• 대한조선학회지
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• 제17권2호
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• pp.11-16
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• 1980

For vibration analysis of stiffened plates the orthotropic plate analogy is commonly accepted. As to stiffened plates in contact with water, however, there is still much uncertainty in estimation of the added mass because of the lack of direct methods. The authors, considering that for added mass of plates there are many reliable data derived theoretically or experimentally available, suggest a method to estimate the added mass of a stiffened plate by combining the mass increase factor, $\beta$, of an equivalent orthotropic plate and the correction factor, $\kappa$, for the effects of stiffeners. The latter is to be derived from systematic experimental investigations. Then, the natural frequency in water, f', can be calculated from that in air, f, by the equation $f'=f/\sqrt{1+\kappa\beta}$. To investigate practical applicability of this method, a systematic experiment was carried out with five uniaxially stiffened plates. Each of them had a plate of same size, $600mm{\times}600mm{\times}3.2mm$, but stiffeners of different size in the web-depth, 41.6mm, 51.2mm or 66.8mm and of different spacing 75mm, 100mm, or 150mm. Natural frequencies were measured under simply supported-edge conditions in both air and water, and corresponding $\kappa$ values derived. In spite of wide variations of web-depth and spcae of stiffeners, the experimental results show that the diversity of $\kappa$ values is not remarkable; mean values of $\kappa$ are 1.31 with standard deviation of 0.025 for the first modes and 1.27 with that 0.077 for the second modes. Hence, the authors concluded that the above $\kappa$ values can be used generally for the cases of uniaxially stiffened plates both sides of which contact with water, and that $\kappa$ values of general use for the cases of cross-stiffened plates may also be obtainable from similar experiments.

• 지구물리와물리탐사
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• 제10권2호
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• pp.154-160
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• 2007

이 논문에서는 Mackie et al. (1994), Sasaki (1999) 및 Nam et al. (2007)이 개발한 3차원 자기지전류 탐사 모델링 알고리듬의 특징을 자세히 비교 분석하고자 한다. Mackie et al. (1994)과 Sasaki (1999)의 알고리듬은 유한차분법(FDM)에 기초한 반면, Nam et al. (2007)의 알고리듬은 변유한요소법(EFEM)에 기초하고 있다. 이들 세 가지 방법으로 COMMEMI 3D-2 모형의 해를 구하고 적분방정식법의 해와 비교하였으며, 또한 세 가지 격자에 대해 세 개 주파수에서 계산시간을 비교하였다. FDM에 기반을 둔 두 가지 기법에서는 EFEM을 이용하는 경우보다 빠른 시간에 해를 계산할 수 있으며 이때 계산된 겉보기비저항과 위상은 전체적으로 적분방정식법의 해와 잘 일치하였으며 이상체 근처에서만 작은 차이를 보인다. 한편 EFEM에 기초한 알고리듬도 비교적 합리적인 시간 내에 매우 정확한 해를 계산할 수 있으며 지형을 포함한 경우에도 해를 계산할 수 있는 장점이 있다.

• Steel and Composite Structures
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• 제25권6호
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• pp.717-726
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• 2017

In this paper, a new simple shear deformation theory for bending analysis of functionally graded plates is developed. The present theory involves only three unknown and three governing equation as in the classical plate theory, but it is capable of accurately capturing shear deformation effects, instead of five as in the well-known first shear deformation theory and higher-order shear deformation theory. A shear correction factor is, therefore, not required. The material properties of the functionally graded plates are assumed to vary continuously through the thickness, according to a simple power law distribution of the volume fraction of the constituents. Equations of motion are obtained by utilizing the principle of virtual displacements and solved via Navier's procedure. The elastic foundation is modeled as two parameter elastic foundation. The results are verified with the known results in the literature. The influences played by transversal shear deformation, plate aspect ratio, side-to-thickness ratio, elastic foundation, and volume fraction distributions are studied. Verification studies show that the proposed theory is not only accurate and simple in solving the bending behaviour of functionally graded plates, but also comparable with the other higher-order shear deformation theories which contain more number of unknowns.

