• Wind and Structures
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• 제32권4호
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• pp.341-354
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• 2021

Articulated towers are one of the class of compliant offshore structures that freely oscillates with wind and waves, as they are designed to have low natural frequency than ocean waves. The present study deals with the dynamic response of a double-pendulum articulated tower under hydrodynamic and aerodynamic loads. The wind field is simulated by two approaches, namely, single-point and multiple-point. Nonlinearities such as instantaneous tower orientation, variable added mass, fluctuating buoyancy, and geometrical nonlinearities are duly considered in the analysis. Hamilton's principle is used to derive the nonlinear equations of motion (EOM). The EOM is solved in the time domain by using the Wilson-θ method. The maximum, minimum, mean, and standard deviation and salient power spectral density functions (PSDF) of deck displacement, bending moment, and central hinge shear are drawn for high and moderate sea states. The outcome of the analyses shows that tower response under multiple-point wind-field simulation results in lower responses when compared to that of single-point simulation.

• 한국해양공학회지
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• 제29권3호
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• pp.217-223
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• 2015

In order to predict the motions of a planing hull in waves, it is necessary to accurately estimate the force components acting on the hull such as the hydrodynamic force, buoyancy, and friction, as well as the wave exciting force. In particular, based on strip theory, hydrodynamic forces can be estimated by the summation of the forces acting on each cross-section of the hull. A non-linear strip method for planing hulls was mathematically developed by Zarnick, and his formula has been used to predict the vertical motions of prismatic planing hulls in regular waves. In this study, several improvements were added to Zarnick's formula to predict the vertical motions of warped planing hulls. Based on calm water model test results, the buoyancy force and moment correction coefficients were modified. Further improvements were made in the pile-up correction. Pile-up correction factors were changed according to variations of the deadrise angles using the results found in previous research. Using the same hull form, captive model tests were carried out in other recent research, and the results were compared with the present calculation results. The comparison showed reasonably good agreements between the model tests and present calculations.

• 터널과지하공간
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• 제8권1호
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• pp.8-16
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• 1998

The first purpose of this study is to verify the application of computer modelling to a enclosed space fire. The second one is to determine temperature distribution for the three different ventilation types in case of a enclosed space fire. The third one is to find out the ventilation direction and ventilation quantity to remove effectively heat and combustion products generated by a fire in variable air volume(VAV) system. Firstly, compared with experimental results of Lawrence Livermore National Laboratory(LLNL), numerical results show good agreements. Secondly, among three different ventilation types, the numerical analyses show the highest temperature distribution in occupied zone(up to 1.8 m from bottom) from firing moment to 100 sec. when supply ducts are placed in ceiling and extract duct is placed close to the bottom on side walls. This is due to disadvantageous position of extract duct in ventilating high temperature air which rise because of buoyancy force. Thirdly, this study finds out effective ventilation direction and ventilation quantity to remove heat and combustion products generated by a fire by using VAV system. $CO_2$ concentration is used as a fire fume removal index. As soon as a fire happens, ventilation direction is changed in order to gather and drive out fire fumes. In case of three times ventilation quantity of ordinary one, $CO_2$ concentration and temperature have begun to decrease at 120 sec. after firing, i.e.fire fumes have begun to be removed.

• 대한기계학회논문집
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• 제16권11호
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• pp.2090-2097
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• 1992

본 연구에서는 밸브판의 거동을 2차의 회전계로 간주하고, 양력 및 항력(drag force)을 구하는데 있어 Reif등이 구한것과 동일한 방법을 이용하되 정지핀에 의해 그 최대 열림각이 제한을 받는 물리적 사실을 고려하고, 밸브를 통과하는 유량도 단위 입 력이 아닌 정현파(sine wave)로 간주하여 밸브판의 동적거동을 해석하였다. 해석대 상의 밸브 모델로는 Bjork-Shiley 27mm 1엽 밸브를 선정하였다.

