• 천문학회보
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• 제39권1호
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• pp.37.2-37.2
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• 2014

Gas in disk galaxies interacts nonlinearly with a underlying stellar spiral potential to form galactic spiral shocks. Numerical simulations typically show that these shocks are unstable to the wiggle instability, forming non-axisymmetric structures with high vorticity. While previous studies suggested that the wiggle instability may arise from the Kelvin-Helmholtz instability or orbit crowding of gas elements near the shock, its physical nature remains uncertain. It was even argued that the wiggle instability is of numerical origin, caused by the inability of a numerical code to resolve a shock that is inclined to numerical grids. In this work, we perform a normal-mode linear stability analysis of galactic spiral shocks as a boundary-value problem. We find that the wiggle instability originates physically from the potential vorticity generation at a distorted shock front. As the gas follows galaxy rotation, it periodically passes through multiple shocks, successively increasing its potential vorticity. This sets up a normal-mode that grows exponentially, with a growth rate comparable to the orbital angular frequency. We show that the results of our linear stability analysis are in good agreement with the those of local hydrodynamic simulations of the wiggle instability.

• 한국군사과학기술학회지
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• 제19권5호
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• pp.638-644
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• 2016

In this research, theoretical study on empirical analysis method to estimate waterdrop's deformation by shock wave is presented. Flow field is calculated using theoretical and empirical relations. Waterdrop's deformation including movement, size, mass, and orientation is modeled using empirical relations derived from existing experimental data. Developed method is applied to specific flight examples with arbitrary flight speed and vehicle's configuration. The flight speed is assumed to Mach number of 2 and 4. The diameter of waterdrop is varied from 1 to 5 mm. Waterdrops along the stagnation line in front of hemispherical nose with the radius of 50 mm and around a cone-shaped side wall with the half angle of 20 degree are considered. It is found that the maximum diameter of the waterdrop is increased up to 2.77 times the initial diameter. The mass is conserved more than 66.7 %. In the case of a cone-shaped side wall, waterdrop's orientation angles defined from the flight direction when the Mach number is 2 and 4 are calculated as 33.0 and 25.6 degree, respectively.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2010년도 제34회 춘계학술대회논문집
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• pp.397-400
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• 2010

충격파 유도 연소장에서의 적응격자기법의 유용성을 확인하기 위하여 화학반응식을 포함한 2차원 Euler 방정식을 이용하여 삼각형 비정렬 적응격자계에서 계산을 수행하였다. 2차원 쐐기형상에 대하여 냉가스 및 열가스 유동 해석을 수행하였다. 적응격자를 이용하여 경사충격파에서 폭굉파로의 천이를 잘 관찰 할 수 있었고, 유도영역, 천이영역, 폭굉영역 등의 특성을 잘 모사하는 것을 확인 할 수 있었다. 본 연구를 통하여 연소장이 포함된 고속압축성 유동장에서의 비정렬 적응격자의 유용성을 확인 할 수 있었다.

• 한국우주과학회:학술대회논문집(한국우주과학회보)
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• 한국우주과학회 2004년도 한국우주과학회보 제13권2호
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• pp.337-340
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• 2004

Upper stage telemetry system of KSLV- I (Korea Space Launch Vehicle I) is composed of MDU (Master Data Unit), RDU (Remote Data Unit), SRU (Shock Recorder Unit) and Transmitter. RDU is the front-end telemetry data acquisition unit which gathers analog/discrete signals from various sensors and other units, and transmits the processed data to MDU via MIL-STD-I553B data bus. In order to acquire useful data from analog signal, signal conditioning circuits, such as anti-aliasing or amplifying, should be implemented. For this purpose, SCM (Signal Conditioning Module) had been developed. This paper describes hardware structure of SCM and analog signal conditioning circuits for various sensors. Also, sampling time scheme for different sampling rates were designed and tested.

• 한국멀티미디어학회:학술대회논문집
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• 한국멀티미디어학회 2003년도 춘계학술발표대회논문집
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• pp.779-782
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• 2003

컴퓨터 관련기술의 눈부신 발전은 기술적인 분야뿐만 아니라 우리의 생활과 문화의 거의 모든 분야의 변화를 초래했다 그 중 하이퍼미디어 분야는 편이성과 오락성을 바탕으로 대중들에게 크게 어필하면서 전문가가 아닌 사용자에게도 컴퓨터에 대한 접근을 더욱 쉽게 유도하였다 이러한 하이퍼미디어의 가능성은 교육분야에도 광범위하게 적용되어 텍스트나 단순한 이미지만으로 학습하기 어려운 부분을 동영상과 사운드, 상호작용을 이용하여 보다 쉽고 편리하며 재미있게 학습할 수 있게 되었다. 본 연구에서 구현된 컴파일러 프론트 엔드 학습모듈은 이러한 하이퍼미디어의 장점을 최대한 살려 컴퓨터 관련학과 학생들에게도 난이도가 높은 '컴파일러'라는 과목을 보다 시각적이고 역동적인 화면과 사운드를 이용하여 학습효과를 극대화하는데 활용할 수 있을 것이다.

