• The Journal of Korean Institute of Communications and Information Sciences
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• v.36 no.5B
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• pp.573-581
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• 2011

The underwater channel is known to offer poor communications channel. The channel medium is highly absorptive and the transmission bandwidth is limited. In addition, the channel is highly frequency selective; the degree of selectiveness depends on a detailed geometry of the channel. Furthermore, the response changes over time as the channel conditions affecting the response such as water temperature, sea surface wind and salinity are time-varying. The transceiver design to deal with the frequency and time selective channel, therefore, becomes very challenging. It has been known that deep fading at certain specific sub-carriers are detrimental to OFDM systems. To mitigate this negative effect, the proposed coded OFDM system employs an LDPC code based modulation. In this paper, we aim 1) to provide a detailed underwater channel model; 2) to design a robust LDPC coded OFDM system; 3) to test the proposed system under a variety of channel conditions enabled by the channel model.

• Atmosphere
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• v.29 no.4
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• pp.501-509
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• 2019

Asian Dust is a meteorological phenomenon that sand particles are raised from the arid and semi-arid regions-Taklamakan Desert, Gobi Desert and Inner Mongolia in China-and transported by westerlies and deposited on the surface. Asian dust results in a negative effect on human health as well as environmental, social and economic aspects. For monitoring of Asian Dust, Korea Meteorological Administration operates 29 stations using a continuous ambient particulate monitor. Kim et al. (2016) developed an automatic quality check (AQC) algorithm for objective and systematic quality check of observed PM₁₀ concentration and evaluated AQC with results of a manual quality check (MQC). The results showed the AQC algorithm could detect abnormal observations efficiently but it also presented a large number of false alarms which result from valid error check. To complement the deficiency of AQC and to develop an AQC system which can be applied in real-time, AQC has been modulated. Based on the analysis of instrument status codes, valid error check process was revised and 6 status codes were further considered as normal. Also, time continuity check and spike check were modified so that posterior data was not referred at inspection time. Two-year observed PM₁₀ concentration data and corresponding MQC results were used to evaluate the modulated AQC compared to the original AQC algorithm. The results showed a false alarm ratio decreased from 0.44 to 0.09 and the accuracy and the probability of detection were conserved well in spite of the exclusion of posterior data at inspection time.

• Ocean Systems Engineering
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• v.3 no.4
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• pp.257-273
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• 2013

Top tensioned riser (TTR) is often used in a floating oil/gas production system deployed in deep water for oil/gas transport. This study focuses on the extension of the existing numerical code, known as CABLE3D, to allow for static and dynamic simulation of a TTR connected to a floating structure through a tensioner system or buoyancy can, and restrained by riser guides at different elevations. A tensioner system usually consists of three to six cylindrical tensioners. Although the stiffness of individual tensioner is assumed to be linear, the resultant stiffness of a tensioner system may be nonlinear. The vertical friction between a TTR and the hull at its riser guide is neglected assuming rollers are installed there. Near the water surface, a TTR is forced to move horizontally due to the motion of the upper deck of a floating structure as well as related riser guides. The extended CABLE3D is then integrated into a numerical code, known as COUPLE, for the simulation of the dynamic interaction among the hull of a floating structure, such as spar or TLP, its mooring system and riser system under the impact of wind, current and waves. To demonstrate the application of the extended CABLE3D and its integration with COUPLE, the numerical simulation is made for a truss spar under the impact of Hurricane "Ike". The mooring system of the spar consists of nine mooring lines and the riser system consists of six TTRs and two steel catenary risers (SCRs).

• Nuclear Engineering and Technology
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• v.24 no.1
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• pp.52-62
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• 1992

Radioactive nuclides deposited on out-of-core surface after the radiation in the core by the transport of corrosion products (CRUD) through the primary coolant system in PWR which is the major plant type in Korea, are leading sources of radiation exposure to plant maintenance personnel. Thus, the optimal chemistry operation method is required for the reduction of radiation exposure by the corrosion products. This study analysed the actual water chemistry operation data of four operating domestic PWRs. And in order to evaluate the coolant chemistry operation data, a computer code which can calculate the activity buildup in the various chemistry conditions of PWR coolant was employed. Through the analysis of comparison between the activity buildup of actual water chemistry operation mode and that of assumed Elevated Li operation mode calculated by the computer code, it was found that the out-of core radioactivity can be reduced by diminishing the deposition of corrosion products on the core in case that the Elevated Li operation mode is applied to the coolant chemistry operation of PWR. And the higher coolant pH operation was shown to have the advantage of the reduction of out-of-core activity buildup if the integrity of system structural materials and fuel cladding is guaranteed.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.39 no.7
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• pp.652-659
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• 2011

A computational system to predict flowfield and infrared signature in aircraft exhaust system is developed. As the first step, a virtual mission profile is considered and an engine is selected through a performance analysis. Then a nozzle that meets the requirement of each mission is designed. The internal flow in the exhaustion nozzle at the maximum thrust is analyzed using a state-of-the-art CFD code. In addition, a system to combine information of the skin temperature distribution of the nozzle and after-body surface with an infrared prediction code is developed. Finally, qualitative results for the infrared signature reduction design are obtained by investigating the infrared signature level under various conditions.