• Smart Structures and Systems
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• 제18권4호
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• pp.683-705
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• 2016

A semi-active algorithm for edgewise vibration control of the spar-type floating offshore wind turbine (SFOWT) blades, nacelle and spar platform is developed in this paper. A tuned mass damper (TMD) is placed in each blade, in the nacelle and on the spar to control the vibrations for these components. A Short Time Fourier Transform algorithm is used for semi-active control of the TMDs. The mathematical formulation of the integrated SFOWT-TMDs system is derived by using Euler-Lagrangian equations. The theoretical model derived is a time-varying system considering the aerodynamic properties of the blade, variable mass and stiffness per unit length, gravity, the interactions among the blades, nacelle, spar, mooring system and the TMDs, the hydrodynamic effects, the restoring moment and the buoyancy force. The aerodynamic loads on the nacelle and the spar due to their coupling with the blades are also considered. The effectiveness of the semi-active TMDs is investigated in the numerical examples where the mooring cable tension, rotor speed and the blade stiffness are varying over time. Except for excessively large strokes of the nacelle TMD, the semi-active algorithm is considerably more effective than the passive one in all cases and its effectiveness is restricted by the low-frequency nature of the nacelle and the spar responses.

• Ocean Systems Engineering
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• 제6권4호
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• pp.345-362
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• 2016

This study aims to develop the numerical method to estimate level ice impact load and investigate the dynamic interaction between an arctic Spar with sloped surface and drifting level ice. When the level ice approaches the downward sloped structure, the interaction can be decomposed into three sequential phases: the breaking phase, when ice contacts the structure and is bent by bending moment; the rotating phase, when the broken ice is submerged and rotated underneath the structure; and the sliding phase, when the submerged broken ice becomes parallel to the sloping surface causing buoyancy-induced fictional forces. In each phase, the analytical formulas are constructed to account for the relevant physics and the results are compared to other existing methods or standards. The time-dependent ice load is coupled with hull-riser-mooring coupled dynamic analysis program. Then, the fully coupled program is applied to a moored arctic Spar with sloped surface with drifting level ice. The occurrence of dynamic resonance between ice load and spar motion causing large mooring tension is demonstrated.

• 한국해양공학회:학술대회논문집
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• 한국해양공학회 2002년도 추계학술대회 논문집
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• pp.83-87
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• 2002

종강도 위주의 일반 상선의 LMC 의 경우는 단지 선박을 l 차원 Beam Model 로 단순화하여 선미로부터 선수까지의 Weight Distribution 과 Buoyancy Distribution 을 계산하여 두 값의 차이를 Shear Force 로 계산하고 Shear Force 적분값을 Bending Moment 로 계산한다. 횡강도가 중요시되는 Barge 선의 경우 Global Transverse Strength 같은 경우에는 위의 식을 적용할 수 있으나 복수의 바지선을 Hinge Type 이 아닌 Fixed Type 으로 고정시켜 사용할 경우 각각의 Connector 에 작용하는 Strength 값이 횡강도의 큰 비중을 차지한다. 일반적인 Load Master Computer 의 경우 이와 같은 계산이 불가능하며 NAPA 와 같은 전용 계산 프로그램의 경우 하나의 Condition 을 계산하는데 소요되는 시간이 많아 실질적인 Monitoring 은 불가능하다. 이에 특수목적의 Load Master Computer(ShipManager-88) 를 제작하게 되었고 이 Program 을 이용하여 Loadout 과 Floatoff 의 Simulation 을 수행하고 Monitoring 하였다. ShipManager-88 은 Barge 선의 종강도 횡강도, Stability, Trim & Draft 등을 계산하며 Sequence 기능으로 실제 LOADOUT 과 FLOATOFF 시의 모의시뮬레이션을 수행해 볼 수 있으며 Online Interlace 제공으로 Tank 에 설치된 센서에서 Level 값을 받아 실시간으로 현재 선박의 상태를 정확하게 계산할 수 있다. 실제 LOADOUT and FLOATOFF 를 수행하면서 Check 한 부분은 종강도, 횡강모 Stability, Deform, Connector Strength, Level 등을 Check 하였고 종방향의 LOADOUT 이 불가능한 Project 를 위해 Transverse LOADOUT 을 이용할 계획이다.