• 한국항공우주학회지
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• 제37권1호
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• pp.82-88
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• 2009

정상파 시스템의 구조는 발열반응으로 상변화를 하는 물질의 연속방정식에 의해 타당성을 검증받는다. 1차원 연속체 충격 구조 분석에서의 이론적 배경을 기반으로, 상변화 현상과 관련된 파의 마이크로 두께를 산출하였다. 상변화를 하는 물질로써, n-heptane은 탄화수소 연료의 증발과 응축 분석에 사용하였고, HMX은 고체 로켓 연료의 용융과 응고 분석에 사용하였다. n-heptane의 증발-응축 면의 산출 두께는 $10^{-2}$ 마이크론 차수이고, 반면에 HMX의 용융-응고 면의 산출 두께는 1 마이크론 차수 이다. 소개된 상면 두께 산출 이론은 실험적으로 얻을 수 없는 방대한 범위의 에너지 물질까지 계산범위를 확장시킬 수 있다.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2000년도 추계 학술대회논문집
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• pp.39-45
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• 2000

Hypersonic analysis on the KSR-III payload configuration has been performed using an axisymmetric Navier-Stokes code. A numerical code based on the Harten and Yee's upwind TVD scheme with simplified curve fits in the equilibrium chemically reacting air was developed. The carbuncle phenomenon on detached shock in front of the payload is controlled by using pressure gradients to tune the dissipation. Equilibrium chemically reacting computations for reentry flight conditions of Mach no. 10.2, 8, 4.9 are presented and compared with the results of calorically perfect gas.

• 대한토목학회논문집
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• 제29권4B호
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• pp.339-346
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• 2009

본 연구에서는 조류와 충격파가 혼재한 해역의 흐름 특성을 3차원 수치모형(Princeton Ocean Model, POM)을 사용하여 파악하였다. 1994년에 완공된 시화방조제의 끝막이 공사 동안 개방구간과 그 주변 해역의 흐름을 모의하기 위하여 POM 모형을 선정하였다. 모의결과는 큰 내 외수위차와 적은 통수단면적으로 인해 발생한 충격파가 창조시와 낙조시에 각각 방조제 내측과 외해로 전파되는 것으로 나타났다. 또한, 충격파 주변에서 흐름분리가 발생하며, 충격파가 개방구간을 통과한 후 더 넓은 지역으로 퍼짐에 따라 충격파의 조류에 대한 영향은 약화되는 것으로 나타났다. 흐름의 종방향 유속분포는 충격파의 영향을 받는 것으로 밝혀졌다. 또한, 낙조시 개방구간 전방에서 수위강하가 발생했으며, 내외수위차가 가장 큰 모의조건에서는 도수현상이 발생하였다. 따라서 충격파가 지배적인 해역의 흐름 특성을 파악하기 위해서는 3차원적 수리해석이 필요하며, 해석결과는 대상해역에서 수행되는 해상공사 및 시설물 관리에 필요한 정보를 제공할 것으로 기대된다.

• 천문학회지
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• 제48권5호
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• pp.285-298
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• 2015

Active Galactic Nuclei (AGN) with bright radio jets offer the opportunity to study the structure of and physical conditions in relativistic outflows. For such studies, multi-frequency polarimetric very long baseline interferometric (VLBI) observations are important as they directly probe particle densities, magnetic field geometries, and several other parameters. We present results from first-epoch data obtained by the Korean VLBI Network (KVN) within the frame of the Plasma Physics of Active Galactic Nuclei (PAGaN) project. We observed seven radio-bright nearby AGN at frequencies of 22, 43, 86, and 129 GHz in dual polarization mode. Our observations constrain apparent brightness temperatures of jet components and radio cores in our sample to > 10^8.01 K and > 10^9.86 K, respectively. Degrees of linear polarization m_L are relatively low overall: less than 10%. This indicates suppression of polarization by strong turbulence in the jets. We found an exceptionally high degree of polarization in a jet component of BL Lac at 43 GHz, with m_L ~ 40%. Assuming a transverse shock front propagating downstream along the jet, the shock front being almost parallel to the line of sight can explain the high degree of polarization.

• Composites Research
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• 제31권3호
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• pp.83-87
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• 2018

An object in the Low Earth Orbit (LEO) is affected by many environmental conditions unlike earth's surface such as, Atomic oxygen (AO), Ultraviolet Radiation (UV), thermal cycling, High Vacuum and Micrometeoroids and Orbital Debris (MMOD) impacts. The effect of all these parameters have to be carefully considered when designing a space structure, as it could be very critical for a space mission. Polybenzimidazole (PBI) is a high performance thermoplastic polymer that could be a suitable material for space missions because of its excellent resistance to these environmental factors. A thin coating of PBI polymer on the carbon epoxy composite laminate (referred as CFRP) was found to improve the energy absorption capability of the laminate in event of a hypervelocity impact. However, the overall efficiency of the shield also depends on other factors like placement and orientation of the laminates, standoff distances and the number of shielding layers. This paper studies the effectiveness of using a PBI coating on the front bumper in a multi-shock shield design for enhanced hypervelocity impact resistance. A thin PBI coating of 43 micron was observed to improve the shielding efficiency of the CFRP laminate by 22.06% when exposed to LEO environment conditions in a simulation chamber. To study the effectiveness of PBI coating in a hypervelocity impact situation, experiments were conducted on the CFRP and the PBI coated CFRP laminates with projectile velocities between 2.2 to 3.2 km/s. It was observed that the mass loss of the CFRP laminates decreased 7% when coated by a thin layer of PBI. However, the study of mass loss and damage area on a witness plate showed CFRP case to have better shielding efficiency than PBI coated CFRP laminate case. Therefore, it is recommended that PBI coating on the front bumper is not so effective in improving the overall hypervelocity impact resistance of the space structure.