• Proceedings of the Korea Contents Association Conference
• /
• 2003.11a
• /
• pp.367-372
• /
• 2003

In the framework of Lorentzian warped products, we study the Friedmann - Robertson - Walker cosmological model to investigate non-smooth curvatures associated with multiple discontinuities involved in the evolution of the universe. In particular we analyze non-smooth features of the spatially flat Friedmann-Robertson-Walker universe by introducing double discontinuities occurred at the radiation- matter and matter-lambda phase transitions in astrophysical phenomenology.

• Journal of the Korea Institute of Military Science and Technology
• /
• v.23 no.2
• /
• pp.125-138
• /
• 2020

One of the most important steps to consider in utilizing micro-satellites for surveillance or reconnaissance operations is the design of the satellite constellation. The Walker-Delta constellation which is commonly used in designing satellite constellations is not ideal for this operation in which military satellites are required to monitor specific regions continuously in a stable manner. This study aims to discuss the methodology for designing a satellite constellation that is capable of monitoring the fixed region at the fixed time each day by using the Sun synchronous Orbit. The BB(Beach Ball) constellation that we propose outperforms the Walker-Delta constellation in terms of robustness and it holds the merit of being simple in its design, thereby making future expansions more convenient. We expect the BB constellation will have a high applicability as the operational concept of military surveillance satellites is established in the near future.

• Journal of the Korean Applied Science and Technology
• /
• v.40 no.2
• /
• pp.342-347
• /
• 2023

The purpose of this study was to determine the comparison of upper arm, trunk and core muscle activities according to different performance in high Plank exercise using weightless exercise equipment. Six males(age, 23.00±0.73 years; height, 172.95±2.05 cm; body mass, 66.83±2.75 kg; and BMI, 22.33±0.72 kg/m²) completed this study as the subjects. Four type's high Plank exercises using weightless exercise equipment were performed(high plank, HP; high plank with air walker, HPAW; high plank with surf board, HPSB; high plank with waist trainer, HPWT). For the EMG analysis, we measured the body muscle activities of right side on the deltoid(DT), triceps brachii(TB), latissimus dorsi(LD), and external oblique(EO). This research's results were as follows. DT, TB, LD, & EO muscle activities were greatest during HPSB(p=.000). Therefore, these results are expected to serve as basic data for high Plank exercise using weightless exercise equipment performance applications in effective exercise programs.

• International Journal of Aeronautical and Space Sciences
• /
• v.15 no.3
• /
• pp.320-334
• /
• 2014

Over the last two decades, numerous experimental and numerical efforts have examined physical phenomena in plasma discharge devices. The physical mechanisms that govern the anomalous electron diffusion from the cathode to the anode in the Hall Effect Thruster (HET) are not fully understood. This work used 1-D numerical method to improve our understanding and gain insight into the effect of the anomalous electron diffusion in the HET. To this end, numerical solutions are compared with various experimental HET performance measurements and the effects of anomalous electron diffusion are analyzed. The relationships between the anomalous electron diffusion and important parameters of the HET are also studied quantitatively. The work identifies the cathode mass flow rate fraction, radial magnetic field distribution, and discharge voltage as significant factors that affect anomalous electron diffusion. Additionally, the study demonstrates a computational process to determine the radial magnetic field distribution required to achieve specific thruster performance goals.

• Korean journal of applied entomology
• /
• v.56 no.2
• /
• pp.203-212
• /
• 2017

Thirty-three species of insect pests including of two-spotted spider mite, Tetranychus urticae Koch, and air-breathing land snail, Acusta despecta Sowerby, were observed on mulberry for producing mulberry fruits in Jeonbuk province from 2015 to 2016. Of these, 8 species of insect pests, including of mulberry thrips, Pseudodendrothrips mori Niwa, are regarded as major insect pests. Of the 25 species observed before harvesting, 6 species-P. mori, Anomoneura mori, Pseudaulacaspis pentagona, Baris deplanata, Menophra atrilineata and T. urticae-are regarded as major insect pests. In contrast, of the 20 species observed after harvesting, 5 species-P. mori, P. pentagona, Glyphodes pyloalis Walker, Hyphantria cunea Drury and T. urticae-are regarded as major insect pests. Eight of the major insect pest species and nine other insect pest species-including Hexcacentrus unicolor Serville, Malcus japonicus Ishihara et Hasegawa, Halyomorpha halys stal, Xylosandrus germanus Blandford, four species of Repidoptera (Bombyx mandarina Moore, Euproctis subflava Bremer, Acronicta major Bremer, Agrotis trifurca Butler), and Acusta despecta Sowerby-were observed at all sites studied, although occurrence of the insect pests observed on mulberry varied from site to site.

• Journal of Aerospace System Engineering
• /
• v.16 no.2
• /
• pp.63-72
• /
• 2022

As interest in microsatellites increases, research has been actively conducted recently on the performance and use, as well as the orbital design and deployment techniques, for the microsatellite constellations. The purpose of this study was to investigate orbital deployment techniques using thrust and differential atmospheric drag control (DADC) for the Walker-delta constellation. When using thrust, the time and thrust required for orbital deployment vary, depending on the separation speed and direction of the satellite with respect to the launch vehicle. A control strategy to complete the orbital deployment with limited performance of the propulsion system is suggested and it was analyzed. As a result, the relationship between the deployment period and the total thrust consumption was derived. It takes a relatively longer deployment time using differential air drag rather than consuming thrusts. It was verified that the satellites can be deployed only with differential air drag at a general orbit of a microsatellite constellation. The conclusion of this study suggests that the deployment strategy in this paper can be used for the microsatellite constellation.

• Journal of Korean Society of Occupational and Environmental Hygiene
• /
• v.25 no.1
• /
• pp.72-81
• /
• 2015

Objectives: The aim of this study was to investigate the exposure and risk assessment of residents near asbestos mines in Korea. Methods: To assess asbestos types and airborne concentrations, air monitoring was performed in the neighborhoods of Kwangcheon (KC) and Sinsuk (SS) mines, which were leading South Korean mines in the past. In addition, activity-based-sampling (ABS) of residents' particular activities were conducted in order to estimate the Excess Lifetime Cancer Risks (ELCRs) for the residents. Conclusions: The average concentration of airborne asbestos in KC was 0.0014 f/cc and 0.0015 f/cc by PCM and TEM, respectively. In SS it was equal at 0.0012 f/cc by PCM and TEM. No statistically significant difference was found in the average concentration of airborne asbestos between the two mines. The average asbestos concentration of ABS was 0.0048 f/cc (PCM) and 0.0042 f/cc (TEM) in KC, while it was 0.0137 f/cc (PCM) and 0.0125 f/cc (TEM) in SS. It was found that the average asbestos concentration of ABS in SS was statistically significantly higher than that of KC (p<0.01). The results of ELCRs by scenario in KC showed that the scenarios of bicycle, car, weed control, weed whacking, child playing in the dirt, and physical training fell within $1{\times}0^{-6}-1{\times}10^{-4}$, which is the acceptable range of ELCR. The scenarios of motorcycle, walker, digging, and field sweeping, however, exceeded the acceptable range. In SS, only the scenario of car fell within the acceptable range, while all of the other scenarios exceeded the acceptable range.