• 한국지반신소재학회논문집
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• 제14권1호
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• pp.43-50
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• 2015

본 연구에서는 현장잔토를 활용할 수 있고 다짐이 불필요하여 관로공사에 유용한 경량기포혼합토를 지중매설관의 뒷채움재로 이용할 경우 관로의 거동을 조사하기 위하여 시험시공을 실시하고 계측결과를 분석하였다. 동시에 동일한 조건의 새만금 준설토사 뒷채움 시공결과와 비교하였다. 연직토압의 경우 경량기포혼합토 뒷채움 시 준설토사 대비 초기 슬러리 상태에서 25.6% 가량 저감되었고 양생 후에는 준설토사 토압의 10% 이내에 불과하였다. 준설토사 뒷채움 시에는 경량기포혼합토 대비 수평토압의 경우 3.6배 이상, 연직 및 수평변위는 각각 3.2배와 2.6배 가량의 차이를 나타내어 경량기포혼합토의 토압 및 변위저감 효과가 우수한 것으로 나타났다. 매설관 상반부에서 측정된 응력의 경우 토사 뒷채움 시 대체로 압축응력이 발생한 반면, 경량기포혼합토를 타설한 경우 초기 슬러리 상태에서 발생한 부력으로 인하여 고정된 양단부를 지점으로 부(-)의 모멘트가 발생함으로써 상반부에 인장응력이 발생하는 결과를 얻었다. 결론적으로, 새만금에서 준설된 토사를 이용한 경량기포혼합토는 인근 개발지에서 매설관 뒷채움재로 활용하는데 있어 준설토사와 비교하여 매우 우수한 재료이나 타설 시 부력에 대한 대책이 필요하다.

• 대한조선학회논문집
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• 제29권4호
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• pp.87-97
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• 1992

극지방의 천연자원의 수송에 필수적인 빙해항행선박은 금후 우리나라의 조선산업이 지향해야 할 기술집약형 선박중 하나로서 국제경쟁에 대비하여 독자적인 연구가 요구되는 분야이다. 본 연구는 빙해항행선박 기본설계의 중요한 인자인 선수부 형상과 평탄빙에서의 쇄빙능력 사이의 상관관계를 밝히는데 목적이 있다. 통상 평탄빙(level ice)에서 선박의 전진속도를 3-4 knots로 볼 때, 탄성으로 취급되는 얼음의 재료특성을 고려하여, 파괴시킬 수 있는 얼음의 최대두께와 파괴된 얼음의 특성길이를 수치적으로 추정하는데 주안점을 두고 있다. 본 연구에서는 빙해항행 선박이 평탄빙에서 연속쇄빙을 하고 있는 상황을 탄성지지기반 위에 놓인 유한 길이의 쐐기보에 작용한 충격하중의 문제로 가정하고 굽힘모멘트에 의해 어떤 위치에서 발생한 최대인장응력이 얼음의 굽힘파괴강도에 도달한다면 그 부분에서 파단이 일어날 가능성이 가장 높다고 판단한다.

• Coupled systems mechanics
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• 제2권3호
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• pp.231-253
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• 2013

The impact of spar-nacelle-blade coupling on edgewise dynamic responses of spar-type floating wind turbines (S-FOWT) is investigated in this paper. Currently, this coupling is not considered explicitly by researchers. First of all, a coupled model of edgewise vibration of the S-FOWT considering the aerodynamic properties of the blade, variable mass and stiffness per unit length, gravity, the interactions among the blades, nacelle, spar and mooring system, the hydrodynamic effects, the restoring moment and the buoyancy force is proposed. The aerodynamic loads are combined of a steady wind (including the wind shear) and turbulence. Each blade is modeled as a cantilever beam vibrating in its fundamental mode. The mooring cables are modeled using an extended quasi-static method. The hydrodynamic effects calculated by using Morison's equation and strip theory consist of added mass, fluid inertia and viscous drag forces. The random sea state is simulated by superimposing a number of linear regular waves. The model shows that the vibration of the blades, nacelle, tower, and spar are coupled in all degrees of freedom and in all inertial, dissipative and elastic components. An uncoupled model of the S-FOWT is then formulated in which the blades and the nacelle are not coupled with the spar vibration. A 5MW S-FOWT is analyzed by using the two proposed models. In the no-wave sea, the coupling is found to contribute to spar responses only. When the wave loading is considered, the coupling is significant for the responses of both the nacelle and the spar.