• 한국수자원학회:학술대회논문집
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• 한국수자원학회 2019년도 학술발표회
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• pp.152-152
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• 2019

This study demonstrates a novel approach of remotely sensed estimates of stream flow at fifteen hydrological station in the Han River Basin, Korea. Multi-temporal data of the European Space Agency's Sentinel-1 SAR satellite from 19 January, 2015 to 25 August, 2018 is used to develop and validate the flow estimation model for each station. The flow estimation model is based on a power law relationship established between the remotely sensed surface area of water at a selected reach of the stream and the observed discharge. The satellite images were pre-processed for thermal noise, radiometric, speckle and terrain correction. The difference in SAR image brightness caused by the differences in SAR satellite look angle and atmospheric condition are corrected using the histogram matching technique. Selective area filtering is applied to identify the extent of the selected stream reach where the change in water surface area is highly sensitive to the change in stream discharge. Following this, an iterative procedure called the Optimum Threshold Classification Algorithm (OTC) is applied to the multi-temporal selective areas to extract a series of water surface areas. It is observed that the extracted water surface area and the stream discharge are related by the power law equation. A strong correlation coefficient ranging from 0.68 to 0.98 (mean=0.89) was observed for thirteen hydrological stations, while at two stations the relationship was highly affected by the hydraulic structures such as dam. It is further identified that the availability of remotely sensed data for a range of discharge conditions and the geometric properties of the selected stream reach such as the stream width and side slope influence the accuracy of the flow estimation model.

• 한국방재학회 논문집
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• 제11권3호
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• pp.157-163
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• 2011

본 연구에서는 대상유역을 지표면과 하천 격자로 분할한 후, Manning 식을 기반으로 각 격자 중심에서 유역 출구점까지의 유하시간을 계산하여 등시간별 체적유량을 산정하였으며, 이로부터 유역의 동수역학적 특성인 특성유속을 고려한 직접유출수문 곡선을 도출하였다. 대상유역은 보청천유역의 산성유역을 선정하였다. 대상유역에 대한 매개변수 산정 결과 흐름누적값은 3800, K는 3.3으로 결정되었다. 보정된 매개변수를 실제사상에 대해 적용하여 검증한 결과 첨두유량과 첨두시간의 평균 절대오차는 각각 3.41%, 0.67 hr로 비교적 양호한 모의결과를 나타냈다. 또한 유효우량에 따른 유하시간별 기여면적 주상도를 분석한 결과 본 연구에서 적용된 모형은 강우강도에 따른 유역의 수리학적 특성을 잘 표현하고 있음을 알 수 있었다.

• Smart Structures and Systems
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• 제24권1호
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• pp.53-65
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• 2019

Misaligned wind-wave and seismic loading render offshore wind turbines suffering from excessive bi-directional vibration. However, most of existing research in this field focused on unidirectional vibration mitigation, which is insufficient for research and real application. Based on the authors' previous work (Sun and Jahangiri 2018), the present study uses a three dimensional pendulum tuned mass damper (3d-PTMD) to mitigate the nacelle structural response in the fore-aft and side-side directions under wind, wave and near-fault ground motions. An analytical model of the offshore wind turbine coupled with the 3d-PTMD is established wherein the interaction between the blades and the tower is modelled. Aerodynamic loading is computed using the Blade Element Momentum (BEM) method where the Prandtl's tip loss factor and the Glauert correction are considered. Wave loading is computed using Morison equation in collaboration with the strip theory. Performance of the 3d-PTMD is examined on a National Renewable Energy Lab (NREL) monopile 5 MW baseline wind turbine under misaligned wind-wave and near-fault ground motions. The robustness of the mitigation performance of the 3d-PTMD under system variations is studied. Dual linear TMDs are used for comparison. Research results show that the 3d-PTMD responds more rapidly and provides better mitigation of the bi-directional response caused by misaligned wind, wave and near-fault ground motions. Under system variations, the 3d-PTMD is found to be more robust than the dual linear TMDs to overcome the detuning effect. Moreover, the 3d-PTMD with a mass ratio of 2% can mitigate the short-term fatigue damage of the offshore wind turbine tower by up to 90%.

