• 한국지반공학회논문집
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• 제33권2호
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• pp.35-47
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• 2017

지반의 열적 거동은 대부분 열전도에 국한되어 연구가 진행되었으며, 자연대류 현상에 대한 연구는 매우 미비한 실정이다. 지반의 온도증가는 간극수의 밀도변화에 따른 부력을 유발하여 자연대류를 발생시키게 된다. 유체역학 관점에서 다공질 재료내의 자연대류 해석의 제약조건에 대하여 논의하고, 거시적 관점에서 완전 결합된 열-수리-역학적인 지배방정식을 이용한 대류현상에 대한 수치해석 기법을 제시하였다. 실내 열전도도 측정을 위한 탐침기 실험에 대한 수치실험은 자연대류를 무시하고 평가된 열전도도의 불확실성에 대하여 논의하고, 모델식과의 오류를 최소화하기 위한 적정한 실험조건을 제시하였다. 해저 전력선의 매설은 해저면 0.2m 깊이에서의 온도상승을 $2^{\circ}C$로 제한하고 있으나, 투수성이 큰 지반재료에 대한 수치해석결과는 기준온도를 초과하는 것으로 나타났다. 해저면의 온도와 열-수리-역학적 물성은 전력선의 매설설계에 중요한 설계인자이며 자연대류의 영향을 고려하여야 한다. 특히, 큰 투수성을 갖는 지반내에 열원이 존재하는 경우, 간극수의 밀도변화에 따른 자연대류가 중요한 열전달의 인자가 되므로 이를 고려한 해석을 수행하여야 한다.

• Journal of Ecology and Environment
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• 제43권2호
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• pp.161-182
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• 2019

Quantifying the amount of carbon pools in forest ecosystems enables to understand about various carbon pools in the forest ecosystem. Therefore, this study was conducted in the Chilimo dry afromontane forest to estimate the amount of carbon stored. The natural forest was stratified into three forest patches based on species composition, diversity, and structure. A total of 50 permanent sample plots of 20 m × 20 m (400 ㎡ ) each were established, laid out on transects of altitudinal gradients with a distance of 100 m between plots. The plots were measured twice in 2012 and 2017. Tree, deadwood, mineral soil, forest floor, and stump data were collected in the main plots, while shrubs, saplings, herbaceous plants, and seedling data were sampled inside subplots. Soil organic carbon (SOC %) was analyzed following Walkely, while Black's procedure and bulk density were estimated following the procedure of Blake (Methods of soil analysis, 1965). Aboveground biomass was calculated using the equation of Chave et al. (Glob Chang Biol_20:3177-3190, 2014). Data analysis was made using RStudio software. To analyze equality of means, we used ANOVA for multiple comparisons among elevation classes at α = 0.05. The aboveground carbon of the natural forest ranged from 148.30 ± 115.02 for high altitude to 100.14 ± 39.93 for middle altitude, was highest at 151.35 ± 108.98 t C ha^-1 for gentle slope, and was lowest at 88.01 ± 49.72 t C ha^-1 for middle slope. The mean stump carbon density 2.33 ± 1.64 t C ha^-1 was the highest for the middle slope, and 1.68 ± 1.21 t C ha^-1 was the lowest for the steep slope range. The highest 1.44 ± 2.21 t C ha^-1 deadwood carbon density was found under the middle slope range, and the lowest 0.21 ± 0.20 t C ha^-1 was found under the lowest slope range. The SOCD up to 1 m depth was highest at 295.96 ± 80.45 t C ha^-1 under the middle altitudinal gradient; however, it was lowest at 206.40 ± 65.59 t C ha^-1 under the lower altitudinal gradient. The mean ecosystem carbon stock density of the sampled plots in natural forests ranged from 221.89 to 819.44 t C ha^-1. There was a temporal variation in carbon pools along environmental and social factors. The highest carbon pool was contributed by SOC. We recommend forest carbon-related awareness creation for local people, and promotion of the local knowledge can be regarded as a possible option for sustainable forest management.

