• 천문학회보
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• 제41권1호
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• pp.50.2-50.2
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• 2016

Continuous accretion of metal-poor gas can explain the discrepancy between the number of observed G-dwarfs and the number predicted by the "simple model" of galactic evolution. The maximum accretion rate estimated based upon approaching high velocity clouds (HVCs) can be up to ${\sim}0.4M_{\odot}{\cdot}yr^{-1}$ which is comparable with the accretion rate required by many chemical evolution models that is at least ${\sim}0.45M_{\odot}{\cdot}yr^{-1}$. However, it is not clear to what extent the exchange of gas between the disk and the cloud can occur when an HVC collides with the galactic disk. Therefore, we examined a series of HVC-Disk collision simulations using the FLASH 2.5 hydrodynamics simulation code. The outcomes of our simulations show that an HVC will more likely take away substances from the galactic disk rather than adding new material to the disk. We define this as an HVC having a "negative fuel rate". Further results in our study also indicate that the process and amount of fuel rate change can have various forms depending on the density, radius and velocity of an approaching HVC. The simulations in our study covers HVCs with a neutral hydrogen volume density from $1.0{\times}10^{-2}cm^{-3}$ to $41.0cm^{-3}$, radius of 200 pc to 1000 pc and velocity in the range between $40km{\cdot}s^{-1}$ and $100km{\cdot}s^{-1}$.

• 한국결정성장학회지
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• 제15권1호
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• pp.10-15
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• 2005

Microwave plasma CVD 다이아몬드/Ti 박막 성장 시 CH₄/H₂ 가스의 유량비율, chuck bias, microwave power 등이 다이아몬드 박막의 구조적 특성과 입자밀도에 미치는 영향에 대하여 조사하였다. 2∼3 CH₄ Vol.% 조건일 때 sp³-결합성의 탄소 neutral 들이 우선적으로 형성되고 sp²-결합성의 탄소 neutral 들이 선택적으로 제거됨에 따라 양질의 다이아몬드 박막을 얻을 수 있었으며, 다이아몬드 입자 증착 기구를 해석하였다. Ti 기판에 걸어준 negative chuck bias가 증가함에 따라 다이아몬드 핵생성이 증진되어 다이아몬드 입자 밀도가 증가하였고, 임계 전압은 약 -50V 임을 확인하였다. 또한, microwave power가 증가함에 따라 미세결정질(micro-crystalline) graphite 층 생성이 제어되고 다이아몬드 층이 형성됨을 확인하였다.

• 대한기계학회논문집
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• 제18권8호
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• pp.1939-1951
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• 1994

This study introduces a new tool breakage detecting technology comprised of an unsupervised neural network combined with adaptive time series autoregressive(AR) model where parameters are estimated recursively at each sampling instant using a parameter adaptation algorithm based on an RLS(Recursive Least Square). Experiment indicates that AR parameters are good features for tool breakage, therefore it can be detected by tracking the evolution of the AR parameters during milling process. an ART 2(Adaptive Resonance Theory 2) neural network is used for clustering of tool states using these parameters and the network is capable of self organizing without supervised learning. This system operates successfully under the wide range of cutting conditions without a priori knowledge of the process, with fast monitoring time.

• Steel and Composite Structures
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• 제15권4호
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• pp.451-466
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• 2013

This paper deals with the multi-objective optimization of tire reinforcement structures such as the tread belt and the carcass path. The multi-objective functions are defined in terms of the discrete-type design variables and approximated by artificial neutral network, and the sensitivity analyses of these functions are replaced with the iterative genetic evolution. The multi-objective optimization algorithm introduced in this paper is not only highly CPU-time-efficient but it can also be applicable to other multi-objective optimization problems in which the objective function, the design variables and the constraints are not continuous but discrete. Through the illustrative numerical experiments, the fiber-reinforced tire belt structure is optimally tailored. The proposed multi-objective optimization algorithm is not limited to the tire reinforcement structure, but it can be applicable to the generalized multi-objective structural optimization problems in various engineering applications.

