• Structural Engineering and Mechanics
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• 제54권5호
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• pp.891-908
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• 2015

For a systematic study on wind-induced vibration characteristics of large hyperbolic cooling towers with different feature sizes, the pressure measurement tests are finished on the rigid body models of three representative cooling towers with the height of 155 m, 177 m and 215 m respectively. Combining the refined frequency-domain algorithm of wind-induced responses, the wind-induced average response, resonant response, background response, coupling response and wind vibration coefficients of large cooling towers with different feature sizes are obtained. Based on the calculating results, the parametric analysis on wind-induced vibration of cooling towers is carried out, e.g. the feature sizes, damping ratio and the interference effect of surrounding buildings. The discussion shows that the increase of feature sizes makes wind-induced average response and fluctuating response larger correspondingly, and the proportion of resonant response also gradually increased, but it has little effect on the wind vibration coefficient. The increase of damping ratio makes resonant response and the wind vibration coefficient decreases obviously, which brings about no effect on average response and background response. The interference effect of surrounding buildings makes the fluctuating response and wind vibration coefficient increased significantly, furthermore, the increase ranges of resonant response is greater than background response.

• 대한전자공학회논문지TC
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• 제48권5호
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• pp.46-53
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• 2011

유한한 기판크기가 E-평면으로 배열된 선형 위상 배열 안테나의 방사 특성에 미치는 영향을 연구하였다. 여러 가지 기판 크기에 따른 active reflection coefficient 와 average active element pattern 으로부터 선형 배열안테나의 방사특성을 예측하였다. E-평면과 만나는 기판 가장자리까지의 거리 변화가 H-평면과 만나는 기판 가장자리까지의 거리 변화에 비해 위상 배열 안테나의 방사 패턴 특성에 미치는 영향이 큰 것을 알 수 있었다.

• Nuclear Engineering and Technology
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• 제43권6호
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• pp.531-546
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• 2011

The backward differentiation formula (BDF) method is applied to a three-dimensional reactor kinetics calculation for efficient yet accurate transient analysis with adaptive time step control. The coarse mesh finite difference (CMFD) formulation is used for an efficient implementation of the BDF method that does not require excessive memory to store old information from previous time steps. An iterative scheme to update the nodal coupling coefficients through higher order local nodal solutions is established in order to make it possible to store only node average fluxes of the previous five time points. An adaptive time step control method is derived using two order solutions, the fifth and the fourth order BDF solutions, which provide an estimate of the solution error at the current time point. The performance of the BDF- and CMFD-based spatial kinetics calculation and the adaptive time step control scheme is examined with the NEACRP control rod ejection and rod withdrawal benchmark problems. The accuracy is first assessed by comparing the BDF-based results with those of the Crank-Nicholson method with an exponential transform. The effectiveness of the adaptive time step control is then assessed in terms of the possible computing time reduction in producing sufficiently accurate solutions that meet the desired solution fidelity.

• Advances in nano research
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• 제7권3호
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• pp.157-167
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• 2019

In this paper, a classical plate model is utilized to formulate the wave propagation problem of magnetostrictive sandwich nanoplates (MSNPs) while subjected to hygrothermal loading with respect to the scale effects. Herein, magnetostriction effect is considered and controlled on the basis of a feedback control system. The nanoplate is supposed to be embedded on a visco-Pasternak substrate. The kinematic relations are derived based on the Kirchhoff plate theory; also, combining these obtained equations with Hamilton's principle, the local equations of motion are achieved. According to a nonlocal strain gradient theory (NSGT), the small scale influences are covered precisely by introducing two scale coefficients. Afterwards, the nonlocal governing equations can be derived coupling the local equations with those of the NSGT. Applying an analytical solution, the wave frequency and phase velocity of propagated waves can be gathered solving an eigenvalue problem. On the other hand, accuracy and efficiency of presented model is verified by setting a comparison between the obtained results with those of previous published researches. Effects of different variants are plotted in some figures and the highlights are discussed in detail.

• International Journal of Naval Architecture and Ocean Engineering
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• 제13권1호
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• pp.718-733
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• 2021

A prediction method, based on the Morison equation as well as Froude-Krylov formula, is presented to simulate the loads acting on the columns and caissons of the semi-submersible platform induced by Internal Solitary Wave (ISW) respectively. Combined with the experimental results, empirical formulas of the drag and inertia coefficients in Morison equation can be determined as a function of the Keulegan-Carpenter (KC) number, Reynolds number (Re) and upper layer depth h₁/h respectively. The experimental and calculated results are compared. And a good agreement is observed, which proves that the present prediction method can be used for analyzing the ISW-forces on the semi-submersible platform. Moreover, the results also demonstrate the layer thickness ratio has a significant effect upon the maximum horizontal forces on the columns and caissons, but both minimum horizontal and vertical forces are scarcely affected. In addition, the incoming wave directions may also contribute greatly to the values of horizontal forces exerted on the caissons, which can be ignored in the vertical force analysis.

• 한국정밀공학회지
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• 제12권9호
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• pp.148-155
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• 1995

The object of this study is to achieve a gteater understanding of meterial removal process and its mechanism. In this study, some applications of finite element techniques are applied to analyze the chip formation and cutting heat generation mechanism of metal cutting. To know the effect of cutting parameters, simulations employed some independent cutting variables change, such as constitutive deformation laws of workpiece and tool material, frictional coefficients and tool-chip contact interfaces, cutting speed, tool rake angles, depth of cut and this simulations also include large elastic-plastic defor- mation, adiabetic thermal analysis. Under a usual plane strain assumption, quasi-static, thermal-mechanical coupling analysis generate detailed informations about chip formation process and cutting heat generation mechanism Some cutting parameters are affected to cutting force, plastic deformation of chip, shear plane angle, chip thickness and tool-chip contact length and reaction force on tool, cutting temperature and thermal behavior. Several aspects of the metal cutting process predicted by the finite element analysis provide information about tool shape design and optimal cutting conditions.

