• Tunnel and Underground Space
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• v.24 no.2
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• pp.155-165
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• 2014

Multiple thermal energy storage (TES) caverns can be used for storing thermal energy on a large scale and for a high-aspect-ratio heat storage design to provide good thermal performance. It may also be necessary to consider the use of multiple caverns with a reduced length when a single, long tunnel-shaped cavern is not suitable for connection to aboveground heat production and injection equipments. When using multiple TES caverns, the separation distance between the caverns is one of the significant factors that should be considered in the design of storage space, and the optimal separation distance should be determined based on a quantitative stability criterion. In this paper, we described a numerical approach for determining the optimal separation distance between multiple caverns for large-scale TES utilization. For reliable stability evaluation of multiple caverns, we employed a probabilistic method which can quantitatively take into account the uncertainty of input parameters by probability distributions, unlike conventional deterministic approaches. The present approach was applied to the design of a conceptual TES model to store hot water for district heating. The probabilistic stability results of this application demonstrated that the approach in our work can be effectively used as a decision-making tool to determine the optimal separation distance between multiple caverns. In addition, the probabilistic results were compared to those obtained through a deterministic analysis, and the comparison results suggested that care should taken in selecting the acceptable level of stability when using deterministic approaches.

• Journal of Preventive Medicine and Public Health
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• v.34 no.2
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• pp.175-181
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• 2001

Objectives : To compare the multiple outcomes of patients with cataract surgery at perioperative time,3-4 months and 12 months after surgery and to assess patient outcomes associated with visual improvement(visual acuity of operated eye, visual function-14(VF-14), symptom score). Methods : For this assessment, a prospective study was conducted with 389 patients who had undergone cataract surgery for either one eye or both eyes. The surgery was peformed by 20 ophthalmologists who were practicing at university hospitals and general hospitals. Patients were interviewed and clinical data were obtained. Doctors were questioned with self-reported questionnaire forms. Medical records were examined in order to measure variables related to the surgical process such as surgical methods and ocular comorbidity. The survey was 'conducted at 4 stages' : preoperative time(389 cases), perioperative time(344 cases, 88.4%), postoperative 3-4 months (343 cases, 88.2%), and postoperative 12 months (281 cases, 72.2%). After excluding cases with incomplete data, 198 cases were enrolled in the study. Patient outcomes was measured for any improvement in the functional outcomes(visual acuity of operated eye, visual function, symptom score) at postoperative 3-4 months. Results : The visual acuity(operated, weighted average), symptom score, VF-14 score, satisfaction with vision, and subjective health status were shown to be improved at the perioperative time, postoperative 3-4 months and 12 mouths. An improvement in the Snellen visual acuity score was observed in 190 patients(96.0%), whereas improvements of the VF-14 score and cataract symptom sure were observed in 151 patients(76.3%) and 179 patients(90.4%), respectively. All three outcome measures demonstrated improvement in 137 patients(09.2%). The improvement of the three functional outcomes at 3-4 months after receiving surgery was associated with a lower level of visual function and a higher level of cataract symptom score at perioperative time, as well as a greater experience level of the surgeon. Conclusions : In this study, the estimates of the proportion of patients benefiting from cataract surgery varied with the outcome measure of benefit. Preoperative VF-14 score, a measure of functional impairment related to vision, and symptom score may be better measures of the benefit derived from cataract surgery than the change in visual acuity.

• Journal of the Korean association of regional geographers
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• v.17 no.4
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• pp.432-442
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• 2011

Climatic variables such as temperature and precipitation tend to vary both in space and in time simultaneously. Thus, it is necessary to include space-time autocorrelation into conventional spatial interpolation methods for reliable time-series mapping. This paper introduces and applies space-time variogram modeling and space-time kriging to generate time-series temperature maps using hourly Automatic Weather System(AWS) temperature observation data for a one-month period. First, temperature observation data are decomposed into deterministic trend and stochastic residual components. For trend component modeling, elevation data which have reasonable correlation with temperature are used as secondary information to generate trend component with topographic effects. Then, space-time variograms of residual components are estimated and modelled by using a product-sum space-time variogram model to account for not only autocorrelation both in space and in time, but also their interactions. From a case study, space-time kriging outperforms both conventional space only ordinary kriging and regression-kriging, which indicates the importance of using space-time autocorrelation information as well as elevation data. It is expected that space-time kriging would be a useful tool when a space-poor but time-rich dataset is analyzed.

