• International Journal of Ocean System Engineering
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• 제1권3호
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• pp.155-164
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• 2011

The second-order motion characteristics of a semi-submersible are investigated in regular waves. A higher-order boundary element method in a frequency domain and a finite element method in a time-domain were applied to the numerical analysis of the nonlinear hydrodynamic force and motion characteristics of semi-submersibles in view point of potential flow. Various aspects of nonlinear effects on the heave and roll of a semi-submersible were numerically investigated and some selected cases were compared with the model test data.

• International Journal of Naval Architecture and Ocean Engineering
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• 제6권4호
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• pp.1000-1013
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• 2014

This paper presents some of the efforts by the authors towards numerical prediction of springing of ships. A time-domain Higher Order Boundary Element Method (HOBEM) based on cubic shape function is first presented to solve a complete second-order problem in terms of wave steepness and ship motions in a consistent manner. In order to avoid high order derivatives on the body surfaces, e.g. mj-terms, a new formulation of the Boundary Value Problem in a body-fixed coordinate system has been proposed instead of traditional formulation in inertial coordinate system. The local steady flow effects on the unsteady waves are taken into account. Double-body flow is used as the basis flow which is an appropriate approximation for ships with moderate forward speed. This numerical model was used to estimate the complete second order wave excitation of springing of a displacement ship at constant forward speeds.

• 한국대기환경학회지
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• 제12권1호
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• pp.29-41
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• 1996

The two-stage numerical model was used to study the relation between three-dimensional local wind model, advection/diffusion model of random walk method and second moment method on Pusan coastal area. The first stage is three dimensional time-dependent local wind model which gives the wind field and vertical dirrusion coefficient. The second stage is advection/diffusion model which uses the results of the first stage as input data. First, wind fields on Pusan coastal area for none synoptic scale wind showed typical land and sea breeze circulation, and convergence zone occured at 1200LST in northern of domain, in succession, moved northward of domain. Emissions from Sinpyeong industrial district were trasnported toward the inland by sea breeze during daytime, and reached the end part of domain about 1800LST. During nighttime, emissions return to sea by land breeze and vertical diffusion also contributes to upward transport. In order to use this model for forecast of air pollution concentration on the Pusan coastal area, it is necessary that computed value must be compared with measured value and wind fields model must also be dealt in detail.

• 대한조선학회논문집
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• 제45권3호
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• pp.258-268
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• 2008

The radiation potential of a ship advancing in waves is studied using the 3D time-domain forward-speed free-surface Green function and the Green integral equation. Numerical solutions are obtained by making use of the 2nd order BEM(Boundary Element Method) which make it possible to take account of the line integral along the waterline in a rigorous manner. The 6 degree of freedom motion memory functions of a hemisphere and the Wigley seakeeping model obtained by direct integration of the time-domain 3D potentials over the wetted surface are presented for various Froude numbers.

• 한국음향학회지
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• 제15권1호
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• pp.23-33
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• 1996

본 논문에서는 bispectra를 위한 고전적 주영역 (classical principal domain)이 2계 Volterra 모델의 출력을 결정짓는데 사용되면 그 출력은 완전하지 못하게 될 것임을 지적한다. 이러한 불완전함은 DFT의 주기적 특성과 관련이 있다. 이런 이유로, 본 논문의 목적은 비선형 시스템의 응답의 추정을 향상기키는 Volterra 커널을 위한 확장된 주영역 (extended principal domain)을 제안하는데 있다. 확장된 주영역을 정의 내리기 위하여, 2차원 DFT와 Volterra 모델의 2계 요소와 정사각형 필터와의 관계를 사용하여 이산 시간 영역 Volterra 모델에서 새로운 이산 주파수 영역의 Volterra 모델을 유도하였다. 확장된 영역이 모델의 출력에 미치는 영향을 DFT의 주기성 측면에서 해석을 하였다. 컴퓨터 모의 실험을 통하여, Volterra 모델링에서 확장된 주영역의 영향을 살펴보았다. 모의 실험 결과에 의하면, Volterra 모델의 출력을 계산하는 과정과 Volterra 모델의 계수를 추정하는데 있어서 매우 중요한 역할을 함을 알 수 있었다.

