• Journal of the Society of Naval Architects of Korea
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• v.28 no.2
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• pp.159-173
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• 1991

This paper describes a potential-based panel method formulated for the analysis of a super-cavitating two-dimensional hydrofoil. The method employs normal dipoles and sources distributed on the foil and cavity surfaces to represent the potential flow around the cavitating hydrofoil. The kinematic boundary condition on the wetted portion of the foil surface is satisfied by requiring that the total potential vanish in the fictitious inner flow region of the foil, and the dynamic boundary condition on the cavity surface is satisfied by requiring thats the potential vary linearly, i.e., the tangential velocity be constant. Green's theorem then results in a potential-based integral equation rather than the usual velocity-based formulation of Hess & Smith type. With the singularities distributed on the exact hydrofoil surface, the pressure distributions are predicted with improved accuracy compared to those of the linearized lilting surface theory, especially near the leading edge. The theory then predicts the cavity shape and cavitation number for an assumed cavity length. To improve the accuracy, the sources and dipoles on the cavity surface are moved to the newly computed cavity surface, where the boundary conditions are satisfied again. This iteration process is repeated until the results are converged. Characteristics of iteration and discretization of the present numerical method are much faster and more stable than the existing nonlinear theories. The theory shows good correlations with the existing theories and experimental results for the super-cavitating flow. In the region of small angles of attack, the present prediction shows and excellent comparison with the Geurst's linear theory. For the long cavity, the method recovers the trends of the Wu's nonlinear theory. In the intermediate regions of the short super-cavitation, the method compares very well with the experimental results of Parkin and also those of Silberman.

• Korean Journal of Optics and Photonics
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• v.6 no.3
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• pp.188-200
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• 1995

A design of unit magnification 2-mirror system with high resolution is presented. It is for soft X-ray(wavelength of 13 nm) projection imaging and suitable for preparation of high density semiconductor chip. In general, a holosymmetric system with unit magnification has the advantage that both coma and distortion are completely eliminated. In our holosymmetric 2-mirror system, spherical aberration is addtionally removed by using two identical paraboloidal mirror surfaces and field curvature aberration is also corrected by balancing Petzval sum and astigmatism which depends on the distance between two mirrors, so that the system is a aplanatic flat-field paraboloidal 2-mirror holosymmetric system. This 2-mirror system is small in size, and has a simple configuration with rotational symmetry about optical axis, and has also small central obscuration. Residual finite aberrations, spot diagrams, and diffraction-based MTF's are analyzed for the check of performances as soft X-ray lithography projection system. As a result, the image sizes for the resolutions of$0.25\mum$and $0.18\mum$are 4.0 mm, 2.5 mm respectively, and depths of focus for those are $2.5\mum$, $2.4\mum$respectively. This system should be useful in the fabrication of 256 Mega DRAM or 1 Giga DRAM. DRAM.

• Korean Journal of Optics and Photonics
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• v.16 no.5
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• pp.423-427
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• 2005

In this paper, we propose easily tooling method for Seidel third order aberration, which are not well utilized in actual design process due to the complication of mathematical operation and the difficulty of understanding Seidel third order aberration theory, even though most insightful and systematic means in pre-designing for the initial data of optimization. First, using paraxial ray tracing and Seidel third order aberration theory, spherical aberration coefficient is derived for a two-mirror system with a finite object distance. The coefficient, which is expressed as a higher-order nonlinear equation, consists of design parameters(object distance, two curvatures, and inter-mirror distance) and effective focal length(EFL). Then, the expressed analytical equation is solved by using a computer with numerical analysis method. From the obtained numerical solutions satisfying the nearly zero coefficient condition($<10^{-6}$), linear fitting process offers a linear relationship called the curvature linear equation between two mirrors. Consequently, this linear equation has two worthy meanings: the equation gives a possibility to obtain initial design data for optimization easily. And the equation shows linear relationship to a two-mirror system with a finite object distance under the condition of corrected third order spherical aberration.

• Korean Journal of Remote Sensing
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• v.37 no.5_1
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• pp.1207-1214
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• 2021

In thisletter, we developed technology which can exclude effect of cloudsto perform remote waterbody detection based on Sentinel-2 optical satellite imagery to calculate the area of ungauged reservoirs and applied to the Hwanggang dam reservoir, a representative ungauged reservoir, to verify usability. The remote waterbody detection technology calculates the cloud blocking ratio by comparing the cloud boundary in the Sentinel-2 imagery and the reservoir boundary first. Next, itselects data whose cloud blocking ratio does not exceed a specific value and calculates NDWI (Normalized Difference Water Index) with selected imagery. In last, it calculatesthe area of the reservoir by counting the number of grids which have NDWI value considered as waterbody within the boundary of the target reservoir and correcting with cloud blocking ratio. To determine cloud blocking ratio threshold forselecting image, we performed the area calculation of Hwanggang dam reservoir from July 2018 to October 2021. As a result, when the cloud blocking ratio threshold wasset 10%, we confirmed that the result with large error due to clouds were filtered well and obtained 114 results that can show changes in Hwanggang dam reservoir area among 220 images.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.32 no.4
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• pp.249-256
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• 2019

