• Korean Journal of Materials Research
• /
• v.21 no.11
• /
• pp.604-610
• /
• 2011

The effect of $BaF_2$ flux in $Y_3Al_5O_{12}:Ce^{3+}$(YAG:Ce) formation was investigated. Phase transformation of $Y_3Al_5O_{12}$(YAG) was characterized by using XRD, SEM, and TEM-EDS, and it was revealed that the sequential formation of the $Y_4Al_2O_9$(YAM), $YAlO_3$(YAP) and $Y_3Al_5O_{12}$(YAG) in the temperature range of 1000-1500$^{\circ}C$. Single phase of YAG was revealed from 1300$^{\circ}C$. In order to find out the effect of $BaF_2$ flux, three modeling experiments between starting materials (1.5$Al_2O_3$-2.5$Y_2O_3$, $Y_2O_3$-$BaF_2$, and $Al_2O_3$-$BaF_2$) were done. These modeling experiments showed that the nucleation process occurs via the dissolution-precipitation mechanism, whereas the grain growth process is controlled via the liquid-phase diffusion route. YAG:Ce phosphor particles prepared using a proposed technique exhibit a spherical shape, high crystallinity, and an emission intensity. According to the experimental results conducted in this investigation, 5% of $BaF_2$ was the best concentration for physical, chemical and optical properties of $Y_3Al_5O_{12}:Ce^{3+}$(YAG:Ce) that is approximately 10-15% greater than that of commercial phosphor powder.

• Journal of Institute of Control, Robotics and Systems
• /
• v.13 no.10
• /
• pp.962-969
• /
• 2007

In this paper, an approach is developed for relative state estimation of satellite formation flying. To estimate relative states of two satellites, the Extended Kalman Filter Algorithm is adopted with the relative distance and speed between two satellites and attitude of satellite for measurements. Numerical simulations are conducted under two circumstances. The first one presents both chief and deputy satellites are orbiting a circular reference orbit around a perfectly spherical Earth model with no disturbing acceleration, in which the elementary relative orbital motion is taken into account. In reality, however, the Earth is not a perfect sphere, but rather an oblate spheroid, and both satellites are under the effect of $J_2$ geopotential disturbance, which causes the relative distance between two satellites to be on the gradual increase. A near-Earth orbit decays as a result of atmospheric drag. In order to remove the modeling error, the second scenario incorporates the effect of the $J_2$ geopotential force, and the atmospheric drag, and the eccentricity in satellite orbit are also considered.

• The Bulletin of The Korean Astronomical Society
• /
• v.40 no.2
• /
• pp.30.1-30.1
• /
• 2015

Radio relics are diffuse radio sources found in the outskirts of galaxy clusters and they are thought to trace synchrotron-emitting relativistic electrons accelerated at shocks. We explore a diffusive shock acceleration (DSA) model for radio relics in which a spherical shock with the parameters relevant for the Sausage radio relic in cluster CIZA J2242.8+5301 impinges on a magnetized cloud containing fossil relativistic electrons. This model is expected to explain some observed characteristics of giant radio relics such as the relative rareness, uniform surface brightness along the length of thin arc-like radio structure, and spectral curvature in the integrated radio spectrum. We find that the observed surface brightness profile of the Sausage relic can be explained reasonably well by shocks with speed $u_s{\sim}3{\times}10^3km/s$ and sonic Mach number $M_s{\sim}3$. These shocks also produce curved radio spectra that steepen gradually over $(0.1-10){\nu}_{br}$ with a break frequency ${\nu}_{br}{\sim}1GHz$, if the duration of electron acceleration is ~60-80 Myr. However, the abrupt increase in the spectral index above ~1.5 GHz observed in the Sausage relic seems to indicate that additional physical processes, other than radiative losses, operate for electrons with the Lorentz factor, ${\gamma}_e$ > $10^4$.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
• /
• 2005.05a
• /
• pp.150-153
• /
• 2005

The rifle impact of human body is affected by geometry of human for rifling. The interaction of human-rifle system influence a firing accuracy. In this paper, impact analysis of human model for standing postures with two B.C. carried out. ADAMS code and LifeMOD is used in impact analysis of human model and modeling of the human body, respectively. On the shooting, human model is affected by rifle impact during the 0.001 second. Also, Because Human Natural frequency is 5-200Hz, human impact is considered during 0.2-0.005 sec. Dut to the Firng test, Performed simulation time for shooting is 0.1 second. Applied constraint condition to human-rifle system is rotating and spherical condition. Also, The resulrt of changin the position of the grip is dfferent from the each other. As the results, The human model of firing was built successfully.

• Proceedings of the KSRS Conference
• /
• 2005.10a
• /
• pp.589-592
• /
• 2005

Gravity characteristics and Moho undulations are investigated in the Korean peninsula by using satellite gravity data. According to the development of satellite geodesy, gravity potential models which have high accuracy and resolution were released. Using the EIGEN-CGOIC model based on low orbit satellite data such as CHAMP and GRACE, geoid and gravity anomaly were calculated by spherical harmonic analysis. The study area is located at $123^{\circ}\sim132^{\circ}E, 33^{\circ}\sim43^{\circ}$N including Korea. Free-air anomalies, which show the effect of terrain, have the values between $-37\sim724 mgal. After Bouguer correction, the range of simple Bouguer anomalies is $-221\sim246$ mgal. Complete Bouguer anomalies after terrain correction increase from continent to marine. This phenomenon is related rise of Moho discontinuity. The cut-frequency for extraction of Moho undulation was determined by power spectrum analysis, and then 3D inversion modeling was implemented. The mean, maximum, minimum, and standard deviation of Moho depth undulation are -26, -36, -8, and 4.9 krn, respectively.