• 제어로봇시스템학회논문지
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• 제8권6호
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• pp.439-444
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• 2002

In this paper, a new model reduction method is proposed to obtain a reduced order model in the frequency domain. The method is developed based on the second-order plus dead time modeling technique. The initial value of the reduced model parameters can be obtained using this method coinciding four point(0, -$\pi$/2, -$\pi$, -3$\pi$/2) on the Nyquist curve. The optimal parameters of the reduced model is obtained through calculation procedure with three steps. It is shown that Nyquist curves and unit step responses of the reduced models of numerical examples closely agree with those of original models.

• 대한조선학회논문집
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• 제47권3호
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• pp.291-305
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• 2010

A three-dimensional time-domain calculation method is of crucial importance in prediction of the motions and wave loads of a ship advancing in a severe irregular sea. The exact solution of the free surface wave-ship interaction problem is very complicated because of the essentially nonlinear boundary conditions. In this paper, an approximate body nonlinear approach based on the three-dimensional time-domain forward-speed free-surface Green function has been presented. The Froude-Krylov force and the hydrostatic restoring force are calculated over the instantaneous wetted surface of the ship while the forces due to the radiation and scattering potentials over the mean wetted surface. The time-domain radiation and scattering potentials have been obtained from a time invariant kernel of integral equations for the potentials which are discretized according to the second-order boundary element method (Hong and Hong 2008). The diffraction impulse-response functions of the Wigley seakeeping model advancing in transient head waves at various Froude numbers have been presented. A simulation of coupled heave-pitch motion of a long rectangular barge advancing in regular head waves of large amplitude has been carried out. Comparisons between the linear and the approximate body nonlinear numerical results of motions and wave loads of the barge at a nonzero Froude number have been made.

• 대한토목학회논문집
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• 제26권5B호
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• pp.539-549
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• 2006

본 연구에서는 포텐셜 이론, 섭동법, 경계요소법에 근간을 둔 이차의 시간영역 수치모델을 개발하고 이를 이용하여 폰툰형 부방파제의 성능을 평가하였다. 다양한 설계조건에 대하여 파력, 운동변위, 자유수면고, 투과율 등의 변화를 고찰하였으며, 파랑의 약 비선형성이 방파제의 동수역학적 특성에 미치는 영향을 분석하는데 주안점을 두었다. 수치모의 수행 결과, 이차의 성분 파는 동유체력, 계류장력, 운동변위에 미치는 영향이 큰 것으로 분석되었으나, 파랑의 약 비선형성이 투과율에 미치는 영향은 매우 작아 선형해석만으로도 파랑제어효율을 평가할 수 있음을 확인하였다. 또한 파랑제어효율이 우수한 수심과 흘수의 비, 파수와 폭과의 관계 등을 제시하였다.

• 산업경영시스템학회지
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• 제10권16호
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• pp.39-51
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• 1987

This paper deals with the case analysis of second order processes under sampled-data. Proportional Integral-Derivative(PID) control, and development of Decision Support Systems(DSS) for such processes. In this paper three techniques were described for identifying the dynamics of closed loop stable processes. The first, called pulse testing is a frequency-domain method, which yields the frequency response diagram of an open loop process. The second is a time-domain method which yields the gain and time constants of the process model. The third technique is based on step response and gives the parameters of PID controllers. The development of DSS design programs consisting of above three techniques will provide very powerful tools in the microcomputer based process control.

• International Journal of Aeronautical and Space Sciences
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• 제2권1호
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• pp.78-92
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• 2001

In this paper, several dynamic systems are modeled using the time domain finite element method. Galerkins' Weak Principle is used to model the general second-order mechanical system, and is applied to a simple pendulum dynamics. Problems caused by approximating the final momentum are also investigated. Extending the research, some dynamic analysis methods are suggested for the hybrid coordinate systems that have both slew and flexible modes. The proposed methods are based on both Extended Hamilton's Principle and Galerkin's Weak Principle. The matrix wave equation is propagated in space domain, satisfying the geometric/natural boundary conditions. As a result, the flexible motion can be obtained compatible with the applied control input. Numerical example is shown to demonstrate the effectiveness of the proposed modeling methods for the hybrid coordinate systems.