This paper presents an optimization framework of electrical impedance tomography for characterizing electrical conductivity profiles of concrete structures in two dimensions. The framework utilizes a partial-differential-equation(PDE)-constrained optimization approach that can obtain the spatial distribution of electrical conductivity using measured electrical potentials from several electrodes located on the boundary of the concrete domain. The forward problem is formulated based on a complete electrode model(CEM) for the electrical potential of a medium due to current input. The CEM consists of a Laplace equation for electrical potential and boundary conditions to represent the current inputs to the electrodes on the surface. To validate the forward solution, electrical potential calculated by the finite element method is compared with that obtained using TCAD software. The PDE-constrained optimization approach seeks the optimal values of electrical conductivity on the domain of investigation while minimizing the Lagrangian function. The Lagrangian consists of least-squares objective functional and regularization terms augmented by the weak imposition of the governing equation and boundary conditions via Lagrange multipliers. Enforcing the stationarity of the Lagrangian leads to the Karush-Kuhn-Tucker condition to obtain an optimal solution for electrical conductivity within the target medium. Numerical inversion results are reported showing the reconstruction of the electrical conductivity profile of a concrete specimen in two dimensions.

• Journal of the Korean Geotechnical Society
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• v.24 no.2
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• pp.77-86
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• 2008

The disturbance zone and measured values are affected by the size of the penetrometer. The local value may be measured by the smaller penetrometer. An ultra small Micro-Cone penetrometer (5mm in outer diameter) is designed and manufactured to characterize soil properties with minimum disturbance during penetration tests. The tip resistance is measured by using stain gauges attached near the Micro-Cone. In addition, the friction sleeve is adopted to effectively remove the skin friction from the tip resistance. Design concern includes the installation of stain gauges, circuits, penetration systems, penetration rate, sampling rate, operating temperature, and calibration. Application tests show that the clay interface, and the soil layers consisting of clay and sand are clearly detected by the Micro-Cone. Furthermore, the cone tip resistances measured by the Micro-Cone and the miniature cone (16mm in outer diameter) are similar. Note the resolution is much higher in the Micro-Cone. This study shows that the Micro-Cone may effectively detect the soil interface with high resolution, and with minimum disturbance.

• Journal of the Korean Geographical Society
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• v.36 no.4
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• pp.474-482
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• 2001

The purpose of this study is to investigate the formation and development processes of beach resort by fabric construction in a island environment. The results are as follows. (1) The research area(Tong-ri beach, Bokil-myon, Chollanam-do)has been transformed to belch by sedimentary environmental change since latter half of 1800's. (2) The mean slope of beach face is 0.96°, and the difference of attitude between beach and mud flat face is 75cm. (3) The mean particle size of beach surface sediment is 3.53$\Phi$. This value is very finer than that of any other beach in Korea peninsula. But its value is coarser than that of mud flat surface sediment. (4) The particle size distribution of core sediment is become changed to fine particle in 70cm depth. This value is corresponded to difference of altitude between beach face and mud flat face. (5) The analysis of aerial photographs after 1970 indicates that sedimentation process was not brisked since 1970's. Consequently, the research ares has been developed by sedimentary environmental change for sea-level rise effect and wave height energy rise effect.

• Korean Journal of Environment and Ecology
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• v.17 no.1
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• pp.44-55
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• 2003

The wetland in the city could be mainly divided into the deep water type and the abandoned paddy type, so this study was conducted to analyze characteristics between the two types of the wetland. The former sample site was located near the Olympic village in Songpa-gu of Seoul, and the latter sample site was in front of the Mt. Bukhan fortress in Eunpyeong-gu of Seoul. The actual vegetation, vascular plants, and avian fauna were researched. In the actual vegetation, the deep water type had the broad surface of water and the emerged plant as Phragmites communis have grown widely, but the abandoned paddy type had the narrow sur-face of water and hydrophyte as Persicaria thunbergii have grown widely. It might be judged because the water depth of the abandoned paddy type were shallow wholly. And the floating-leaved plants and the free-floating planktonic plants were not observed such as Nymphaea tetragona var. angusta, Lemna paucicostata in the abandoned paddy type wetland. The wild birds were mainly observed at the edge of the wetland(at the edge of woodland) in the abandoned paddy type, but were observed equally in the deep water type. 28 families 433 species were observed at the former site and 32 families 365 species were observed at latter site. It was judged that the various topographical structure(habitat diversity) might make all items various.

