• Composites Research
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• v.20 no.4
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• pp.42-50
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• 2007

This paper describes the results of experiments and numerical simulation studies on the low-velocity impact damage of two different sandwich composite panels for application to bodyshell and floor structure of the BIMODAL tram vehicle. Square test samples of 100mm sides were subjected to low-velocity impact loading using an instrumented testing machine at four impact energy levels. Part of this work presented is focused on the finite element analysis of low-velocity impact response onto a sandwich composite panels. It is based on the application of explicit finite element (FE) analysis codes LS-DYNA 3D to study the impact response of sandwich structures under low-velocity impact conditions. Material testing was conducted to determine the input parameters for the metallic and composite material model, and the effective equivalent damage model for the orthotropic honeycomb materials. Numerical and experimental results showed a good agreement for damage area and the depth of indentation of sandwich composite panels created by the impact loading.

• Journal of Energy Engineering
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• v.29 no.2
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• pp.1-9
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• 2020

In recent days, fuel cell has received attention from the world as an alternative power source to hydrocarbon used in automobile engines. With the industrial advances of fuel cell, There have been a lot of researches actively conducted to find a way of generating hydrogen. Among many hydrogen production methods, Solid Oxide Electrolysis Cell(SOEC) is not only a basic way but also environment-friendly method to produce hydrogen gas. Solid Oxide Electrolysis Cell has lower electrical energy demands and high thermal efficiency since it is possible to operate under high temperature and high pressure conditions. For these reasons, experimental researches as well as studies on numerical modeling for Solid Oxide Electrolysis Cell have been under way. However, studies on numerical modeling are relatively less enough than experimental accomplishments and have limited performance prediction, which mostly is considered as a result from inadequate effects of electrochemical properties by temperature and pressure. In this study, various experimental studies of commercial Membrane Electrode Assembly (MEA) composed of Ni-YSZ (40wt%, Ni-60 wt% YSZ)/8-YSZ (TOSOH, TZ8Y)/LSM (La_0.9Sr_0.1MnO₃) was utilized for improving effectiveness of SOEC model. After numerically analyzing effects of electrochemical properties according to operating temperature, causing the largest deviation between experiments and simulation are that Charge Transfer Coefficient (CTC), exchange current density, diffusion coefficient, electrical conductivity in SOEC. Analyzing temperature effect on parameter used in overpotential model is conducted for modeling of SOEC. cross-validation method is adopted for application of various MEA and evaluating feasibility of model. As a result, the study confirm that the numerical model of SOEC based on structured process of effectiveness evaluation makes performance prediction better.

• Applied Chemistry for Engineering
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• v.24 no.6
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• pp.587-592
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• 2013

In this study, we prepared pelletized porous carbon adsorbent (PCA) according to the different pyrolysis temperature using activated carbon and polyvinyl alcohol (PVA) as a binder for the removal of toluene, which is one of the representative volatile organic compounds (VOCs). We investigated physical characteristics of PCA using FE-SEM, BET, TGA and evaluated their adsorption capacity for toluene using GC. It was confirmed that the formability of pellets composed of the activated carbon, PVA and solvent of mass mixing ratio was 1 : 0.2 : 0.8 was the most outstanding. Toluene adsorption capacity was evaluated by measuring the maximum time when more than 99% of toluene adsorbed on the pellet. The specific surface area of the adsorbent pyrolyzed at $300^{\circ}C$ was measured as 4.7 times in $941.9m^2/g$ compared to that of the unpyrolyzed pellet. Micropore volume and toluene adsorption capacity of PCA increased fivefold to be 0.30 cc/g and thirteenfold to be 26 hours compared to that of the unpyrolyzed pellet, respectively. These results were attributed to the change of pore size and specific surface area due to the PVA content and the different pyrolysis temperature.

• Korean Journal of Construction Engineering and Management
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• v.16 no.2
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• pp.46-53
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• 2015

Electrical construction cost index is composed of the cost of albor and material. The producer price index is used to the cost of material. The Bank of Korea restructured the formation method and the basic period of the producer price index in 2013. Because fixed-weighted method can't faithfully reflect industrial structure changes. The weighted value and price index of fixed-weighted method is fixed on the basicp eriod. Electrical construction cost index is changed from fixed-weighted method to chain-weighted method in september 2014, because of these on the need. But the change of organization in formation method changes the weighted value. So there is the need of analysis about the statistical continuity of electrical construction cost index. This study is focused on the time series analysis between fixed-weighted and chain-weighted electrical construction cost index. We uses unit root test, cointegration test, regression analysis of long and short term equation, fitness for the estimation of static forecast as time series analysis. We verify that chain-weighted electrical construction cost index can be replaced to fixed-weighted construction cost index accounting analyses result. So users of it recognize that chain-weighted electrical construction cost index has statistical continuity.

