• Ocean Systems Engineering
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• 제9권1호
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• pp.21-42
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• 2019

In this paper, the Finite-Analytic Navier-Stokes (FANS) code is coupled with an in-house finite-element code to study the dynamic interaction between a floating buoy and its mooring system. Hydrodynamic loads on the buoy are predicted with the FANS module, in which Large Eddy Simulation (LES) is used as the turbulence model. The mooring lines are modeled based on a slender body theory. Their dynamic responses are simulated with a nonlinear finite element module, MOORING3D. The two modules are coupled by transferring the forces and displacements of the buoy and its mooring system at their connections through an interface module. A free-decay model test was used to calibrate the coupled method. In addition, to investigate the capability of the present coupled method, numerical simulations of two degree-of-freedom vortex-induced motion of a CALM buoy in uniform currents were performed. With the study it can be verified that accurate predictions of the motion responses and tension responses of the CALM buoy system can be made with the coupling CFD-FEM method.

• Ocean Systems Engineering
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• 제6권4호
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• pp.305-324
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• 2016

Wave load prediction at zero forward speed using finite depth Green function is a well-established method regularly used in the offshore and marine industry. The forward speed approximation in deep water condition, although with limitations, is also found to be quite useful for engineering applications. However, analysis of vessels with forward speed in finite water depth still requires efficient computing methods. In this paper, a method for analysis of wave induced forces and corresponding motion on freely floating three-dimensional bodies with low to moderate forward speed is presented. A finite depth Green function is developed and incorporated in a 3D frequency domain potential flow based tool to allow consideration of finite (or shallow) water depth conditions. First order forces and moments and mean second order forces and moments in six degree of freedom are obtained. The effect of hull flare angle in predicting added resistance is incorporated. This implementation provides the unique capability of predicting added resistance in finite water depth with flare angle effect using a Green function approach. The results are validated using a half immersed sphere and S-175 ship. Finally, the effect of finite depth on a tanker with forward speed is presented.

• Steel and Composite Structures
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• 제4권6호
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• pp.437-452
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• 2004

A simplified seismic design procedure for steel structures with buckling-restrained braces (BRB) was proposed based on the energy balance concept and the equal energy assumption. The input seismic energy was estimated from a design spectrum, and the elastic and hysteretic energy were computed using energy balance concept. The size of braces was determined so that the hysteretic energy demand was equal to the hysteretic energy dissipated by the BRB. The validity of using equivalent single-degree-of-freedom systems to estimate seismic input and hysteretic energy demand in multi story structures with BRB was investigated through time-history analysis. The story-wise distribution pattern of hysteretic energy demands was also obtained and was applied in the design process. According to analysis results, the maximum displacements of the 3-story structure designed in accordance with the proposed procedure generally coincided with the target displacements on the conservative side. The maximum displacements of the 6- and 8-story structures, however, turned out to be somewhat smaller than the target values due to the participation of higher vibration modes.

• Smart Structures and Systems
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• 제2권2호
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• pp.127-140
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• 2006

In this study, a wavelet-based covariance-driven system identification technique is proposed for damage assessment of structures under ambient excitation. Assuming the ambient excitation to be a white-noise process, the covariance computation is shown to be able to separate the effect of random excitation from the response measurement. Wavelet transform (WT) is then used to convert the covariance response in the time domain to the WT magnitude plot in the time-scale plane. The wavelet coefficients along the curves where energy concentrated are extracted and used to estimate the modal properties of the structure. These modal property estimations lead to the calculation of the stiffness matrix when either the spectral density of the random loading or the mass matrix is given. The predicted stiffness matrix hence provides a direct assessment on the possible location and severity of damage which results in stiffness alteration. To demonstrate the proposed wavelet-based damage assessment technique, a numerical example on a 3 degree-of-freedom (DOF) system and an experimental study on a three-story building model, which are all under a broad-band excitation, are presented. Both numerical and experimental results illustrate that the proposed technique can provide an accurate assessment on the damage location. It is however noted that the assessment of damage severity is not as accurate, which might be due to the errors associated with the mode shape estimations as well as the assumption of proportional damping adopted in the formulation.

• 한국해양공학회지
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• 제26권1호
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• pp.17-26
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• 2012

A coupled analysis of a floating crane barge with a crane wire and hanging structure is carried out in thetime domain. The motion analysis of the crane barge is based on the floating multi-body dynamics, and thecrane wire is modeled as a simple spring tension. The hanging structure is assumed to be a rigid body with 3 degree-of-freedom translational motion. In this study, numerical simulations were conducted at three different stages. First, the developed code was validated by comparing the time-domain motion response of a crane barge with the frequency-domain results. Then, a coupled analysis of a crane barge and simple structure hanging by the crane wire was performed using the present scheme. The motion response and wire tension from the present calculations are compared with the results of OrcaFlex. The agreement between the two sets of results isfairly good. Last, lowering simulations in regular and irregular waves were conducted considering buoyancy changes in the hanging structure. The effects of the wave conditions, structure's weight, wire length, and lowering speed on the wire tension are considered.

