• 응용통계연구
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• 제34권5호
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• pp.711-721
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• 2021

이 논문에서는 불균형 자료에 대한 분류 분석에서 불순도지수를 이용하여 임계값을 조정하는 방법에 대해 알아본다. 이항자료에 대한 분류에서는 소수범주를 Positive, 다수범주를 Negative라고 하면, 일반적으로 사용하는 0.5 기준으로 범주를 정하면 불균형 자료에서는 특이도는 높은 반면 민감도는 상대적으로 낮게 나오는 경향이 있다. 소수범주에 속한 개체를 제대로 분류하는 것이 상대적으로 중요한 문제에서는 민감도를 높이는 것이 중요한데 이를 분류기준이 되는 임계값을 조정을 통해 높이는 방법에 대해 알아본다. 기존연구에서는 G-mean이나 F1-score와 같은 측도를 기준으로 임계값을 조정했으나 이 논문에서는 CHAID의 카이제곱통계량, CART의 지니지수, C4.5의 엔트로피를 이용하여 최적임계값을 선택하는 방법을 제안한다. 최적임계값이 여러 개 나올 수 있는 경우 해결방법을 소개하고 불균형 분류 예제로 사용되는 데이터 분석을 통해 0.5를 기준으로 ?(무엇?)을 때와 비교하여 어떤 개선이 이루어졌는지 등을 분류성능측도로 알아본다.

• Smart Structures and Systems
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• 제33권3호
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• pp.189-199
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• 2024

To properly extract the strain components under varying operational conditions is very important in bridge health monitoring. The abnormal sensor readings can be correctly identified and the expected operational performance of the bridge can be better understood if each strain components can be accurately quantified. In this study, strain components under varying load conditions, i.e., temperature variation and live-load variation are evaluated based on field strain measurements collected from a real concrete box-girder bridge. Temperature-induced strain is mainly regarded as the trend variation along with the ambient temperature, thus a smoothing technique based on the wavelet packet decomposition method is proposed to estimate the temperature-induced strain. However, how to effectively extract the vehicle-induced strain is always troublesome because conventional threshold setting-based methods cease to function: if the threshold is set too large, the minor response will be ignored, and if too small, noise will be introduced. Therefore, an autoencoder framework is proposed to evaluate the vehicle-induced strain. After the elimination of temperature and vehicle-induced strain, the left of which, defined as the model error, is used to assess the operational performance of the bridge. As empirical techniques fail to detect the degraded state of the structure, a clustering technique based on Gaussian Mixture Model is employed to identify the damage occurrence and the validity is verified in a simulation study.

• 한국응용곤충학회지
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• 제50권4호
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• pp.289-293
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• 2011

파정식 초기 파총채벌레의 경제적피해허용수준을 설정하기 위하여 온실에서 파총채벌레 접종기간에 따른 파의 생육, 피해 및 수량을 조사하였다. 파 정식초기에는 파총채벌레 접종기간(즉 가해정도)에 따른 수량감소율은 5일, 10일, 15일, 20일 접종기간에서 각각 17.0%, 53.3%, 38.4%, 80.8%로 증가하였다. 파총채벌레 발생정도와 파 수량감소 관계를 구명하기 위하여 파총채벌레 누적밀도일(Cumulative Insect Days, CID)과 수량 감수량(%) 간 회귀분석을 실시한 결과 비선형식인 logistic 모형에 잘 적용되었다. 이 식으로부터 경험적인 수익한계(Gain Threshold) 5%와 통상적 상품화율(93%)를 감안한 수량 감소율 12% 수준에서 파총채벌레 경제적피해허용수준은 30 CID로 추정되었다. 또한 요방제밀도는 경제적피해수준의 80%가 되는 24 CID로 추정되었다. 본 결과는 더 개선된 모형이 개발될 때까지 파 생육초기 파총채벌레 관리에 유용하게 활용될 수 있을 것으로 판단되었다. 또한 파 노지포장에서 생육후기(정식 후 120일 접종) 파총채벌레 피해가 파 수량에 미치는 영향을 검토한 결과, 파 생육후기에는 낮은 밀도에서 오히려 수량이 증가하였고, 그 후 밀도가 더 증가함에 따라 수량이 감소하는 등 전형적인 보상적 반응(over-compensatory response)을 보였다.

