• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.20 no.2
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• pp.180-186
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• 2011

This paper describes a micro electrical discharge machining (MEDM) technique that uses an insulated tool in combination with ultrasonic vibration to drill micro holes. As the machining depth becomes deeper, the dispersion of debris and circulation of the dielectric fluid are difficult to occur. Consequently, machining becomes unstable in the machining region and unnecessary electrochemical dissolution and secondary discharge sparking occur at the tool side face. To reduce the amount of unnecessary side machining, an insulated tool was used. Ultrasonic vibration was applied to the MEDM work fluid to better remove debris. Through these methods, a $1000\;{\mu}m$ thick stainless steel plate was machined by using a $73\;{\mu}m$ diameter electrode. The diameters of the hole entrance and exit were $96\;{\mu}m$ and $88\;{\mu}m$, respectively. It took only 351s to completely drill one hole.

• Journal of the Korean Society of Visualization
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• v.18 no.2
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• pp.51-58
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• 2020

Research on shock structures of supersonic jet through visualization experiments in low-pressure environment have not been actively conducted. Therefore, in this study, shock waves and supersonic jets were analyzed and compared by numerical analysis and Schlieren technique at low-pressure. Schlieren technique is commonly used to visualize the shock waves generated by density gradient as interferometric methods. Pressure ratio of entrance and ambient was set around 4 to observe moderate under-expanded jet. For validation of experimental and numerical results, the shock structure and frequency were compared. In the case of ST and C nozzle, the results were shown that the difference of shock cell distance was within 10%. The Mach number gradually decreased due to energy reduction, and the error rate was within 7%. D nozzle was not fitted to be observing the shock structure. Because the interface between rarefaction fan and supersonic jet was ambiguous and oscillating phemenoma occurred at end of jet, the supersonic jet in low ambient pressure was observed and analyzed.

• Culinary science and hospitality research
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• v.14 no.3
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• pp.123-135
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• 2008

The purpose of this study is to investigate the priority order in the factors influencing college choice of students who are majoring in culinary art in Chungcheong-do. For the study, we set the decision-making factors of upper hierarchies and nineteen bottom hierarchies based on the literature review and employed the analytical hierarchical process(AHP). As a result, the first considering factor among 4 upper hierarchies for college choice was the educational environments of the department(0.378). The next came in the order named as follows: college and university grade(0.263), the educational environments of colleges or universities(0.244), recommendation(0.115). The first considering factor among the educational environments of the department was the aptitude for the major(0.323). The first considering factor in college and university grade was the entrance competitive rate(0.397). The first considering factors among the educational environments of the colleges or universities were scholarships and tuition fees(0.325). The first considering factor in recommendation was the recommendation of a high school teacher(0.295). This results of this study will contribute to the development of colleges and universities under the turbulent changes of educational environments.

• IEMEK Journal of Embedded Systems and Applications
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• v.9 no.3
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• pp.127-135
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• 2014

On lot of ships, Ballast Water Treatment Systems(BWTS) are in use, or the newly built ship should be mandatory installation by 2017. It is IMO(International Maritime Organization) regulations. Because the vessel is in long distance, It is managed this system only in the vessel and it may occur considerable cost of maintenance when a problem arises from BWTS. We describe in this paper how to monitor treated ballast water to make sure the ballast water and BWTS on a ship is in proper condition, but before the port entrance. In addition, analysing and forecasting trend signal to prevent failure. Also we can expect reduce the cost of BWTS maintenance. We can monitor the sensor data value's change which collected through satellite from BWTS. And we can observate the ballasting and de-ballasting state of the vessel easily.

• The Mathematical Education
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• v.49 no.3
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• pp.329-341
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• 2010

In this paper, we analyzed the freshmen's achievements on general mathematics their GPA based on 'basic mathematics diagonal test score'. Also, we studied the achievements of students who were not passed the 'Basic Mathematics Diagonal Test (BMDT)' and had to take supplementary lessons to improve their mathematics abilities four times a week during the first semester of academic year 2008 in Mokpo National University. Before taking college entrance exam, high school students had to choose two types of scholastic area. One is on 'Ga' or 'Na' in mathematics and the other is on Natural Science or Social Science. According to the types, we classified the freshman-Ga or Na and NS or SS. We found some facts. First, a few of Ga and NS freshmen had low score on the BMDT. Second, Na and NS freshman got higher score than Na and SS freshmen on the BMDT. Third, Ga and NS freshmen who passed the BMDT got higher score on the general mathematics than those who failed the BMDT. Finally, there are correlations between achievements of general mathematics and a curriculum of freshmen who were passed test after taking supplementary lessons.