• 대한기계학회논문집A
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• 제25권11호
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• pp.1785-1795
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• 2001

A multi-layered brick element fur the finite element method is developed for analyzing the three-dim-ensionally layered composite structures subjected to both thermal and mechanical boundary conditions. The element has eight nodes with one degree of freedom for the temperature and three for the display-ements at each node, and can contain arbitrary number of layers with different material properties with-in the element; the conventional element should contain one material within an element. Thus the total number of nodes and elements, which are needed to analyze the multi-layered composite structures, can be tremendously reduced. In solving the global equation, a partitioning technique is used to obtain the temperature and the displacements which are caused by both the mechanical boundary conditions and temperature distributions. The results by using the developed element are compared wish the commercial package, ANSYS and the conventional finite element methods, and they are in good agreement. It is also shown that the Number of nodes and elements can be tremendously reduced using the element without losing the numerical accuracies.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2004년도 춘계학술대회논문집
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• pp.101-104
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• 2004

This paper addresses uncertainty issues encountered recently in measuring head vibration using the conventional 6-axis or 9-axis bite-bar Those conventional bite-bars are shown to present insufficient information to measure a complete 6 degree-of-freedom motion of head vibration. In order to overcome such limit, a theoretical measurement model that consists of four 3-axis linear accelerometers is suggested (Theoretical backgrounds presented in this paper shall have been addressed in the international congress of ICA 2004 in this April). It is shown to enable the direct measurement of three angular acceleration components and six angular velocity-dependent nonlinear terms. In audition to the three linear acceleration terms, those nine angular motion-dependent ones are found to make it possible to evaluate the general head vibration for a given position. To examine the feasibility of the proposed method, a newly designed 12-axis bite-bar was developed. Detailed experimental results obtained using the developed 12-axis bite-bar are illustrated in the presentation of this paper, which illustrates what amount of measurement accuracy provides. But, this paper provides more detailed experimental data and extended uncertainty factors.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2012년도 춘계학술대회 논문집
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• pp.324-329
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• 2012

Isolation tables are widely used for precision equipments and their isolation performances have been usually expressed and evaluated by transsmissibility. However, transmissibility is a concept for 1-degree of freedom(DOF) system. In practice, isolation tables are supproted by more than 4 springs. Each spring is subjected to vertical and horizontal ground vibrations, and also the table has more than 1-DOF. Therefore, isolation tables should be treated as multi-input/multi-output(MIMO) system of which isolation performance is expressed by transmissibility matrix. However, the matrix is too complicated to be an index for a system. In this paper, maximum singular value of transmissibility matrx is suggested as a simple performance index of a MIMO isolation system. Physical meaning of singular value is explained using a simple a 2-DOF isolation table. Furthermore, maximum singular values of passive, 3-DOF active and 6-DOF active isolation tables are obtained through experiments, and their meaning are explained and compared with each other.

• 한국정밀공학회지
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• 제20권8호
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• pp.205-212
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• 2003

In this study, in order to investigate a modeling technique of the structure with bolted joints, four kinds of finite element model are introduced; a solid bolt model, a coupled bolt model, a spider bolt model, and no bolt model. All proposed models take account on prestrained effect and contact behavior of flanges to be joined. Among these models, a solid bolt model, which is modeled by using a 3-D solid element and a surface-to-surface contact element between the head/nut and the flange interfaces, has the best accurate responses compared with the experimental results. In addition, coupled bolt model, which couples the degree of freedom between the head/nut and the flange, shows the best effectiveness and usefulness in view of computational time and memory usage. Finally, the bolt model proposed here is adopted for structural analysis of a large diesel engine of a ship consisting of several parts which is connected by long stay bolts.

• 한국지능시스템학회논문지
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• 제14권3호
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• pp.298-303
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• 2004

본 논문은 임의의 입력지연을 갖는 Takagi-Sugeno (T-S) 퍼지 시스템의 관측기 기반 출력궤환 제어 시스템을 논의한다. 설계된 연속시간 T-S 퍼지 관측기 시스템을 영차의 샘플/홀드 함수를 이용하여 이산시간 관측기를 설계한다. 이때 플랜트와 관측기의 출력에러가 제어기를 통하여 궤환되기 때문에 이산화 과정에서 발생한 에러를 보정할 수 있다. 여기에서 시스템의 제어 입력은 임의로 변화하는 유한개의 상태를 갖는 마코프 확률과정으로 표현한다. 생성된 시스템의 확률적 안정 가능성 조건은 선형 행렬 부등식의 형태로 표현한다. 이러한 결과를 2자유도 헬리콥터의 모델에 대한 모의실험을 통하여 효용성을 확인한다.