• 천문학회지
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• 제37권4호
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• pp.237-241
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• 2004

We perform numerical experiments on supernova-driven turbulent flows in order to see whether or not supernovae playa major role in driving turbulence in the interstellar medium. In a $(200pc)^3$ computational box, we set up, as initial conditions, uniformly magnetized gas distributions with different pairs of hydrogen number densities and magnetic field strengths, which cover the observed values in the Galactic midplane. We then explode supernovae at randomly chosen positions at a Galactic explosion rate and follow up the evolution of the supernova-driven turbulent flows by integrating numerically the ideal MHD equations with cooling and heating terms. From the numerical experiments we find that the density-weighted velocity dispersions of the flows are in the range of 5-10 km $s^{-l}$, which are consistent with the observed velocity dispersions of cold and warm neutral media. Additionally, we find that strong compressible flows driven by supernova explosions quickly change into solenoidal flows.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2016년도 제50회 동계 정기학술대회 초록집
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• pp.96-96
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• 2016

Nanotechnology mostly employs nano-materials and nano-structures with distinctive properties based on their size, structure, and composition. It is quite difficult to produce nano-materials and nano-structures with identical sizes, structures, and compositions in large quantities, because of spatiotemporal fluctuation of production processes. In other words, fluctuation is the bottleneck in nanotechnology. We propose three strategies to suppress such fluctuations: employing 1) difference between linear and nonlinear phenomena, 2) difference in time constants, and 3) nucleation as a bottleneck phenomenon. We are also developing nano- and micro-scale guided assembly using plasmas as a plasma nanofabrication.1-5) We manipulate nano- and micro-objects using electrostatic, electromagnetic, ion drag, neutral drag, and optical forces. The accuracy of positioning the objects depends on fluctuation of position and energy of an object in plasmas. Here we evaluate such fluctuations and discuss the mechanism behind them. We conducted in-situ evaluation of local plasma potential fluctuation using tracking analysis of fine particles (=objects) in plasmas. Experiments were carried out with a radio frequency low-pressure plasma reactor, where we set two quartz windows at the top and bottom of the reactor. Ar plasmas were generated at 200 Pa by applying 13.56MHz, 450V peak-to-peak voltage. The injected fine particles were monodisperse methyl methacrylate-polymer spheres of $10{\mu}m$ in diameter. Fine particles were injected into the reactor and were suspended around the plasma/sheath boundary near the powered electrode. We observed binary collision of fine particles with a high-speed camera. The frame rate was 1000-10000 fps. Time evolution of their distance from the center of mass was measured by tracking analysis of the two particles. Kinetic energy during the collision was obtained from the result. Potential energy formed between the two particles was deduced by assuming the potential energy plus the kinetic energy is constant. The interaction potential is fluctuated during the collision. Maximum amplitude of the fluctuation is 25eV, and the average is 8eV. The fluctuation can be caused by neutral molecule collisions, ion collisions, and fluctuation of electrostatic force. Among theses possible causes, fluctuation of electrostatic force may be main one, because the fine particle has a large negative charge of -17000e and the corresponding electrostatic force is large compared to other forces.

• 천문학회보
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• 제46권1호
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• pp.50.2-51
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• 2021

Stars form exclusively in cold and dense molecular clouds. To fully understand star formation processes, it is hence a key to investigate how molecular clouds form out of the surrounding diffuse atomic gas. With an aim of shedding light in the process of the atomic-to-molecular transition in the interstellar medium, we analyze Arecibo HI emission and absorption spectral pairs along with TRAO/PMO 12CO(1-0) emission spectra toward 58 lines of sight probing in and around molecular clouds in the solar neighborhood, i.e., Perseus, Taurus, and California. 12CO(1-0) is detected from 19 out of 58 lines of sight, and we report the physical properties of HI (e.g., central velocity, spin temperature, and column density) in the vicinity of CO. Our preliminary results show that the velocity difference between the cold HI (Cold Neutral Medium or CNM) and CO (median ~ 0.7 km/s) is on average more than a factor of two smaller than the velocity difference between the warm HI (Warm Neutral Medium or WNM) and CO (median ~ 1.7 km/s). In addition, we find that the CNM tends to become colder (median spin temperature ~ 43 K) and abundant (median CNM fraction ~ 0.55) as it gets closer to CO. These results hints at the evolution of the CNM in the vicinity of CO, implying a close association between the CNM and molecular gas. Finally, in order to examine the role of HI in the formation of molecular gas, we compare the observed CNM properties to the theoretical model by Bialy & Sternberg (2016), where the HI column density for the HI-to-H2 transition point is predicted as a function of density, metallicity, and UV radiation field. Our comparison shows that while the model reproduces the observations reasonably well on average, the observed CNM components with high column densities are much denser than the model prediction. Several sources of this discrepancy, e.g., missing physical and chemical ingredients in the model such as the multi-phase ISM, non-equilibrium chemistry, and turbulence, will be discussed.

