• Journal of Korean Society of Coastal and Ocean Engineers
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• v.5 no.3
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• pp.212-220
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• 1993

A numerical model for the transformation of irregular waves in a coastal area is developed, which takes account of shoaling, refraction, diffraction, bottom friction and wave breaking. The governing equations are the usual energy conservation equation and kinematic conservation equations, but to consider the diffraction effects additional terms are included in the usual kinematic conservation or wave number equations. A linear superposition technique is used to represent the spectral formation. and an explicit formula is developed for the estimation of friction factor of irregular waves. A breaking criterion of component waves, which is the modified form of the Kitaigorodskii saturation relation, is employed to restrict the growth of shoaling waves in very shallow waters. The model was applied to a laboratory test and satisfactory agreement was obtained between the computation and measurement.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.39 no.12
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• pp.566-574
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• 2002

In this paper, the scattered field from a perfectly conducting fractal surface by the Monte-Carlo moment method was computed. An one-dimensional fractal surface was generated by using the fractional Brownian motion model. Back scattering coefficients are calculated with different values of the spectral parameter(S$\_$0/), and fractal dimension(D) which determine characteristics of the fractal surface. The number of surface realization for the computed field, the point number, and the width of surface realization are set to be 80, 2048, and 64L, respectively. In order to verify the computed results these results are compared with those of small perturbation methods, which show good agreement between them.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.28 no.2A
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• pp.63-69
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• 2003

In this paper, the scattered field from a perfectly conducting fractal surface by Finite-Difference Time-Domain(FDTD) method was computed. A one-dimensional fractal surface was generated by using the fractional Brownian motion model. Back scattering coefficients are calculated with different values of the spectral parameter(S0), fractal dimension(D) which determine characteristics of the fractal surface. The number of surface realization for the computed field, the point number, and the width of surface realization are set to be 80, 1024, 16λ, respectively. In order to verify the computed results these results are compared with those of small perturbation methods, which show good agreement between them.

• Journal of the Korea Concrete Institute
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• v.22 no.6
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• pp.843-850
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• 2010

The dynamic modulus of elasticity of concrete can be determined nondestructively using impact echo test as prescribed in KS F 2437. The fundamental longitudinal frequency of the concrete cylinders with free-free boundary condition was estimated by the wavelet transform theory. The advantage of the wavelet transform over either a pure spectral or temporal decomposition of the signal is that the features of the pertinent signals can be characterized in the time-frequency plane. For the concrete mix design utilized in this study, no significant difference between the dynamic and the static moduli of elasticity was observed. This was contrary to the perceived general notion of having the dynamic modulus considerably higher than the static modulus. It has been shown that the modulus from static and dynamic by impact echo test are comparable to each other fairly well, when the effect of strain level was properly taken into account. In this experimental test, it was shown that the dynamic modulus is approximately equal to the tangent modulus at $1{\times}10^{-4}$ strain level.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.30 no.2
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• pp.153-158
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• 2017

In this paper, we take into account topology optimization problems considering spatial randomness in the material property of elastic modulus. Based on 88 lines MATLAB Code, Monte Carlo analysis has been performed for MBB(messerschmidt-$b{\ddot{o}}lkow$-blohm) model using 5,000 random sample fields which are generated by using the spectral representation scheme. The random elastic modulus is assumed to be Gaussian in the spatial domain of the structure. The variability of the volume fraction of the material, which affects the optimum topology of the given problem, is given in terms of correlation distance of the random material. When the correlation distance is small, the randomness in the topology is high and vice versa. As the correlation distance increases, the variability of the volume fraction of the material decreases, which comply with the feature of the linear static analysis. As a consequence, it is suggested that the randomness in the material property is need to be considered in the topology optimization.

• The Journal of Engineering Geology
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• v.14 no.1
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• pp.29-46
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• 2004

It is important to identify geometries of fracture that act as a conduit of fluid flow for characterization of ground water flow in fractured rock. Fracture geometries control hydraulic conductivity and stream lines in a rock mass. However, we have difficulties to acquire whole geometric data of fractures in a field scale because of discontinuous distribution of outcrops and impossibility of continuous collecting of subsurface data. Therefore, it is needed to develop a method to describe whole feature of a target fracture geometry. This study suggests a new approach to develop a method to characterize on the whole feature of a target fracture geometry based on the Fourier transform. After sampling of specimens along a target fracture from borehole cores, effective frequencies among roughness components were selected by the Fourier transform on each specimen. Then, the selected effective frequencies were averaged on each frequency. Because the averaged spectrum includes all the frequency profiles of each specimen, it shows the representative components of the fracture roughness of the target fracture. The inverse Fourier transform is conducted to reconstruct an averaged whole roughness feature after low pass filtering. The reconstructed roughness feature also shows the representative roughness of the target subsurface fracture including the geometrical characteristics of each specimen. It also means that overall roughness feature by scaling up of a fracture. In order to identify the characteristics of permeability coefficients along the target fracture, fracture models were constructed based on the reconstructed roughness feature. The computation of permeability coefficient was performed by the homogenization analysis that can calculate accurate permeability coefficients with full consideration of fracture geometry. The results show a range between $10^{-4}{\;}and{\;}10^{-3}{\;}cm/sec$, indicating reasonable values of permeability coefficient along a large fracture. This approach will be effectively applied to the analysis of permeability characteristics along a large fracture as well as identification of the whole feature of a fracture in a field scale.