• Journal of Ocean Engineering and Technology
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• v.21 no.5
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• pp.55-60
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• 2007

Bellows are mechanical components which prevent the damage of system by absorption of the vibration and the displacement of axle and radial direction. Thermal piping system is expanded by the fluid of the high temperature from the heat engine inside. At this time, bellows prevent the damage of the piping due to the thermal expansion. Recently, design of bellows is required to fit some other operational environments which are not suggested in the E.J.M.A code book. And it is difficult to produce and to maintain bellows of high temperature and high pressure bemuse of its complicated shape and this causes the manufacturing cost to rise. The objective of this study is to determine optimum shape of bellows which can endure in the high temperature and high pressure. The maximum stress has an effect on the fatigue life of bellows, therefore it needs to be minimized. This study attempts to find a shape which minimizes the stress occurring in the bellows by the design of experiment. The model used in this study is not presented in the E.J.M.A code book, therefore, from the result of design of experiment we find the factors which give effects on the characteristic value and we presents the recession model using the RSM, which can predict the characteristic values depending on the change of factor values.

• Proceedings of the Korean Society of Medical Physics Conference
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• 2002.09a
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• pp.291-293
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• 2002

Accurate dose calculation in radiation treatment planning is most important for successful treatment. Since human body is composed of various materials and not an ideal shape, it is not easy to calculate the accurate effective dose in the patients. Many methods have been proposed to solve the inhomogeneity and surface contour problems. Monte Carlo simulations are regarded as the most accurate method, but it is not appropriate for routine planning because it takes so much time. Pencil beam kernel based convolution/superposition methods were also proposed to correct those effects. Nowadays, many commercial treatment planning systems, including Pinnacle and Helax-TMS, have adopted this algorithm as a dose calculation engine. The purpose of this study is to verify the accuracy of the dose calculated from pencil beam kernel based treatment planning system Helax-TMS comparing to Monte Carlo simulations and measurements especially in inhomogeneous region. Home-made inhomogeneous phantom, Helax-TMS ver. 6.0 and Monte Carlo code BEAMnrc and DOSXYZnrc were used in this study. Dose calculation results from TPS and Monte Carlo simulation were verified by measurements. In homogeneous media, the accuracy was acceptable but in inhomogeneous media, the errors were more significant.

• Fire Science and Engineering
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• v.25 no.5
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• pp.1-7
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• 2011

In this study, standards and test methods for building materials of domestic and foreign countries were compared and analyzed to propose evaluation methods for the fire-retardant performance of HANOK components (Traditional house). IBC and NFPA codes recently have been adopted in the US, and the properties such as critical heat flux, fire spread index and smoke density are being used as an evaluation reference. In Europe, the unified Euroclass has been adopted and the surface flammability, prototype fire test or cone calorimeter test are conducted for the performance test. Japan has the similar system as Korea where the class is classified into 3 grades. We tried to study a quantitative evaluation method of fire retardant performance for the HANOK components based on the analysis results of the several countries' standards and test methods for building materials.

• Journal of the Korean Institute of Gas
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• v.5 no.3 s.15
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• pp.1-8
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• 2001

This study was carried out for the purpose of determination of maintenance period and investigation of weak point due to freeze when the gas heater of KOGAS valve station Is not operated in winter season. 3-dimensional non-linear numerical simulation was conducted in order to predict the time and location which bath water in heater reaches to ice point. FLUENT V 5.0, commercial code, is used for thermal fluid flow analysis. We thought this was problem of heat conduction solving the energy equation and modeled gas heater by using the real geometry and scale for performing the 3-dimensional simulation. It was analyzed complex heat transfer phenomena considering convection due to air on surface, conduction in insulation material, natural convection of liquid in heater and heat loss through the pipe.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.25 no.7
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• pp.1139-1146
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• 2001

The reactor pressure vessel(RPV) is usually cladded with stainless steel to prevent corrosion and radiation embrittlement, and a number of subclad cracks have been found during an in-service-inspection. These subclad cracks should be assured for a safe operation under normal conditions and faulted conditions such as pressurized thermal shock(PTS). Currently available integrity assessment procedure for an RPV, ASME Code Sec. XI, are built on the basis of linear fracture mechanics (LEFM). In PTS condition, however, thermal stress and mechanical stress give rise to high tensile stress at the cladding and elastic-plastic behavior is expected in this area. Therfore, ASME Code Sec. XI is overly conservative in assessing the structural integrity under PTS condition. In this paper, the fracture parameter (stress intensity factor, K, and RT(sub)NDT) from elastic analysis using ASME Sec. XI and finite element method were validated against 3-D elastic-plastic finite element analyses. The difference between elastic and elastic-plastic analysis became significant with increasing crack depth. Therfore, it is recommended to perform elastic-plastic analysis for the accurate assessment of subclad cracks under TPS which causes plastic deformation at the cladding.