• KIEE International Transactions on Electrophysics and Applications
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• v.3C no.3
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• pp.91-98
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• 2003

The importance of improving the quality of electric power is being strongly raised, owing to an increasing use of sensitive and small-sized electronic devices and systems. The transient over-voltages on low-voltage power distribution systems are induced by direct or indirect lightning return strokes. These can cause damage and/or malfunction of the utility systems for home automation, office automation, factory automation, medical automation, etc. The behaviors of lightning overvoltages transferred through the transformer to the low-voltage distribution systems using a Marx generator were experimentally investigated. Furthermore, the coupling mechanisms of lightning overvoltages transferred to the low-voltage systems were clearly illustrated through a theoretical simulation using a Pspice program. The overvoltages in low-voltage ac power systems are rarely limited by the application of the surge arrester to the primary side of the distribution transformer. A superior surge protection scheme is to install surge protection devices at the service entrance switchboard and/or at the load devices in TN power systems.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 2017.10a
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• pp.96-97
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• 2017

When a new student, a guest, or a student who first visited a school wants to know information about the school, he / she searches through a smartphone or a tablet. However, if you visit the homepage of the school, you do not know exactly where the information you want to find is located, and you spend a lot of time. In this paper, we develop a school guidance system using Alexa with speech recognition function. Divide the school guidance system into college introduction, major, college activities, and entrance information topics, and fill in the details by topic. In the lambda function of Amazon Web Services, we use Node.js to create information on topics and provide information to users by voice.

• Korea-Australia Rheology Journal
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• v.17 no.4
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• pp.157-164
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• 2005

Hydrodynamic characteristic of surfactant drag-reducing flows is still not fully understood. This work investigated the temperature and diameter effect on hydrodynamic characteristic of cationic surfactant drag reducing flows in pipes. Solution of oleyl bishydroxyethyl methyl ammonium chloride (Ethoquad O/12), 900 ppm, as a cationic surfactant and sodium salicylate (NaSal), 540 ppm, as a counter-ion was tested at 12, 25, 40, and $50^{\circ}C$ in pipes with diameter of 13, 25, and 40 mm. Drag reduction effectiveness of this surfactant solution was evaluated in 25 mm pipe from 6 to $75^{\circ}C$. Rheological characteristic of this solution was measured by stress control type rheometer with cone-and-plate geometry. Scale-up laws proposed by previous investigators were used to evaluate the flow characteristic of the solution. It was found that this surfactant solution has clear DR capability until $70^{\circ}C$. Result of this work suggested that temperature has a significant influence in changing the hydrodynamic entrance length of surfactant drag reducing flows. From rheological measurement, it was found that the solution exhibits Shear Induced Structure at all temperatures with different degree of peak viscosity and critical shear rate.

• The Journal of Natural Sciences
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• v.12 no.1
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• pp.103-110
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• 2002

The aim of this study was to analysis the landscape equipments of 42 school gardens in Taejon metropolitan city. Outdoor educational landscape equipments were found out at 10 schools out of 42 schools. The materials of main entrance were consisted of steel and stone at 19 schools. The stands at front of the main ground were existed at 21 schools. The pergolas and benches were existed at 20 schools. The living material fences were found out at 21 schools. Plant nameplates were found out at 22 schools. But the contents of the nameplates were so poor. The wetland and roof garden of the school were not existed. School landscape equipments should be introduced by the distinction of elementary school, middle school, and high school as well as by the space of the school site.

• The Bulletin of The Korean Astronomical Society
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• v.46 no.1
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• pp.38.3-38.3
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• 2021

We are developing an optomechanical design of infrared telescope for the CubeSat and Unmanned Aerial Vehicle (UAV) which adapts the Linear Astigmatism Free- Three Mirror System in the confocal off-axis condition. The small entrance pupil (diameter of 40 mm) and the fast telescope (f-number of 1.9) can survey large areas. The telescope structure consists of three mirror modules and a sensor module, which are assembled on the base frame. The mirror structure has duplex layers to minimize a surface deformation and physical size of a mirror mount. All the optomechanical parts and three freeform mirrors are made from the same material, i.e., aluminum 6061-T6. The Coefficient of Thermal Expansion matching single material structure makes the imaging performance to be independent of the thermal expansion. We investigated structural characteristics against external loads through Finite Element Analysis. We confirmed the mirror surface distortion by the gravity and screw tightening, and the overall contraction/expansion following the external temperature environment change (from -30℃ to +30℃).