• 한국정보통신학회논문지
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• 제23권10호
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• pp.1234-1239
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• 2019

본 논문에서는 추가 설비 없이 기존 CCTV 영상을 디지털 영상 처리를 통하여 미세먼지 농도를 측정하는 방법을 제안한다. 영상처리 알고리즘은 노이즈 제거, 샤프닝, ROI 지정, 엣지 강도 계산, HSV 변환을 통한 보정 순으로 구성되며 C++ OpenCV 라이브러리를 이용해 구현하였다. 한달동안 캡쳐한 CCTV 이미지들에 본 알고리즘을 적용한 결과 ROI 영역에 대해 계산된 엣지 강도는 미세먼지 농도와 밀접한 관계가 있는 것으로 나타났다. 두 데이터간 상관관계를 추론하고자 MATLAB을 이용하여 거듭제곱 방정식 형태의 추세선을 수립하였으며 그 추세선으로부터 이탈한 데이터 포인트들의 개수는 12.5% 내외로 나타나 전체적으로 약 87.5%의 정확도를 보였다.

• 한국의학물리학회지:의학물리
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• 제31권3호
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• pp.54-62
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• 2020

Dose calculation algorithms play an important role in radiation therapy and are even the basis for optimizing treatment plans, an important feature in the development of complex treatment technologies such as intensity-modulated radiation therapy. We reviewed the past and current status of dose calculation algorithms used in the treatment planning system for radiation therapy. The radiation-calculating dose calculation algorithm can be broadly classified into three main groups based on the mechanisms used: (1) factor-based, (2) model-based, and (3) principle-based. Factor-based algorithms are a type of empirical dose calculation that interpolates or extrapolates the dose in some basic measurements. Model-based algorithms, represented by the pencil beam convolution, analytical anisotropic, and collapse cone convolution algorithms, use a simplified physical process by using a convolution equation that convolutes the primary photon energy fluence with a kernel. Model-based algorithms allowing side scattering when beams are transmitted to the heterogeneous media provide more precise dose calculation results than correction-based algorithms. Principle-based algorithms, represented by Monte Carlo dose calculations, simulate all real physical processes involving beam particles during transportation; therefore, dose calculations are accurate but time consuming. For approximately 70 years, through the development of dose calculation algorithms and computing technology, the accuracy of dose calculation seems close to our clinical needs. Next-generation dose calculation algorithms are expected to include biologically equivalent doses or biologically effective doses, and doctors expect to be able to use them to improve the quality of treatment in the near future.

• 한국정보통신학회논문지
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• 제26권11호
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• pp.1637-1643
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• 2022

최근 영상 및 보안, 시스템의 지능화 등 디지털 시장의 거대화 및 다양화로 인하여 이에 사용되는 영상처리기술의 수요 역시 증가하고 있다. 특히, CT, MRI, X-ray, 스캐너와 같이 잡음에 민감하게 반응하는 경우, 영상교정 및 복원을 위해 잡음 처리가 필수적으로 이루어져야 한다. 이중 대표적인 Salt and Pepper 잡음은 기존에도 연구가 활발히 진행되었지만, 여전히 잡음이 아주 많은 경우 상세 정보와 에지가 만족스럽지 못하고 흐려지는 한계를 가진다. 따라서 본 논문은 유클리드 거리 식에 따른 가중치를 부분 마스크에 적용하고, 원본과 가장 유사한 비잡음 화소만을 유효 화소로 사용하는 알고리즘을 제안하였다. 제안한 알고리즘은 설계한 부분 마스크의 내부 화소 상태 및 마스크의 훼손 정도에 따라 필터의 종류를 결정하기 때문에, 훼손이 심한 환경에서도 우수한 잡음 제거 능력을 나타내었다.

• Structural Engineering and Mechanics
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• 제86권1호
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• pp.1-16
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• 2023

The free vibration of temperature-dependent functionally graded plates (FGPs) resting on a viscoelastic foundation is investigated in this paper using a newly developed simple first-order shear deformation theory (FSDT). Unlike other first order shear deformation (FSDT) theories, the proposed model contains only four variables' unknowns in which the transverse shear stress and strain follow a parabolic distribution along the plates' thickness, and they vanish at the top and bottom surfaces of the plate by considering a new shape function. For this reason, the present theory requires no shear correction factor. Linear steady-state thermal loads and power-law material properties are supposed to be graded across the plate's thickness. Uniform, linear, non-linear, and sinusoidal thermal rises are applied at the two surfaces for simply supported FGP. Hamilton's principle and Navier's approach are utilized to develop motion equations and analytical solutions. The developed theory shows progress in predicting the frequencies of temperature-dependent FGP. Numerical research is conducted to explain the effect of the power law index, temperature fields, and damping coefficient on the dynamic behavior of temperature-dependent FGPs. It can be concluded that the equation and transformation of the proposed model are as simple as the FSDT.