• The Bulletin of The Korean Astronomical Society
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• v.46 no.1
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• pp.53.2-53.2
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• 2021

Quasar luminosity function (QLF) shows the active galactic nucleus (AGN) demography as a result of the combination of the growth and the evolution of black holes, galaxies, and dark matter halos along the cosmic time. The recent wide and deep surveys have improved the census of high-redshift quasars, making it possible to construct reliable ultraviolet (UV) QLFs at 2 < z < 6 down to M1450 = -23 mag. By parameterizing these up-to-date observed UV QLFs that are the most extensive in both luminosity and survey area coverage at a given redshift, we show that the UV QLF has a universal shape, and their evolution can be approximated by a pure density evolution (PDE). In order to explain the observed QLF, we construct a model QLF employing the halo mass function, a number of empirical scaling relations, and the Eddington ratio distribution. We also include the outshining of AGN over its host galaxy, which made it possible to reproduce a moderately flat shape of the faint end of the observed QLF (slope of ~ -1.1). This model successfully explains the observed PDE behavior of UV QLF at z > 2, meaning that the QLF evolution at high redshift can be understood under the framework of halo mass function evolution. The importance of the outshining effect in our model also implies that there could be a hidden population of faint AGNs (M1450 > -24 mag), which are buried under their host galaxy light.

• Journal of the Korean Society of Combustion
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• v.8 no.3
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• pp.15-23
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• 2003

Partial quenching structure of turbulent diffusion flames in a turbulent mixing layer is investigated by the method of flame hole dynamics in order to develop a prediction model for turbulent flame lift off. The essence of flame hole dynamics is derivation of the random walk mapping, from the flame-edge theory, which governs expansion or contraction of flame holes initially created by local quenching events. The numerical simulation for flame hole dynamics is carried out in two stages. First, a direct numerical simulation is performed for constant-density fuel-air channel mixing layer to obtain the turbulent flow and mixing fields, from which a time series of two dimensional scalar dissipation rate array is extracted at a fixed virtual flame surface horizontally extending from the end of split plate to the downstream. Then, the Lagrangian simulation of the flame hole random walk mapping projected to the scalar dissipation rate array yields temporally evolving turbulent extinction process and its statistics on partial quenching characteristics. The statistical results exhibit that the chance of partial quenching is strongly influenced by the crossover scalar dissipation rate while almost unaffected by the iteration number of the mapping that can be regarded as a flame-edge speed.

• Proceedings of the KSME Conference
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• 2003.04a
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• pp.2219-2226
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• 2003

Two perforated plates are placed in parallel and staggered arrangements with a gap distance of 2 times of the hole diameter, and initial crossflow passes between the plates. Both the injection and effusion hole diameters are 10 mm, and the Reynolds number based on the hole diameter and hole-to-hole pitch are fixed to 10,000 and 6 times of the hole diameter, respectively. To investigate the effect of crossflow, the flow rate of crossflow is changed from 0.2 to 2 times of that of the impinging jet. A naphthalene sublimation method is used to determine the local heat/mass transfer coefficients on the upward facing surface of the effusion plate. With the initial crossflow, the heat/mass transfer rates on the effusion (target) plate decrease as the velocity of crossflow increases, since the crossflow induces the locally low transfer regions formed at the mid-way between the effusion holes. However, the impingement/effusion cooling with crossflow presents higher heat/mass transfer rates than the array jet impingement cooling with the same initial crossflow.

• The Bulletin of The Korean Astronomical Society
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• v.43 no.1
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• pp.45.2-45.2
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• 2018

CS molecule is known to be adsorbed onto dust in cold dense cores, causing its significant depletion in the center region of cores. This study is aimed to investigate the depletion of CS molecule with optically thin $C^{34}S$ molecular line observations, including significance of its differentiation depending on the evolutionary status of the dense cores. We mapped five evolved starless cores, L1544, L1552, L1689B, L694-2 and L1197 using two molecular lines, $C^{34}S$ (J=2-1) and $N_2H^+$ (J=1-0) with NRO 45 m telescope. The $H^2$ column density and temperature structures of each targets were obtained by SED fitting for Herschel continuum images and the internal number density profiles by model fitting. All of the integrated intensity maps of $C^{34}S$ show depletion holes and 'semi-ring-like' distribution, indicating that the depletion of CS is clear and general. The radial profiles of CS abundance also show significant decrease towards the core center, while $N_2H^+$ abundance is almost constant or enhanced. We find that the more evolved cores with higher $H^2$ density tend to have a stronger depletion of CS. Our data strongly support claims that CS molecule generally depletes out in the central regions of starless dense cores and such chemical differentiation is closely related to their evolution.

• Macromolecular Research
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• v.11 no.5
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• pp.357-367
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• 2003

The linear low density polyethylene (LLDPE)/zeolite composite using novel inorganic filler, zeolite, is prepared by a conventional compounding procedure using a twin-screw extruder. The observed scanning electron microscopic (SEM) morphology shows a good dispersion and adhesion of zeolite in the LLDPE matrix. The mechanical properties in terms of the Young's modulus, the yield stress, the impact strength, and the elongation at break were enhanced with a successive increment of zeolite content up to 40 wt%. The X-ray diffraction measurement is of supportive for the improved mechanical properties and the complex melt viscosity is as well. Upon applying a certain level of strain on the composites, the dewetting, the air hole formation and its growth are characterized. The dewetting originates around the filler particles at low strain and induces elliptical micropores upon further stretching. The microporosity such as the aspect ratio, the number and the total area of the air holes is also characterized. Thus, the composites loaded 40 % zeolite and 300 % elongation may be applicable for breathable microporous films with improved modulus, impact and yield stress, elongation at break, microporosity and air hole properties.