• Proceedings of the Korean Quaternary Association Conference
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• 2006.11a
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• pp.23-25
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• 2006

제4기 만빙기에서 후빙기 사이의 기후변화는 인류문명에 있어 정착생활 및 농경활동과 더불어 잉여생산물 저장을 위한 토기제작 등과 같은 생활상의 변화를 가져 온 계기가 되었다(F.M. Chambers, 1993; Guoyu Ren, 1998; Mori Y, 2002; Martin Bell et al., 2004). 식물화석, 화분, 식물규소체 등의 대리자료(proxy data)를 이용하여 이시기의 식생환경을 밝히는 것은 고생태복원분야 뿐 아니라 당시의 환경변수들 을 추정하여 인간활동에 어떠한 영향을 미쳤는지에 대한 중요한 정보를 제공해준다. 그러나 홀로세 초기 환경은 인간에게 지대한 영향을 미쳤으나 후기로 올수록 인간 스스로가 환경을 변화시키는 주체가 되었다(Martin Bell et al., 2004). Meyer&Turner(1994)는 인간활동이 자연적인 영향보다 생물구에 시계열적인 변화를 가져왔다고 하였으며, R.B Singh(1995)는 현재뿐 아니라 과거까지 다양한 인위 적인 간섭이 전지구적인 환경변화를 야기 시키는 주된 원인이라고 하였다. 따라서 환경을 복원한다는 것은 바로 그 지역을 생활 터전으로 삼았던 당시 선사인들의 생활상을 간접적으로 들여다 볼 수 있는 기회를 제공해준다. 현재까지 국내 고고학 유적지에서 이루어진 대부분의 발굴조사는 유물과 유구를 토대로 당시 선사인의 생활상을 밝히는데 중점을 두었으나 최근 인간과 영향을 주고 받았을 환경에 관심을 갖고 이를 통해 역으로 인간활동을 추적하려는 다수의 연구들이 제4기학, 고고학, 지리학, 인류학, 생물학, 농학, 기후학 등 다양한 분야에서 학제간 연구로 진행되고 있다(황상일 등, 1999; 조현종, 2000; 김주용 등, 2002; 최정민, 2004; 윤순옥 등, 2005). 그러나 이러한 연구에 있어서 어떠한 대리자료(proxy data)를 어떻게 고고학 유적지에 적절하게 적용시킬 것인지 대리자료의 조성변화나 값의 해석에 있어서 기후인자와 인간영향을 어떻게 분리해 낼 것인지에 대한 문제점이 남는다. 예를 들어 화분분석의 경우 1차적인 환경변화에 따른 식생조성변화를 반영한다. 인간이 활발한 농경활동을 하게 되면서 주변식생을 제거하게 되고, 제거된 나대지에는 재배작물과 잡초들이 자라게 된다. 그리고 이들 화분은 주변 소택지에 퇴적물과 함께 퇴적되어 화분분석 결과 벼과(科)(Gramineae)와 문화지표수종이라 부르는 여뀌속(屬)(Persicaria), 국화과(科)(Composite), 쑥속(屬)(Artemisia), 쐐기풀속(屬)(Urtica) 등의 화분에 있어 확연한 조성변화를 보여준다. 그러나 화분을 이용한 분석은 토탄층이나 유기질 퇴적물이라는 한정된 토양에서만 분석이 가능하며, 조성변화에 있어서 역시 기후와 인간활동의 영향 모두를 반영하기 때문에 이를 구분해 주는 명확한 기준이 필요하다. 따라서 본 연구에서는 이러한 다양한 대리자료 중 고고학 유적지에 적용 가능한 동식물 화석 및 미화석, 지리, 지질학적 자료에 어떠한 것들을 적용할 수 있는지를 살피고, 몇 가지 대리자료를 중심으로 선사인의 농경활동에 대해 살펴보고자 한다.

• The Korean Journal of Financial Management
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• v.26 no.3
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• pp.199-226
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• 2009

We examine the intertemporal relation between risk and return in the Korean stock market and foreign exchange market based on the two factor ICAPM framework. The standard GARCH model and the GJR(1993) model are employed to estimate conditional variances of the stock returns and foreign exchange rates. The covariance between the rates of stock returns and changes in the exchange rates are estimated by the constant conditional correlation model of Bollerslev(1990) and the dynamic conditional correlation model of Engle(2002). The multivariate GARCH in mean model and quasi-maximum likelihood estimation method, consequently, are applied to investigate riskreturn relation jointly. We find that the estimated coefficient of relative risk aversion is negative and statistically significant in the post-financial crisis sample period in the Korean stock market. We also show that the expected stock returns are negatively related to the dynamic covariance with foreign exchange rates. Both estimated parameters of conditional variance and covariance in the foreign exchange market, however, are not statistically significant. The GJR model is better than the standard GARCH model to estimate the conditional variances. In addition, the dynamic conditional correlation model has higher explanatory power than the constant correlation model. The empirical results of this study suggest following two points to investors and risk managers in hedging and diversifying strategies for their portfolios in the Korean stock market: first, the variability of foreign exchange rates should be considered, and second, time-varying correlation between stock returns and changes in foreign exchange rates supposed to be considered.

• Elastomers and Composites
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• v.34 no.2
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• pp.135-146
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• 1999

The thermal degradation kinetics of SBR and tire were studied using a conventional thermogravimetric analysis in the stream nitrogen at a heating rate of 5, 10, 15, $20^{\circ}C/min$, respectively. Thermogravimetric curves and their derivatives were analyzed using various analytical methods to determine the kinetic parameters. The degradation of the SBR and tire was found to be a complex process which has multi-stages. The Friedman method gave average activation energies for the SBR and tire of 247.53kJ/mol and 230.00kJ/mol, respectively. Mean-while, the Ozawa method Eave 254.80kJ/mol and 215.76kJ/mol. It would appear that either. Friedman's differential method or Ozawa's integral method provided satisfactory mathematical approaches to determine the kinetic parameters for the degradation of the SBR and tire. Approximately 86% and 55% of oil products were obtained at a final temperature of $700^{\circ}C$ and a heating rate of $20^{\circ}C/min$ for the SBR and tire respectively.