• Journal of the Korea Computer Graphics Society
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• v.23 no.3
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• pp.65-75
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• 2017

As a 360-degree image carries information of all directions, it often has too much information. Moreover, in order to investigate a 360-degree image on a 2D display, a user has to either click and drag the image with a mouse, or project it to a 2D panorama image, which inevitably introduces severe distortions. In consequence, investigating a 360-degree image and finding an object of interest in such a 360-degree image could be a tedious task. To resolve this issue, this paper proposes a method to find a region of interest and produces a 2D naturally looking image from a given 360-degree image that best matches a description given by a user in a natural language sentence. Our method also considers photo composition so that the resulting image is aesthetically pleasing. Our method first converts a 360-degree image to a 2D cubemap. As objects in a 360-degree image may appear distorted or split into multiple pieces in a typical cubemap, leading to failure of detection of such objects, we introduce a modified cubemap. Then our method applies a Long Short Term Memory (LSTM) network based object detection method to find a region of interest with a given natural language sentence. Finally, our method produces an image that contains the detected region, and also has aesthetically pleasing composition.

• Proceedings of the Korea Water Resources Association Conference
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• 2011.05a
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• pp.351-351
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• 2011

최근 지구온난화와 더불어 이상기후가 대두됨에 따라 기상 예측이 더욱더 중요시되고 있다. 또한 이전부터 가뭄 및 홍수와 같은 기상현상으로 인한 피해 사례가 빈번하였으며, 이로 인하여 물 관리의 어려움을 겪고 있다. 한 예로 이상기후가 유난히 잦았던 2010년 여름철 경우 평년보다 발달한 북태평양고기압의 영향으로 여름철 92일 가운데 81일의 전국 평균기온이 평년보다 높게 나타났다. 또한 강우 일수가 평년에 비해 7.4일 많은 44.2일을 기록하였으며, 국지성 집중호우 사례가 빈번하였다. 또한 8월 9일 발생한 태풍 `뎬무'를 포함해서 한 달 동안 3개의 태풍이 한반도에 영향을 끼치는 이례적인 사례가 발생하였다. 따라서 본 연구는 이러한 기상재해에 따른 물 관리를 장기적으로 대비하고자 고해상도 전지구 모델 GME를 이용하여 2010년 여름철 강수 예측을 실시하였다. 강수 예측에 사용된 전지구 모델 GME는 기존의 카테시안 격자체계를 가진 모델과 달리 전구를 삼각형으로 구성된 20면체로 격자화 한 Icosahedral-hexagonal grid 격자체계로 구성되어 있어, 해상도 증가에 용이할 뿐만 아니라, HPC(High Performance Computing)환경에서 효율성이 높은 장점을 가지고 있다. 본 계절 예측을 수행함에 있어 발생하는 잡음을 최소화하고자, Time-lag 기법을 이용하여 5개의 앙상블 멤버로 구성되어있으며, 이를 비교 분석하기위해 Climatology를 이용하여 총 10개의 앙상블 멤버로 규준실험을 수행하였다. 선행 연구에 따르면 1개월 이상의 장기 적분의 경우 초기조건보다 외부 강제력이 더 중요한 역할을 한다고 연구된 바 있다. (Yang et al., 1998) 특히 계절 변동성의 경우 대기-해양간의 상호작용에 의해 지배되며, 이를 고려하여 본 연구는 해수면 온도를 경계 자료로 사용하여 계절 예측을 수행하였다. 앞서 말한 실험 계획을 바탕으로 하여 나온 결과를 통해 동아시아지역 및 한반도 도별 강수 및 온도 변수에 대해 순별 및 월별 카테고리맵 분석을 실시하여 한눈에 보기 쉽게 나타냈다. 또한 주요 도시별 강수량 및 온도의 시계열 분석을 실시하여 시간이 지남에 따라 나타나는 변동성을 확인하였다. 계절 예측 결과에서 온도의 경우 평년보다 높게 나타났으며, 이는 실제 온도 예측과도 유사한 패턴을 가졌다, 강수의 경우 7월부터 8월 중순까지 평년보다 다소 적게 모의되었으며, 8월 하순경 회복하는 것으로 예측하였다. 따라서 본 계절 강수 예측은 다소 역학 모델이 가지는 한계를 가지고 있으나, 실제와 비교하여 어느 정도의 경향성이나 패턴에 있어 유사성을 보임을 확인하였으며, 이를 장기적 차원의 물관리를 함에 있어 참고 및 활용 가능할 것으로 예상한다.