous pore structures were obtained. Microstructure and mechanical properties of the AZ31 Mg alloy foams were investigated by optical microscopy, SEM and UTM. The results showed that pore structure and pore distribution were much better than those fabricated at lower temperatures. The compression behavior of the AZ31 Mg alloy foam behaved as typical porous materials. As the foaming temperature increased from $660^{\circ}C$ to $750^{\circ}C$, the compressed strength also increased. The AZ31 Mg alloy foam with a foaming temperature of $720^{\circ}C$ had the best energy absorption. The energy absorption value of Mg foam was 15.52 $MJ/m^3$ at a densification strain of 52%. Furthermore, the high energy absorption efficiencies of the AZ31 Mg alloy foam kept at about 0.85 in the plastic plateau region, which indicates that composite foam possess a high energy absorption characteristic, and the Vickers hardness of AZ31 Mg alloy foam decreased as the foaming temperature increased.

• Journal of Astronomy and Space Sciences
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• 제30권4호
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• pp.335-344
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• 2013

TRiplet Ionospheric Observatory-CubeSat for Ion, Neutron, Electron & MAgnetic fields (TRIO-CINEMA) is a CubeSat with 3.14 kg in weight and 3-U ($10{\times}10{\times}30$ cm) in size, jointly developed by Kyung Hee University and UC Berkeley to measure magnetic fields of near Earth space and detect plasma particles. When a satellite is launched into orbit, it encounters ultra-high vacuum and extreme temperature. To verify the operation and survivability of the satellite in such an extreme space environment, experimental tests are conducted on the ground using thermal vacuum chamber. This paper describes the temperature control device and monitoring system suitable for CubeSat test environment using the thermal vacuum chamber of the School of Space Research, Kyung Hee University. To build the chamber, we use a general purpose thermal analysis program and NX 6.0 TMG program. We carry out thermal vacuum tests on the two flight models developed by Kyung Hee University based on the thermal model of the TRIO-CINEMA satellite. It is expected from this experiment that proper operation of the satellite in the space environment will be achieved.

• 한국주조공학회지
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• 제26권6호
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• pp.267-271
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• 2006

Metal foam is a class of attractive materials, which exhibits unique combinations of physical, mechanical, thermal, electrical and acoustic properties. In particular, it is light and good at absorbing energy, which makes it attractive in automotive and aerospace applications weight is critical. In this paper, the Mg alloy foam was prepared by melt foaming method by addition of calcium as thickening agent, and $TiH_2$ or $CaCO_3$ powder as blowing agent. The macrostructural observation of foamed Mg showed that the pore structures of Mg alloy foam made by $CaCO_3$ as blowing agent were much better than that of foams made by $TiH_2$ as blowing agent. In addition, this paper showed the possible reason of fabrication magnesium alloy foam in proportion to blowing agent and the porosity range was about 40 to 76% as results value.

• 한국정보과학회논문지:데이타베이스
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• 제29권2호
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• pp.99-109
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• 2002

ROLAP(Relational Online Analytical Processing)은 다차원적 데이타 분석을 위한 제반 기술로써, 전사적 데이타 웨어하우스로부터 고부가가치를 창출하는데 필수적인 기술이다. 질의처리 성능을 높이기 위해서 대부분의 ROLAP 시스템들은 집계 테이블들을 미리 계산해 둔다. 이를 큐브 생성이라고 하며, 이 과정에서 기존의 방법들은 데이타를 여러 차례 정렬해야 하고 이는 큐브 생성의 성능을 저하시키는 큰 요인이다. (1)은 MOLAP 큐브 생성 알고리즘을 통해 간접적으로 ROLAP 큐브를 생성하는 것이 훨씬 빠르다는 것을 보였다. 본 연구에서도 MOLAP 큐브 생성 알고리즘을 사용한 신속하고 확장적인 ROLAP 큐브 생성 알고리즘을 제시하였다. 분석할 입력 사실 테이블을 적절하게 조각내어 메모리 효율을 높였고, 집계 테이블들을 최소 부모 집계 테이블로부터 생성하도록 하여 큐브 생성 시간을 단축하였다. 제안한 방법의 효율성은 실험을 통해 검증하였다.