• International Journal of Aeronautical and Space Sciences
• /
• v.15 no.3
• /
• pp.241-257
• /
• 2014

The modern development in design of airships and aerostats has led to unconventional configurations quite different from the classical ellipsoidal and spherical ones. This new class of air-vehicles presents a mass-to-volume ratio that can be considered very similar to the density of the fluid displaced by the vehicle itself, and as a consequence, modeling and simulation should consider the added masses in the equations of motion. The concept of added masses deals with the inertia added to a system, since an accelerating or decelerating body moving into a fluid displaces a volume of the neighboring fluid. The aim of this paper is to provide designers with the added masses matrix for more than twenty Lighter Than Air vehicles with unconventional shapes. Starting from a CAD model of a given shape, by applying a panel-like method, its external surface is properly meshed, using triangular elements. The methodology has been validated by comparing results obtained with data available in literature for a known benchmark shape, and the inaccuracies of predictions agree with the typical precision required in conceptual design. For each configuration, a CAD model and a related added masses matrix are provided, with the purpose of assisting the practitioner in the design and flight simulation of modern airships and scientific balloons.

• Journal of Institute of Control, Robotics and Systems
• /
• v.18 no.11
• /
• pp.989-996
• /
• 2012

The purpose of this paper is to analyze GPS (Global Positioning System) satellite orbital mechanics, and then to propose a novel long-term GPS satellite orbit prediction scheme including virtual planet perturbation. The GPS orbital information is a necessary prerequisite to pinpointing the location of a GPS receiver. When a GPS receiver has been shut down for a long time, however, the time needed to fix it before its reuse is too long due to the long-standing GPS orbital information. To overcome this problem, the GPS orbital mechanics was studied, such as Newton's equation of motion for the GPS satellite, including the non-spherical Earth effect, the luni-solar attraction, and residual perturbations. The residual perturbations are modeled as a virtual planet using the least-square algorithm for a moment. Through the modeling of the virtual planet with the aforementioned orbital mechanics, a novel GPS orbit prediction scheme is proposed. The numerical results showed that the prediction error was dramatically reduced after the inclusion of virtual planet perturbation.

• Proceedings of the Korean Society of Propulsion Engineers Conference
• /
• 2008.05a
• /
• pp.187-192
• /
• 2008

A simplified model for an isolated aluminum particle burning in air is presented. Burning process consists of two stages, ignition and quasi-steady combustion (QSC). In ignition stage, aluminum which is inside of oxide film melts owing to the self heating called heterogeneous surface reaction (HSR) as well as the convective and radiative heat transfer from ambient air until the particle temperature reaches melting point of oxide film. In combustion stage, gas phase reaction occurs, and quasi-steady diffusion flame is assumed. For simplicity, 1-dimesional spherical symmetric condition and flame sheet assumption are also used. Extended conserved scalar formulations and modified Shvab-Zeldovich functions are used that account for the deposition of metal oxide on the surface of the molten aluminum. Using developed model, time variation of particle temperature, masses of molten aluminum and deposited oxide are predicted. Burning rate, flame radius and temperature are also calculated, and compared with some experimental data.

• Journal of ILASS-Korea
• /
• v.11 no.2
• /
• pp.89-97
• /
• 2006

An inviscid axisymmetric model capable of predicting droplet bouncing and the detailed pre-impact motion, influenced by the ambient pressure, has been developed using boundary element method (BEM). Because most droplet impact simulations of previous studies assumed that a droplet was already in contact with the impacting substrate at the simulation start, the previous simulations could not accurately describe the effect of the gas compressed between a failing droplet and the impacting substrate. To properly account for the surrounding gas effect, an effect is made to release a droplet from a certain height. High gas pressures are computationally observed in the region between the droplet and the impact surface at instances just prior to impact. The current simulation shows that the droplet retains its spherical shape when the surface tension energy is dominant over the dissipative energy. When increasing the Weber number, the droplet surface structure is highly deformed due to the appearance of the capillary waves and, consequently, a pyramidal surface structure is formed; this phenomenon was verified with our experiment. Parametric studies using our model include the pre-impact behavior which varies as a function of the Weber number and the surrounding gas pressure.

• Transactions of the Korean Society for Noise and Vibration Engineering
• /
• v.13 no.10
• /
• pp.743-750
• /
• 2003

The equivalent source method(ESM) is used for the calculation of the internal pressure field for an enclosure which can have arbitrary boundary conditions and nay include internal objects which scatter the sound field. The advantage of using ESM is that it requires relatively low computing cost and is easy to model the internal diffracting objects. Typical ESM modeling uses two groups of equivalent source positions. One group includes the first order images of the source inside the enclosure. The Positions of the other group are usually on a spherical surface some distance outside the enclosure. The normal velocity on the surfaces of the enclosure walls is evaluated at a larger number of positions than there are equivalent sources. The sum of the squared difference between this velocity and the expected is minimized by adjusting the strength of the equivalent sources. This study is on the optimal far field sources positions when using the equivalent source method. In general, the far field sources are evenly distributed on a surface of a virtual sphere which is centered at the enclosure with a sufficiently large radius. In this study. optimal far field source locations are searched using simulated annealing method for various radii of spheres where far field sources are located. Simulation results showed that optimally located sources with adequate distance away from the enclosure center gave better result than sources with even distribution even with a smaller number of far field sources.