• Proceedings of the Korean Society of Postharvest Science and Technology of Agricultural Products Conference
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• 2003.04a
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• pp.120-121
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• 2003

과수원에서 재배되는 배는 과수원 내의 위치, 시비, 토양 등의 요인에 따라 다양한 품질을 나타내며, 당도와 숙도의 편차가 크기 때문에 과수농가에서는 경험에 의존하여 적정 숙기로 판단되는 배를 수확하고 있다. 그러나 과학적이지 못한 사실에 기초한 수확 관행은 시장유통되는 배에 대하여 소비자들의 신뢰성 저하를 초래하게 되고 소비 감소와 더불어 농가 소득 감소로 이어지게 된다. 최근, 전국의 청과물 산지유통센터에는 근적외선을 이용하여 과일 내부의 당도, 산도, 결함 등을 실시간으로 판정할 수 있는 비파괴 선별기가 보급되고 있으나 이는 수확이후의 선별.규격화 유통을 위한 것이다. 본 연구에서는 이와는 달리, 수확 이전, 즉 재배 단계에서 배의 당도와 숙도를 판정하여 수확적기를 판단할 수 있도록 나무에 매달린 배에 대하여 가시광선과 근적외선 반사스펙트럼을 측정할 수 있고 이를 이용하여 당도와 숙도가 판정가능한 휴대형 센서를 개발하였으며, 개발된 시작기를 이용하여 당도판정의 가능성을 시험하였다. 휴대형 당도판정센서는 광원과 광섬유프로브, 광검출부, 당도판정부, 전원공급부로 구성된다. 광원은 할로겐램프(6V)를 이용하였고, 광섬유프로브는 동심원 형태로서 외부의 광섬유를 통하여 광원에서 시료로 빛이 조사되게 하고, 내부의 광섬유를 통하여 광검출기로 확산반사되는 광이 전달될 수 있도록 하였다. 전원공급부는 휴대와 충전이 가능한 배터리(12V, 2AH)와 이 배터리에서 정전류가 광원으로 보내어 질 수 있도록 제작된 회로로 구성하였다. 당도 판정을 위하여 518nm에서 1046nm의 파장대역에서의 반사스펙트럼을 이용하였고, 레퍼런스로써 백색 테플론 구를 제작하여 사용하였다. 수원 농산물 도매시장에서 판매중인 2002년산 신고 배를 구매하고, 시작기를 이용하여 총 113개의 배에 대한 반사스펙트럼을 측정하였다. 다음으로 굴절당도계로 당도값을 측정하고 반사스펙트림을 이용하여 당도값을 예측하기 위한 부분최소제곱회귀(PLSR)모델을 개발하였다. 여기서 모델의 정밀도는 교차검정법을 이용하여 검증하였다. 시료 표면과 광섬유프로브와의 접촉상태 불균일, 광원의 시간에 따른 경시 변화, 과일 형상의 차이 등에 의하여 측정된 반사스펙트럼은 상당한 변이를 나타내었으므로 이를 보정하기 위하여 반사 스펙트럼은 다분산보정처리하여 이용하였다. 당도 예측용 PLSR모델 개발의 결과, 모델의 결정계수($R^2$)는 0.67, SEC는 $\pm$0.4brix.로 나타났으며, 교차검정에 의한 미지 시료의 예측에서 총 113개의 미지 시료에 대한 결정계수는 0.57, SEP는 $\pm$ 0.46brix.로 나타났으며, 이는 현장에서 충분히 활용가능할 것으로 판단되었다. 금후, 전체 시스템의 부피와 중량을 줄이고 각 부분품들의 전력소모의 최소화할 수 있도록 개선할 계획이다.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.25 no.1
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• pp.81-89
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• 2012

This study propose the vibration-based damage monitoring scheme for steel girder bolt-connection member by using wireless acceleration sensor node. In order to achieve the objective, the following approaches are implemented. Firstly, wireless acceleration sensor node is described on the design of hardware components and embedded operation software. Secondly, the vibration-based damage monitoring scheme of the steel girder bolt-connection member is described. The damage monitoring scheme performed global damage occurrence alarming and damage localization estimation by the acceleration response feature analysis. The global damage alarming is applied to the correlation coefficient of power spectral density. The damage localization estimation is applied to the frequency-based damage detection technique and the mode-shape-based damage detection technique. Finally, the performance of the vibration-based damage monitoring scheme is evaluated for detecting the bolt-connection member damage on a lab-scale steel girder.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.28 no.2
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• pp.153-160
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• 2015

The objective of this study is to construct the TPU aerodynamic database with wind tunnel test data of overall wind loads and responses on tall buildings. In this study, wind tunnel tests were conducted to investigate characteristics of wind forces and the effect of wind load combination by cross-correlation analysis among along-wind overturning moment, across-wind overturning moment and torsional moment on a tall building with various side ratios(D/B=0.33, 0.50, 0.77, 0.83, 0.91, 1.0, 1.1, 1.2, 1.3, 2.0 and 3.0) for different terrain roughnesses. The results of wind tunnel tests were compared with those of past literatures. As a result, there was no significant effects of changing of terrain roughnesses on moment coefficients and power spectral densities of across-wind overturning moment coefficients and torsional moment coefficients with various side ratios. Further, these results were good agreement with those of past literatures. From cross-correlation analysis, the across-wind overturning moment coefficients were highly correlated with the torsional moment coefficients. The results of this study will be helpful for practical designers in preliminary design stage.

• Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
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• 2006.11a
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• pp.461-464
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• 2006

The caisson of the inner wall type has a weak point that reflecting wave is big. Therefore it has been studied that the research of the decreasing reflecting wave using installation the perforated wall in front of caisson to decrease of that weak point. In this study, we analyzed the characteristic of reflection horizontal and diamond style vertical slit caisson using hydraulic model test. According to the results of experiments, we could confirm that diamond style vertical caisson has a reflection coefficient which has lower than horizontal caisson of the reflection coefficient of 5~10%.