• Wind and Structures
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• 제37권1호
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• pp.57-78
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• 2023

In this study, a generalized three-degree-of-freedom (3-DoF) analytical model is formulated to predict linear aerodynamic instabilities of a prism under quasi-steady (QS) conditions. The prism is assumed to possess a generic cross-section exposed to turbulent wind flow. The 3-DoFs encompass two orthogonal horizontal directions and rotation about the prism body axis. Inertial coupling is considered to account for the non-coincidence of the mass center and the rotation center. The aerodynamic force coefficients-drag, lift, and moment-depend on the Reynolds number based on relative flow velocity, angle of attack, and the angle between the wind and the cable. Aerodynamic forces are linearized with respect to the static equilibrium configuration and mean wind velocity. Routh-Hurwitz and Liénard and Chipart criteria are used in the eigenvalue problem, yielding an analytical solution for instabilities in galloping and static divergence types. Additionally, the minimum structural damping and stiffness required to prevent these instabilities are numerically determined. The proposed 3-DoF instability model is subsequently applied to a conductor with ice accretion and a full-scale dry inclined cable. In comparison to existing models, the developed model demonstrates superior prediction accuracy for unstable regions compared with results in wind tunnel tests.

• 한국광학회지
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• 제17권1호
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• pp.89-93
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• 2006

방향성 결합기의 소멸비 특성 개선을 위해 입사도파로의 접근각도를 조절하는 방안의 가능성과 오차한계를 살펴보았다. 입사도파로의 접근각도가 크면 단순한 모드분석 이론의 틀에 오차를 유발하게 되지만 적절하게 조절하면 오차를 극소화하면서도 입사되는 광모드의 결합계수를 미세 조절함으로써 소멸비를 개선하는 효과가 있었다. 급격한 접근각도의 설정 시에는 비록 그 각도가 작아서 유출모드(leak mode)에 의한 손실이 있기 전이더라도 광모드의 과잉쏠림(field-profile overshooting)에 의해 각종 특성의 열화가 생기기 시작하는 것을 발견하고 그 영향을 분석하였다. 진행 광모드의 변화는 BPM(Beam Propagation Method)으로 전산 모사하였으며, 직선형 및 곡선형의 두 가지 입사도파로 구조에 대해 특성변화를 비교하여 소멸비 특성을 최적화하는 조건을 얻어내었다.

• 비파괴검사학회지
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• 제36권5호
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• pp.353-362
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• 2016

가스터빈 블레이드는 고온 고압의 환경 아래 장시간 가동하기 위하여 초합금 모재에 세라믹 코팅으로 이루어진 열차폐코팅(thermal barrier coating, TBC)은 필수요소이다. 하지만 TBC 또한 가스터빈 가동 중 일정 열화온도 및 가동시간에서 top coat의 박리현상이 일어난다. TBC의 박리는 블레이드의 손상과 직결되므로 가스 터빈의 안정적인 가동을 위해서 TBC의 박리 평가가 선행되어야 한다. 기존 비파괴평가 기법 연구는 산화알루미늄층(thermally grown oxide, TGO)의 생성 유무 또는 완전 박리의 정성적 평가가 이루어져 왔다. 본 연구에서는 TBC 박리를 정량적으로 평가하기 위해 초음파검사의 C-scan기법을 이용한 TBC의 부분박리손상 map을 구현하였다. 시편들은 $1,100^{\circ}C$로 등온열화하여 각각 열화시간을 변화시킨 시편들을 사용하였다. 단일 탐촉자를 이용한 펄스-에코법으로 C-scan을 수행하였고 TBC 내 부분박리를 검출하기 위해 초음파를 수침법으로 시편에 수직 탐상하였다. 그리고 Rogers-Van Buren과 Kim의 이론 반사식을 이용하여 부분박리영역 지름이 1 mm부터 6 mm까지 부분박리지수를 도출했다. 이를 적용하여 각 부분박리지수에 따른 부분박리 손상 map을 영상화하였다. TBC는 열화시간이 증가할수록 부분박리지수에 관계없이 부분박리영역이 모두 증가함을 확인할 수 있었다. 또한 단일 시편 내에서 부분박리지수가 증가할수록 부분박리영역이 감소하는 것을 확인하였다. 부분박리손상 map의 부분박리영역에 따른 분포를 이용하여 TBC의 완전박리 기준과 잔여 수명을 또한 도출할 수 있었다.

• 한국터널지하공간학회 논문집
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• 제10권4호
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• pp.313-327
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• 2008

화재 발생 시 지하구조물의 열-역학상호거동이 정확히 고려되지 못하고 있으며, 이로 인해 일반적인 열전달 이론에 근거한 수치해석 시 화재로 인한 구조물의 손상정도가 과소 평가될 수 있는 문제점이 있다. 따라서 본 연구에서는 화재 발생 시 지하구조물의 열-역학 상호거동을 모사하기 위한 유한요소 기반의 수치모델을 새롭게 개발하였다. 특히, 화재로 인한 구조물의 단면 손실을 모사하기 위한 요소제거모델을 제안하였고 대류 경계조건을 적용하였다. 이때 요소 내의 최대 온도가 해석 시에 설정한 임계온도 이상이 되면 요소가 제거되도록 설정하였다. 모형 화재시험 결과와 해석 결과를 비교한 변수해석을 통하여, RABT와 RWS 화재 시나리오 조건에 대한 최적의 임계온도, 요소크기, 온도에 따른 대류열전달계수 조건 등을 제시하였다.