• 한국수자원학회논문집
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• 제54권5호
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• pp.311-320
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• 2021

농업용수는 우리나라의 용수사용량 중에 61%를 차지하고 있으며, 효율적인 수자원관리를 달성하기 위한 핵심적인 관리목표 중 하나이다. 본 연구에서는 댐과 저수지 등을 관리하는 실무자가 수문상황에 따른 하천에서의 농업용수 임계사용량을 간편하게 추정하기 위한 방법론을 제안하고자 하였다. 이를 위하여, 4대강 권역의 주요 27개 하천유역을 대상으로 하여 유역의 유출량과 하천수 사용실적을 수집하였다. 하천에서의 농업용수 사용과 예년대비 유출율이 유의미한 상관관계가 있음을 나타내었으며, 분위회귀를 이용하여 농업용수 최대사용을 대표하는 임계곡선으로 제시하고 27개 하천유역에 적용하였다. 최종적으로 예년대비 2개월 유출율에 따른 농업용수 최대사용량대비 사용률로 표현되는 두 가지 임계곡선과 식을 제시하였다. 또한, 유역특성을 비교하여 삼림면적 대비 농지면적의 비를 기준으로 임계곡선 유형을 결정할 수 있도록 하였으나, 추가적인 자료 수집과 분석이 필요할 것으로 판단된다. 본 연구에서 제시한 방법은 하천 농업용수 임계사용량을 간편하게 추정하고 활용할 수 있으므로 수자원관리 실무에서 유용하게 활용될 수 있을 것으로 기대한다.

• 정보교육학회논문지
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• 제11권1호
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• pp.91-98
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• 2007

인터넷 통신의 발달 및 워드프로세서의 기능 향상으로 인해 일선 교육현장에서의 표절은 심각한 문제가 되고 있다. 본 연구에서는 C, C++, Java 등으로 작성된 프로그램 소스 코드들의 유사도를 측정하는 방법을 제시하고, 소스 코드를 계층적으로 군집화하고 표절 결과를 수형도로 시각화하는 방법을 제시한다. 채점자는 시각화된 수형도를 보고 임계값을 설정하여 표절 그룹을 분리할 수 있다. 실제 데이터에서 효과를 알아보기 위해서 학부 1학년생 컴퓨터 개론 및 실습과목 강의 중에 제출된 과제물 프로그램을 이용하여 실험해 보았으며, 유용하고 현실성 있는 방법임을 확인하였다.

• 농업과학연구
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• 제43권4호
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• pp.507-521
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• 2016

Landslides are one of the most common natural hazards causing significant damage and casualties every year. In Korea, the increasing trend in landslide occurrence in recent decades, caused by climate change, has set off an alarm for researchers to find more reliable methods for landslide prediction. Therefore, an accurate landslide-susceptibility assessment is fundamental for preventing landslides and minimizing damages. However, analyzing the stability of a natural slope is not an easy task because it depends on numerous factors such as those related to vegetation, soil properties, soil moisture distribution, the amount and duration of rainfall, earthquakes, etc. A variety of different methods and techniques for evaluating landslide susceptibility have been proposed, but up to now no specific method or technique has been accepted as the standard method because it is very difficult to assess different methods with entirely different intrinsic and extrinsic data. Landslide prediction methods can fall into three categories: empirical, statistical, and physical approaches. This paper reviews previous research and surveys three groups of landslide prediction methods.

• 한국지진공학회:학술대회논문집
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• 한국지진공학회 2002년도 춘계 학술발표회 논문집
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• pp.3-11
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• 2002

Source parameters for forty nine recent earthquakes occurred in and around Korean Peninsula are determined and the relations among them are studied. The corner frequency and seismic moment are estimated from three different methods. The spectral fitting of the source displacement spectrum with the $\omega$-square source model of Brune(1970) and Snoke(1987)'s method are applied to all events and empirical Green's function method for two events are adopted. The source parameters determined in this study show different values depending on the adopted method and on the stations of which seismograms are recorded. It is interpreted that the disagreements principally originate from insufficient consideration of source radiation pattern and attenuation and amplification according to path direction. The corner frequencies and seismic moments are averaged to exclude the directional effects and other source parameters are estimated from the mean corner frequency and seismic moment. The static stress drops estimated in this study tend to be independent of seismic moment or magnitude for events above a certain size. For earthquakes with the size less than about 3.0$\times$10$^{21}$dyne-cm(nearly same as M$_{L}$=3.7), the stress drop tends to decrease with the decreasing moment. This fact suggests a breakdown of scaling law of source parameters below the threshold magnitude. The moment magnitudes calculated from source parameters appear to be slightly larger than the Richter's local magnitudes in the range above M$_{L}$=3.5.3.5.