• Korean Journal of Materials Research
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• v.8 no.10
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• pp.961-968
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• 1998

In order to investigate the effects of substrate bias voltage on the formation of$ZnS-SiO_2$ protective film in phase change optical disk by R.F. magnetron sputtering method, thin dielectric film was formed on Si wafer and Corning glass by using ZnS(80mol%)-$SiO_2$(20mol%)t arget under argon gas. In this study, the Taguchi experimental method was applied in order to obtain optimum conditions with reduced number of experiments and to control numerous variables effectively. At the same time this method can assure the reproducibility of experiments. Optimum conditions for film formation obtained by above method were target RF power of 200 W. substrate RF power of 20 W, Ar pressure of 5 mTorr. sputtering time of 20 min.. respectively. The phase of specimen was determined by using XRD and TEM. The compositional analysis of specimen was performed by XPS test. In order to measure the thermal resistivity of deposited specimen, annealing test was carried out at $300^{\circ}C$ and $600^{\circ}C$. For the account of void fraction in thin film, the Bruggeman EMA(Effective Medium Approximation) method was applied using the optical data obtained by Spectroscopic Ellipsometry. According to the results of this work, the existence of strong interaction between bias voltage and sputtering time was confirmed for refractive index value. According to XRD and TEM analysis of specimen, the film structure formed in bias voltage resulted in more refined structures than that formed without bias voltage. But excess bias voltage resulted in grain growth in thin film. It was confirmed that the application of optimum bias voltage increased film density by reduction of void fraction of about 3.7%.

• The Korean Journal of Financial Management
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• v.7 no.1
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• pp.1-20
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• 1990

In most of studies on market efficiency, the stability of risk measures and the normality of residuals unexplained by the pricing model are presumed. This paper re-examines stock splits, taking the possible violation of two assumptions into accounts. The results does not change the previous studies. But, the size of excess returns during the 2-week period before announcements decreases by 43%. The results also support that betas change around announcements and the serial autocorrelation of residuals is caused by events. Based on the results, the existing excess returns are most likely explained as a compensation to old shareholders for unwanted risk increases in their portfolio, or by uses of incorrect betas in testing models. In addition, the model suggested in the paper provides a measure for the speed of adjustment of the market to the new information arrival and the intensity of information contents.

• Korean Journal of Materials Research
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• v.9 no.12
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• pp.1245-1251
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• 1999

Phase-change optical disk very rapid recording, high densification of data, resulting in high feedback rate and good C/N(carrier to noise) ratio of a feedback signal. However, repetitive thermal energy may cause the deformation of a disk or the lowering of an eliminability and a cyclability of the recording. The lowering of the cyclability can be reduced by insertion of thin layer of ZnS-$SiO_2$ dielectric thin film in appropriate disk structure between the upper and lower part of the recording film. Using the Taguchi method, optimum conditions satisfying both the optimized quality characteristic values and the scattering values for film formation were found to be the target R.F. power of 200W, the substrate R.F. power of 20W, the Ar pressure of 6mTorr, and the electrode distance of 6cm. From the refractive index data, the existence of the strong interaction between the electrode distance and Ar pressure was confirmed, and so was the large effect of the electrode distance on transmittance. According to the analysis of TEM and XRD, the closer the electrode distance was, the finer was the grain size due to the high deposition rate. However, the closer electrode distance brought the negative effect on the morphology of the film and caused the reduction of transmittance. AFM and SEM analyses showed that the closer the electrode distance was, the worse was the morphology due to the high rate of the deposition. Under optimum condition, the deposited thin film showed a good morphology and dense microstructure with less defects.

• Applied Chemistry for Engineering
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• v.30 no.4
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• pp.453-459
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• 2019

In this study, the influence factors on the stability of the O/W (oil in water) emulsions prepared with coconut oil and the nonionic mixed surfactant (Tween 80-Span 80) were evaluated. The concentration and HLB value of the nonionic mixed surfactant, and the degree of agitation were used as manufacture factors. The stability of prepared O/W emulsions were measured with the mean droplet size, zeta-potential, emulsion stability index (ESI), and thermal instability index (TII). The mean droplet size of the prepared O/W emulsions was from 100 to 200 nm. As the concentration of mixed surfactant and the homogenization speed increased, the droplet sizes decreased, while the zeta-potential values increased. The effect of HLB values increased in the order of 6.0, 10.0 and 8.0, and at the HLB value of 8 the smallest mean droplet size as 120 nm was obtained whereas the largest value of the zeta-potential between 10 and 60 mV. From the results of ESI and TII, the stability of prepared O/W emulsions increased in order of 6.0, 10.0 and 8.0 of HLB values, and ESI and TII values were above 80% and below 20% respectively at HLB value of 8.0.