• Journal of the Korean Regional Science Association
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• v.39 no.1
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• pp.53-66
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• 2023

Due to COVID-19, the external activities of urban residents have greatly shrunk, causing a lot of damage to the commercial district, such as a decrease in population and sales. The downturn in commercial districts means the collapse of the infrastructure of the national economy, and can have serious side effects on the local economy and individual lives. Therefore, it is necessary to look at the alley commercial area, which is closely related to the national local economy, and pay attention to the damage and stagnation of the alley commercial area where small business owners are concentrated. The purpose of this study is to classify alley commercial districts into growth commercial districts and decline commercial districts by using commercial sales time series data and DTW time series group analysis for the pre- and post-COVID-19 period. The main findings of the study are as follows. First, using the time series data on commercial sales before and after COVID-19, the alley commercial districts were divided into growth commercial districts and decline commercial districts, and it was confirmed that the distribution of growth commercial districts and decline commercial districts was regionally different. Therefore, it is necessary to actively manage commercial districts in areas where many declining commercial districts are distributed, and it is required to prepare policies for each region in consideration of the spatial distribution of declining commercial districts. Second, during the COVID-19 period, face-to-face essential industries, density of guest facilities, and population density negatively affected the sustainability of commercial districts, which is the opposite of previous studies. This is the result of empirically confirming the specificity of the COVID-19 period and the negative effects of the integrated economy, and can be used as basic data for effective commercial district management and policy preparation in the event of a national disaster in the future. Third, the characteristics of the background of the commercial district had a significant effect on the sustainability of the commercial district, and the negative effect of the attracting facilities inducing population concentration in the background area was found. This suggests that it is necessary to consider the characteristics of the background as well as the inside of the commercial district when establishing policies to revitalize the commercial district and support small business owners in a national disaster situation.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.15 no.1
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• pp.86-98
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• 2014

This study examined a long-standing issue with its perverse results in the Korean capital markets, such as any variant financial profiles over time, affecting capital structure for the firms belonging to the chaebols. It may be of interest to identify these components from the perspectives of international investors and domestic policy makers to implement their contingent strategies on the target leverage, since the U.S. financial turmoils in the late 2000s. Regarding the evidence from the three hypothesis tests on the firms in the chaebols, this research found that the control variabels measuring profitability, business risk, and non-debt tax shields, showed their statistically significant relationships with the different types of a debt ratio. While FCFF(free cash flow to the firm) showed its significant influence to discriminate between the firms in the chaebols and their counterparts, not belonging to the chaebols, BDRELY as the ratio of liabilities to total assets, comprising the enhanced 'Dupont' system, only showed its statistically significant effect on leverage in the context of the parametric and nonparametric tests. In line with the results obtained from the present research, one may expect that a firm in the Korean chaebol, may control or restructure its present level of capital structure to revert to its target optimal capital structure towards maximizing the shareholders' wealth.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2011.11a
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• pp.187-199
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• 2011

The main objective of this study was investigation of natural gas flames in a lean premixed swirl-stabilized dump combustor with an attention focused on the effect of the various fuel-air mixing section geometry on the combustion instability characteristics. The multi-channel dynamic pressure transducers were located on the combustor and inlet mixing section region to observe combustion pressure oscillation and difference phase at each dynamic pressure measurement results. Dynamic pressures were also measured to investigate characteristics of combustion at the same time. The combustor and mixing section length was varied in order to have different acoustic resonance characteristics from 800 to 1800 mm in combustor and 470, 550, 870 mm in mixing section. We observed two dominant instability frequencies in this study. Lower frequencies were obtained at lower equivalence ratio region and it was associated with a fundamental longitudinal mode of combustor length. Higher frequencies were observed in higher equivalence ratio conditions. It was related to secondary longitudinal mode of coupled with the combustor and mixing section. In this instability characteristics, pressure oscillation of mixing section part was larger than pressure oscillation of combustor. As a result, combustion instability was strongly affected by acoustic characteristics of combustor and mixing section geometry.