• 천문학회보
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• 제44권2호
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• pp.53.4-53.4
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• 2019

We reconstruct the underlying dark matter (DM) density distribution of the local universe within 20Mpc/h cubic box by using the galaxy position and peculiar velocity. About 1,000 subboxes in the Illustris-TNG cosmological simulation are used to train the relation between DM density distribution and galaxy properties by using UNet-like convolutional neural network (CNN). The estimated DM density distributions have a good agreement with their truth values in terms of pixel-to-pixel correlation, the probability distribution of DM density, and matter power spectrum. We apply the trained CNN architecture to the galaxy properties from the Cosmicflows-3 catalogue to reconstruct the DM density distribution of the local universe. The reconstructed DM density distribution can be used to understand the evolution and fate of our local environment.

• Computational Structural Engineering : An International Journal
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• 제1권2호
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• pp.89-96
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• 2001

A study on the topology optimization of a Hard-Disk-Driver(HDD) actuator arm is presented. The purpose of the present wert is to increase the natural frequency of tole first lateral mode of the HDD actuator arm under the constraint of total moment of inertia, so as to facilitate the position control of the high speed actuator arm. The first lateral mode is an important factor in the position control process. Thus the topology optimization for 2-D model of the HDD actuator arm is considered. A new objective function corresponding to multieigenvalue optimization is suggested to improve the solution of the eigenvalue optimization problem. The material density of the structure is treated as the design variable and the intermediate density is penalized. The effects of different element types and material property functions on the final topology are studied. When the problem is discretized using 8-node element of a uniform density, tole smoothly-varying density field is obtained without checker-board patterns incurred. AS a result of 7he study, an improved design of the HDD actuator arm is suggested. Dynamic characteristics of the suggested design are compared computationally with those of the old design. With the same amount of the moment of inertia, the natural frequency of the first lateral mode of the suggested design is subsequently increased over the existing one.

• Carbon letters
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• 제14권1호
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• pp.40-44
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• 2013

The bipolar plate is the most important and most costly component of proton exchange membrane fuel cells. The development of a suitable low density bipolar plate is scientifically and technically challenging due to the need to maintain high electrical conductivity and mechanical properties. Here, bipolar plates were developed from different particle sizes of natural and expanded graphite with phenolic resin as a polymeric matrix. It was observed that the particle size of the reinforcement significantly influences the mechanical and electrical properties of a composite bipolar plate. The composite bipolar plate based on expanded graphite gives the desired mechanical and electrical properties as per the US Department of Energy target, with a bulk density of 1.55 $g.cm^{-3}$ as compared to that of ~1.87 $g.cm^{-3}$ for a composite plate based on natural graphite (NG). Although the bulk density of the expanded-graphite-based composite plate is ~20% less than that of the NG-based plate, the I-V performance of the expanded graphite plate is superior to that of the NG plate as a consequence of the higher conductivity. The expanded graphite plate can thus be used as an electromagnetic interference shielding material.

• 한국자기학회지
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• 제23권6호
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• pp.188-192
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• 2013

수직 자기기록 매체로 알려진 CoPt 자성막을 전기도금법으로 제작하여 자기상호작용이 활성화 부피와 면적에 미치는 영향을 조사하였다. 모든 시료의 상호작용 기구는 쌍극자 상호작용이었으며, 낮은 전류밀도에서 제작한 시료일수록 두께가 두꺼울수록 상호작용 세기가 증가하였다. 한편 활성화 부피는 낮은 전류밀도에서 제작한 시료가 더 컸으나 두께에 따른 증감현상은 뚜렷하지 않았다. 그러나 활성화 면적은 상호작용의 세기가 강할수록 전류밀도가 낮을수록 감소하는 경향을 보였다.