• Animal cells and systems
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• v.5 no.1
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• pp.45-50
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• 2001

Rana ovarian follicles consist of oocyte, vitelline envelope, granulosa cells, and theca/epithelial layer. Using scanning electron microscopy, the surface structure of each follicular component was investigated. Changes in oocyte surface during oocyte maturation were also examined. Theca/epithelial layer was almost transparent and some blood vessels and granulosa cells were observed underneath in intact follicle. The number of granulosa cells was estimated to be 6700-7200 per oocyte. The granulosa cells partially overlapped each other and their microvilli penetrated the vitelline membrane via holes present in the vitelline envelope and seemed to be linked to oocyte microvilli. After removal of the vitelline envelope by microforcep, oocyte microvilli were observed on the surface of the devitellined oocyte. The oocyte microvilli formed partial clusters on the surface of white spot area which appears iust before germinal vesicle breakdown (GVBD), whereas they were evenly distributed in other areas. The microvilli became shorter and less dense with oocyte maturation. The lengths of oocyte microvilli in the immature and mature oocyte were 1.5 $\mu$m and 0.6 $\mu$m, respectively. The present study suggests a fundamental structural change occurring on the oocyte surface during maturation.

• Journal of Institute of Control, Robotics and Systems
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• v.9 no.5
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• pp.398-406
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• 2003

In modular fixturing systems, a given part or a subassembly is fixed by placing fixture elements such as locators and clamps adequately on a plate with a lattice of holes. It is known that the minimal number of point contacts to restrict translational and rotational motions on a plane is four and the type of three locators and a clamp(3L/1C) is the minimal future. Brost and Goldberg developed the complete algorithm to automatically synthesize 3L/1C types of fixtures which satisfy the condition of form closure. Due to the nature of the fixture, the clearance between the fixture and the part is extremely small. It is hard to load the part repeatedly and accurately for human as well as for robot. However the condition of loadability has not been taken into account in the B&G algorithm. In this paper, a new method to decide a given fixture to be loadable by using configuration space is proposed. A method to plan for a part to be loaded by using compliance safely even in the presence of control and sensing uncertainty is proposed is well.

• Proceedings of the Materials Research Society of Korea Conference
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• 2011.05a
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• pp.239.1-239.1
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• 2011

Dye sensitized solar cells (DSC) are promising devices for inexpensive, nontoxic, transparent, and large-scale solar energy conversion. Generally thick $TiO_2$ nanoporous films act as efficient photoanodes with their large surface area for absorbing light. However, electron transport through nanoparticle networks causes the slowdown and the loss of electron transport because of a number of interparticle boundaries inside the conduction path. We have studied DSCs with precisely dimension-controlled $TiO_2$ nanotubes array as photoanode. $TiO_2$ nanotubes array is prepared by template-directed fabrication method with atomic layer deposition. Well-ordered nanotubes array provides not only large surface area for light absorbing but also direct pathway for electrons with minimalized grain boundaries. Large enlongated anatase grains in the nanotubes could enhance the conductivity of electrons, but also suppress the recombination with holes through defect sites during diffusion into the electrode. To study the effect of grain boundaries, we fabricated two kinds of nanotubes which have different grain sizes by controlling deposition conditions. And we studied electron conduction through two kinds of nanotubes with different grain structures. The solar cell performance was studied as a function of thickness and grain structures. And overall solar-to-electric energy conversion efficiencies of up to 7% were obtained.

• Journal of The Korean Astronomical Society
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• v.29 no.spc1
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• pp.119-122
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• 1996

The case for a massive black hole in the center of the Galaxy is reassessed using improved modeling techniques and observational data. A dark mass of ${\~}{\times} 10^6$ Mo is present within 0.2 pc of the Galactic center. However, the available data can be modeled, without appealing to a massive black hole, using an extended distribution of dark stellar remnants (neutron stars and stellar mass black holes) provided that the stellar initial mass function in the central parsec is deficient in stars less massive than $\~$1 Mo. Such a situation may be a natural consequence of repeated gas build-up followed by starbursts in the central region. A clear distinction between this and the massive central black hole model cannot be made using red giant tracers outside 0.2 pc due to uncertainties in the radial velocity dispersion distribution. The cluster of massive early-type emission-line stars in the central parcsec more effectively probe the mass distribution close to Sgr A $\ast$, but their small number and partial rotational support complicate mass determinations. Proper motion determinations for stars within 0.5' of Sgr A$\ast$ may be the most effective means of unambiguously determining the mass distribution in the immediate vicinity of the Galactic center.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.7
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• pp.1851-1865
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• 1994

The objective of the present study is to predict the film cooling effectiveness by a row of holes at various injection ratios and injection angles. Numerical calculations have been performed to investigate the characteristics of flow and temperature distributions in a region near the down-stream of injection hole including the region of adverse pressure gradient. The elliptic turbulent 3-dimensional governing equations with variable thermal properties using the low-Reynolds number k-$\bar{varepsilon}$ model was solved by SIMPLE algorithm. The results showed that the presence of adverse pressure gradient and secondary vortex in the region near the downstream of injection hole induces large temperature gradent. The $45^{\circ}$ injection has higher averaged film cooling effectiveness than $60^{\circ}$ injection. But neverthless the $90^{\circ}$ injection has greater deviation from a flat plate than $45^{\circ}$ and $60^{\circ}$ injection, the $90^{\circ}$ injection has higher averaged film cooling effectiveness than $45^{\circ}$ and $60^{\circ}$ injection in the region near the downstream of injection hole.