• Journal of the Korean Society of Propulsion Engineers
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• v.16 no.4
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• pp.57-69
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• 2012

The main objective of this study was investigation of natural gas flames in a lean premixed swirl-stabilized dump combustor with an attention focused on the effect of the various fuel-air mixing section geometry on the combustion instability characteristics. The combustor and mixing section length was varied in order to have different acoustic resonance characteristics from 800 to 1800 mm in combustor and 470, 550, 870 mm in mixing section. We observed two dominant instability frequencies in this study. Lower frequencies were associated with a fundamental longitudinal mode of combustor length. Higher frequencies were related to secondary longitudinal mode of coupled with the combustor and mixing section. As a result, combustion instability was strongly affected by acoustic characteristics of combustor and mixing section geometry.

• Proceedings of the Korea Water Resources Association Conference
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• 2023.05a
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• pp.362-362
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• 2023

한정된 수자원의 이용 및 관리로 매년 물 부족과 물 배분 의사결정 문제가 발생하고 있다. 50년간(1965~2014년) 수자원의 총량은 약 1.2배 증가한 반면 인구수 약 1.8배, 생·공·농업용수의 수요는 약 5배가 증가(국회입법조사처, 2018) 했을 뿐 아니라, 기후변화의 영향으로 인한 강수량의 변화와 지역별 편차가 커져 지속가능한 물관리 필요성이 증대되고 있다. 따라서 효율적인 물관리를 위해서는 관리부처가 분절되어 있는 물순환 계통의 데이터를 통합하는 것이 우선시되어야 하고 이를 통해 물순환 모니터링/평가/예측 기술을 개발할 수 있다. 본 연구에서는 생활용수 물순환 계통 통합 DB를 정의 및 구축하였다. 도시의 관점에서 물순환 시스템을 순차적으로 물 유입(수원~취수장)/전달(정수장~급수지역)/유출(하(폐)수처리장~방류구)의 개념으로 설정하고 DB정의서를 마련하였다. 연구대상지는 가뭄이 장기화가 되고 있는 전라남도중 물순환 계통이 비교적 단순한 네트워크로 형성되어 있는 함평군 도시지역으로 선정하였다. 연구 기간은 총 5년(2017년 1월 1일~2021년 12월 31일)이고 일 단위 실계측자료 위주의 원자료를 구축하였다. 이를 이상치 탐지, 제거, 대체의 과정을 거쳐 품질 보정하고 정제된 시계열 자료에 대한 특성 분석을 하였다. 그 결과, 물순환 계통 내 주요 지점 간의 상관관계 및 지연시간을 통한 물흐름의 시계열적 특성을 파악할 수 있었으며 모형의 적합도를 판단하는 데 활용되는 통계량과 유의미하지 않은 잔차의 자기상관성을 볼 때 물 유입-전달-유출의 단기 예측을 위한 ARIMA(Auto-regressive Integrated Moving Average) 모형의 구축도 가능할 것으로 판단되었다. 다만 여름철 발생하는 방류량의 첨두값을 설명하기 위해서는 강우에 의한 불명수 발생으로 증가하는 방류량을 묘사할 수있어야 하므로 향후에는 물순환계통 외 해당 지역의 불명수(강우 효과)도 하수 방류량의 주요 입력 요인으로 추가 검토할 필요가 있다.

• The Korean Journal of Applied Statistics
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• v.6 no.2
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• pp.191-200
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• 1993

This study introduced speical characteristics of an epidemiologic study on blood pressure and compared several statistical methods for evaluating the tracking phenomenon of blood pressure for Korean children. While correlation coefficients adjusted for measurement error are commonly used for the evaluation of tracking, it is hard to interpretate the results when correlation functions for lag-difference are not monotonous. McMahan defined a tracking as maintenance of relative rank over time and calculated tracking index usng growth curve model. The tracking index in McMahan's model is complicate to calculate, and it is hard to determine the degree of growth curve parameter. Blomqvist showed the relationship between the rate of change and the initial value. This concept could be extended for the evaluation of tracking. However, it is not so easy to interpretate the estimates in his model when those are non-positive.