• Asian-Australasian Journal of Animal Sciences
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• 제24권8호
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• pp.1173-1177
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• 2011

In this study wool follicle characteristics of Iranian sheep breeds were investigated. A total of 242 male and female sheep of different age from 5 breeds were studied. Samples of skin were taken from the right midside to determine follicle characteristics. Overall averages for males and females were $3.7{\pm}0.1$ and $3.5{\pm}0.1$ for S/P ratio; $3.2{\pm}0.1$ and $3.6{\pm}0.1$ for primary follicle density; $12.1{\pm}0.7$ and $13.8{\pm}0.4$ for secondary follicle density; $15.3{\pm}0.7$ and $17.4{\pm}0.4$ for total primary plus secondary follicle density; $2.3{\pm}0.2$ and $3.5{\pm}0.3$ for percentage of inactive secondary follicles. Significant differences were found in some follicle characteristics between sheep breeds. Results of this study showed that sheep breeds raised in different parts of Iran characterized by a low S/P ratio and follicle density could be classed as carpet wool breeds.

• 대한기계학회논문집A
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• 제24권7호
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• pp.1801-1809
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• 2000

A study on the topology optimization of Hard-Disk-Driver(HDD) actuator arm in free vibration is presented. The purpose of this research is to increasse the natural frequency of the first lateral mode of the HDD actuator arm under the constraint of total moment of inertia, so as to facilitate the position control of high speed actuator am. The first lateral mode is an important factor in the position control process. Thus the topology optimization for 2-D model of the HDD actuator arm is considered. A new objective function corresponding to multieigenvalue optimization is suggested to improve the solution of the eigenvalue optimization problem. The material density of the structure is treated as the design variable and the intermediate density is penalized. The effects of different element types and material property functions on the final topology are studied. When the problem is discretized using 8-node element of a uniform density, the smoothly-varying density field is obtained without checker-board patterns incurred. As a result of the study an improved design of the HDD actuator arm is suggested. Dynamic characteristics of the suggested design are compared computationally with those of the old design. With the same amount of the moment of inertia, the natural frequency of the first lateral mode or the suggested design is subsequently increased over the existing one.

• 한국산림과학회지
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• 제95권5호
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• pp.595-600
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• 2006

The plot design influences the budgets and the precision of forest inventory results. The objective of this study is to determine the efficiency of estimating forest variables such as tree density, basal area, volume, and species richness based on various plot sizes using fixed-area plot sampling in the natural deciduous forest of Pyeong-Chang County, Gang-won Province, Korea. In this study, 108 reference plots were established with a fixed plot size and shape of 0.09 ha ($30m{\times}30m$). In order to determine the optimal plot size for the interest of variables, each sample plot was established using different shapes (square, circle, and rectangle) and was divided into different plot sizes from 100 to $900m^2$. The mean relative difference (MRD) for the sum of the basal area and volume, and tree density per hectare decreased as plot size increased. But the MRD for three variables were only below 13% at the plot size of $500m^2$. Species richness for each reference stand observed ranging from 2 to 15 species, demonstrated highly positive significant relationships with plot size. The minimum plot size for the estimation of tree density, the sum of the BA and volume was determined to be about $400m^2$, whereas the estimation of species richness required a minimum plot size of $500m^2$.

• Elastomers and Composites
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• 제56권1호
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• pp.6-11
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• 2021

In order to develop an ultra-high-density elastomeric material for substitution of steel dynamic dampers, a new curing system and technique for high-loading of the filler were examined in this study. Mechanochemical modification of chloroprene rubber (MAH-g-CR) using an internal mixer was carried out with maleic anhydride (MAH) as a reactive monomer. The optimum amount of MAH was 10 phr and the efficient grafting of MAH on CR could be achieved at a mixing temperature of 100℃. After preparing MAH-g-CR, 50 mol% epoxidized natural rubber (ENR 50) was blended with MAH-g-CR to develop a "self-curable rubber blend system" via reaction between the functional groups of the elastomeric matrices without the curing agent and additives. The content of ENR 50 was fixed at 30 wt.% throughout evaluation of the curing behavior of the MAH-g-CR/ENR blend. Tungsten powder was added to the MAH-g-CR/ENR matrix up to 60 vol.% to obtain ultra-high-density, and the maximum density obtained was 7.57 g/㎤. Stable ts2 (scorch time) and t90 (90% cure time) could be obtained even when tungsten powder was incorporated up to 60 vol.%. In addition, the tensile strength and damping properties of MAH-g-CR/ENR containing 60 vol.% of tungsten were better than those of CR containing 60 vol.% of tungsten.