• 공업화학
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• 제31권4호
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• pp.347-351
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• 2020

재료는 방사선과 상호작용을 통해 그 물리적, 화학적 특성이 변화하며 여러 방사선 중에서 전하를 띄고 있지 않아 침투깊이가 깊은 중성자 조사에 의한 금속소재의 조사손상은 원자력발전소의 안전과 관련해서 오랜 기간 동안 집중적인 연구대상이었다. 중성자 조사에 의한 조사손상은 초반 피코 초 스케일에서 벌어지는 원자단위의 점결함의 생성으로 시작되며 그 이후의 전개 양상은 전위 고리나 공극과 같은 미세구조상 결함으로 확인될 수 있다. 이러한 미세구조 상 결함의 형상과 분포에 따라 소재의 특성에 미치는 효과는 상이하게 된다. 그러므로 중성자 조건에 따른 미세구조를 예측하는 것은 매우 중요한 일로, 본 논문에서는 중성자 조사에 의한 재료 내의 미세구조 발달에 대해 리뷰한 뒤 조사된 소재의 미세구조 변화 예측에 널리 사용될 수 있는 상장 모델에 대해 간략히 소개하였다.

• 한국식품저장유통학회지
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• 제5권3호
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• pp.247-255
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• 1998

본 연구에서는 독숙기와 성숙기의 감 과실에 외부 에서 에틸렌($50\mu\ell.L^{-1}$)을 처리하였을 때 처리시간에 따라 과실의 경도, 에틸렌 발생량, 세포벽 성분의 변 화 그리고 세포벽 팩틴의 주사슬을 형성하는 polyuuronide(PU) 및 곁사슬을 형 성하는 polysaccharide (PS)의 가용화와 분해가 어떻게 달라지는지를 알고저 하였다. 에틸렌을 처리하였을 때 녹숙기와 성숙기 감 모두 처리 시간이 경과될 수록 과실의 경도가 급격히 감 소하였으며, 감소의 정도는 녹숙기 감에서 더 심하였 다. 녹숙기에 에틸렌을 12시간 처리한 과실에서는 처 리 3일 후부터 에틸렌이 발생하기 시작하였으며, 에 멸렌 발생량은 24시간 에틸렌 처리에서는 최대 $16,000\mu\ell.L^{-1}$에 달하였다. 성숙기의 감에서는 24시 간 동안 에틸렌을 처리하여도 처리후 7일까지는 에 틸렌발생이 없었다. 에틸렌 처리 시간의 증가에 따른 세포벽 펙틴물질의 변화를 보면, 증류수, 0.05M N NazC03, 4M 및 8M KOH 가용성분획에서는 연화가 진행됨에 따라 펙틴함량은 감소하였다. 각 분획별 비 섬유성 중성당의 종류를 보면 0.05M CDTA 및 0.05M $Na_2CO_3$ 가용성 분획에서의 주요 구성당은 arabino않 와 galactose이었고, 4M KOH 가용성 분획에서는 glucose, galactose 및 xylose였다. 에틸렌 처리 시간별 P PU와 PS의 가용화와 분해정도를 보면, 증류수, CDTA, $Na_2CO_3$ 및 4M KOH 가용성 분획에서는 에틸 렌 처리 시간이 진행됨에 따라 고분자 PU와 PS의 가 용화와 저분자화가 진행되었다.

• 한국우주과학회:학술대회논문집(한국우주과학회보)
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• 한국우주과학회 2009년도 한국우주과학회보 제18권2호
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• pp.42.3-43
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• 2009

Triplets of identical cubesats will be built to carry out the following scientific objectives: i) multi-observations of ionospheric ENA (Energetic Neutral Atom) imaging, ii) ionospheric signature of suprathermal electrons and ions associated with auroral acceleration as well as electron microbursts, and iii) complementary measurements of magnetic fields for particle data. Each satellite, a cubesat for ion, neutral, electron, and magnetic fields (CINEMA), is equipped with a suprathermal electron, ion, neutral (STEIN) instrument and a 3-axis magnetometer of magnetoresistive sensors. TRIO is developed by three institutes: i) two CINEMA by Kyung Hee University (KHU) under the WCU program, ii) one CINEMA by UC Berkeley under the NSF support, and iii) three magnetometers by Imperial College, respectively. Multi-spacecraft observations in the STEIN instruments will provide i) stereo ENA imaging with a wide angle in local times, which are sensitive to the evolution of ring current phase space distributions, ii) suprathermal electron measurements with narrow spacings, which reveal the differential signature of accelerated electrons driven by Alfven waves and/or double layer formation in the ionosphere between the acceleration region and the aurora, and iii) suprathermal ion precipitation when the storm-time ring current appears. In addition, multi-spacecraft magnetic field measurements in low earth orbits will allow the tracking of the phase fronts of ULF waves, FTEs, and quasi-periodic reconnection events between ground-based magnetometer data and upstream satellite data.