• Structural Monitoring and Maintenance
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• 제4권2호
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• pp.115-131
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• 2017

Classical flutter of wind turbine blades indicates a type of aeroelastic instability with fully attached boundary layer where a torsional blade mode couples to a flapwise bending mode, resulting in a mutual rapid growth of the amplitudes. In this paper the monitoring problem of onset of flutter is investigated from a detection point of view. The criterion is stated in terms of the exceeding of a defined envelope process of a specific maximum torsional vibration threshold. At a certain instant of time, a limited part of the previously measured torsional vibration signal at the tip of blade is decomposed through the Empirical Mode Decomposition (EMD) method, and the 1st Intrinsic Mode Function (IMF) is assumed to represent the response in the flutter mode. Next, an envelope time series of the indicated modal response is obtained in terms of a Hilbert transform. Finally, a flutter onset criterion is proposed, based on the indicated envelope process. The proposed online flutter monitoring method provided a practical and direct way to detect onset of flutter during operation. The algorithm has been illustrated by a 907-DOFs aeroelastic model for wind turbines, where the tower and the drive train is modelled by 7 DOFs, and each blade by means of 50 3-D Bernoulli-Euler beam elements.

• Coupled systems mechanics
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• 제7권4호
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• pp.407-420
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• 2018

In solid-liquid two phase flow, the knowledge of how descending solid particles affected by the presence of downstream wall is important. This work studies at what interstitial distance the velocity of a vertically descending sphere is affected by a downstream wall as a consequence of wall-modified hydrodynamic forces through a validated dynamic model. This interstitial distance-the hydrodynamic coupling distance ${\delta}_c-is$ found to decay monotonically with the approach Stokes number St which compares the particle inertia to viscous drag characterized by the quasi-steady Stokes' drag. The scaling relation ${\delta}_c-St-1$ decays monotonically as literature below the value of St equal to 10. However, the faster diminishing rate is found above the threshold value from St=10-40. Furthermore, an empirical relation of ${\delta}_c-St$ shows dependence on the drop height which clearly indicates the non-negligible effect of unsteady hydrodynamic force components, namely the added mass force and the history force. Finally, we attempt a fitting relation which embedded the particle acceleration effect in the dependence of fitting constants on the diameter-scaled drop height.

• 천문학회보
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• 제36권2호
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• pp.92.2-92.2
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• 2011

In view of the planned NASA's and ESA's Solar Probe Plus and Solar Orbiter missions, respectively, to probe the inner heliosphere and the Sun's corona, it is timely to investigate outstanding problems associated with the solar wind. Among them is the temperature anisotropy problem. As the solar wind expands into the interplanetary space, the density and magnetic field decreases radially, thus leading to temperature anisotropy ($T_{\parallel}{\gg}T_{\perp}$). However, the measured temperature anisotropy can at times be characterized by $T_{\perp}$ > $T_{\parallel}$, while at other times the measured $T_{\parallel}/T_{\perp}$ is much milder than predicted by adiabatic theory. Physical reasons remain poorly understood. This notwithstanding, it is known from plasma physics that for $T_{\perp}$ > $T_{\parallel}$ electromagnetic ion-cyclotron (EMIC) and mirror instabilities are excited, while for $T_{\parallel}$ > $T_{\perp}$, fire-hose instability is excited. By constructing the threshold conditions for various instabilities, one may construct a closure relation that may be useful for modeling the solar wind. In the present paper we discuss theoretical construction of the anisotropy-beta relation by means of quasi-linear theories of these instabilities. The present work complements previous efforts on the basis of linear theory, hybrid simulations, and empirical fits of observations.