• 정보보호학회논문지
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• 제21권2호
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• pp.3-11
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• 2011

$F_{3^m}$에서의 Tate 페어링 또는 ${\eta}_T$ 페어링 알고리즘 계산을 위하여 효율적인 세제곱근 계산은 매우 중요하다. $x^{1/3}$의 다항식 표현 중 0이 아닌 계수들의 개수를 $x^{1/3}$의 헤밍웨이트라 할 때, 이 헤밍웨이트가 세제곱근 연산의 효율성을 결정하게 된다. O. Ahmadi 등은 $f(x)=x^m+ax^k+b$ (a, $b{\in}F_3$)가 $F_3[x]$의 삼항 기약다항식이라 할 때, $F_{3^m}=F_3[x]/(f)$을 생성하는 모든 삼항 기약다항식에 대하여 $x^{1/3}$의 헤밍웨이트를 계산하였다. 본 논문에서는 Shifted Polynomial Basis(SPB)가 기존의 결과보다 $x^{1/3}$의 헤밍웨이트를 낮출 수 있음을 보이며, 모듈로 감산 연산이 필요 없는 가장 적합한 SPB를 제공한다.

• Wind and Structures
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• 제37권1호
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• pp.25-38
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• 2023

Due to the complexity and difficulty in meeting the multiphase flow complexity, similarity, and multiscale characteristics, the mechanism of snow drift is so complicated that the snow deposition prediction is still inaccurate and needs to be far improved. Meanwhile, the validation of prediction methods is also limited due to a lack of field-measured data about snow deposition. To this end, a field measurement activity about snow deposition around a cube with time was carried out, and the snow accumulation process was measured under blowing snow conditions in northwest China. The maximum snow depth, snow profile, and variation in snow depth around the cube were discussed and analyzed. The measured results indicated three stages of snow accumulation around the cube. First, snow is deposited in windward, lateral and leeward regions, and then the snow depth in windward and lateral regions increases. Secondly, when the snow in the windward region reaches its maximum, the downwash flow erodes the snow against the front wall. Meanwhile, snow range and depth in lateral regions have a significant increase. Thirdly, a narrow road in the leeward region is formed with the increase in snow range and depth, which results in higher wind speed and reforming snow deposition there. The field measurement study in this paper not only furthers understanding of the snow accumulation process instead of final deposition under complex conditions but also provides an important benchmark for validating prediction methods.

• 항공우주시스템공학회지
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• 제17권2호
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• pp.94-102
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• 2023

큐브위성용 태양전지판 분리장치는 열선 또는 저항소자의 발열로 나일론선을 절단하여 구조물의 구속을 해제하는 나일론선 절단방식 구속분리장치가 주로 적용되고 있다. 일반적으로 태양전지판 조립체의 발사하중을 고려한 구조해석을 수행하여 구속분리장치의 설계가 이루어지고 있으나, 발사 이후 궤도 열환경에 대한 구속분리장치의 열적 검토 및 분석사례는 전무한 실정이다. 따라서, 본 논문에서는 현재 개발중인 큐브위성 STEP Cube Lab-II에 적용되는 나일론선 절단 기반 구속분리장치의 열적 안정성 평가를 수행하고자 한다. 위성이 POD (Picosatellite Orbital Deployer)에서 사출되고 태양전지판이 전개되기까지의 분리장치 온도 분포를 검토하여 분리장치의 허용온도 범위 내에서 안정적인 구속 분리가 될 수 있도록 분리장치에 대한 궤도 열해석을 수행하였다. 또한, 열해석 결과를 기반으로 열진공시험을 수행하여 분리장치의 설계를 검증하였다.

• 천문학논총
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• 제12권1호
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• pp.185-196
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• 1997

We present a code which identifies individual clouds in crowded region using IMFORT interface within Image Reduction and Analysis Facility (IRAF). We define a cloud as an object composed of all pixels in longitude, latitude, and velocity that are simply connected and that lie above some threshold temperature. The code searches the whole pixels of the data cube in efficient way to isolate individual clouds. Along with identification of clouds it is designed to estimate their mean values of longitudes, latitudes, and velocities. In addition, a function of generating individual images (or cube data) of identified clouds is added up. We also present identified individual clouds using a $^{12}CO$ survey data cube of Galactic Anticenter Region (Lee et al. 1997) as a test example. We used a threshold temperature of $5\sigma$ rms noise level of the data With a higher threshold temperature, we isolated subclouds of a huge cloud identified originally. As the most important parameter to identify clouds is the threshold value, its effect to the size and velocity dispersion is discussed rigorously.