• 한국해안·해양공학회논문집
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• 제31권5호
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• pp.278-293
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• 2019

파고와 주기 결합분포는 그 공학적 가치에도 불구하고, 주기에 대한 해석 모형의 부재로 인해 파고 분포에 비해 상대적으로 소홀히 다루어져, 현재 비선형성이 주기분포에 미치는 영향에 대해서도 서로 다른 의견이 상존한다. 이에 비해 파고 분포의 경우, 많은 노력이 이루어져 성과가 상당하나, 아직 이러한 성과는 파고와 주기 결합분포로 확대되지 못하였다. 본 논문에서는 이러한 문제를 해결하기 위해 먼저 파고와 주기의 결합분포를 조건부 주기 분포와 파고 분포의 곱으로 정의하였다. 이어 비선형 불규칙 파동계에서의 파고 분포, 임의의 대역폭을 지니는 비선형 불규칙 파랑계에서의 파고분포를 유도하고, 이를 Longuet-Higgins(1975, 1983), Cavanie et al.(1976)의 조건부 주기확률분포와 결합하여 새로운 파고와 주기 결합분포를 제시하였다. 검증과정은 Wallops 스펙트럼에 기초하여 수치 모의된 파랑시계열자료와 경사가 1:15인 단조해안에서 진행되는 불규칙 파랑 천수과정 수치모의를 통해 얻은 강비선형 파랑자료를 활용하여 수행되었으며, 모의 결과 finite banded waves를 대상으로 한 파고 분포와 Cavanie et al. (1976)의 조건부 주기 확률분포를 활용하는 경우 가장 근접한 결과를 얻을 수 있었다.

• 대한조선학회지
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• 제24권3호
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• pp.1-8
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• 1987

In this paper, the localized finite element method(LFEM) is applied to 3-dimensional ship motion problems in water of infinite depth. The LFEM used here is based on the functional constructed by Bai & Yeung(1974). To test the present numerical scheme, a few vertical axisymmetric bodies are treated by general 3-dimensional formulation. The computed results of hydrodynamic coefficients for a few vertical spheroids and vertical circular cylinders show good agreement with results obtained by others. The advantages of the present numerical method compared with the method of integral equation are as follows; (i) The cumbersome existence of irregular frequencies in the method of conventional integral equation is removed. (ii) The final matrix is banded and symmetric and the computation of the matrix elements is comparatively easier, whereas the size of the matrix in the present scheme is much larger. (iii) In the future research, it is possible to accommodate with the nonlinear exact free surface boundary condition in the localized finite element subdomain, whereas the linear solution is assumed in the truncated(far field) subdomain.

• The Journal of the Acoustical Society of Korea
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• 제18권4E호
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• pp.20-25
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• 1999

To get accurate natural frequencies of an engine crankshafts, finite element equations of motion are developed, taking real geometries of the shaft into account. For the crankshaft with wide crank webs, a specialized rotating web element is developed. This includes the effects of rotary inertia, gyroscopic moment, and shear. After the finite element equations are constructed, eigenvalues are extracted from the system equations to get natural frequencies, based on the Sturm sequence method which exploits the banded forms of the system matrices to reduce computations. The scheme developed can be used for the free vibration analysis of any type of spinning structures which include skew symmetric gyroscopic moment matrix in the system matrices. The results are compared with experimental data in order to confirm the study.

• 한국해양공학회:학술대회논문집
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• 한국해양공학회 2006년 창립20주년기념 정기학술대회 및 국제워크샵
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• pp.127-134
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• 2006

Based on a weakly non-Gaussian theory the occurrence probability of freak waves is formulated in terms of the number of waves in a time series and the surface elevation kurtosis. Finite kurtosis gives rise to a significant enhancement of freak wave generation in comparison with the linear narrow banded wave theory. For fixed number of waves, the estimated amplification ratio of freak wave occurrence due to the deviation from the Gaussian theory is 50% - 300%. The results of the theory are compared with laboratory and field data.

• Structural Engineering and Mechanics
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• 제55권5호
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• pp.1055-1086
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• 2015

The dynamic response of two-dimensional unbounded domain on the rigid bedrock in the time domain is numerically obtained. It is realized by the modified scaled boundary finite element method (SBFEM) in which the original scaling center is replaced by a scaling line. The formulation bases on expanding dynamic stiffness by using the continued fraction approach. The solution converges rapidly over the whole time range along with the order of the continued fraction increases. In addition, the method is suitable for large scale systems. The numerical method is employed which is a combination of the time domain SBFEM for far field and the finite element method used for near field. By using the continued fraction solution and introducing auxiliary variables, the equation of motion of unbounded domain is built. Applying the spectral shifting technique, the virtual modes of motion equation are eliminated. Standard procedure in structural dynamic is directly applicable for time domain problem. Since the coefficient matrixes of equation are banded and symmetric, the equation can be solved efficiently by using the direct time domain integration method. Numerical examples demonstrate the increased robustness, accuracy and superiority of the proposed method. The suitability of proposed method for time domain simulations of complex systems is also demonstrated.

• 대한기계학회논문집A
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• 제24권5호
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• pp.1215-1230
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• 2000

Parallel Approaches using Cray T3E which is NIPP (Massively Parallel Processors) machine are presented for the efficient computation of the finite element analysis of 3-D shape rolling processes. D omain decomposition method coupled with parallel linear equation solver is used. Domain decomposition is applied for obtaining element tangent stifffiess matrices and residual vectors. Direct and iterative parallel algorithms are used for solving the linear equations. Direct algorithm is_parallel version of direct banded matrix solver. For iterative algorithms, the well-known preconditioned conjugate gradient solver with Jacobi preconditioner is also employed. Moreover a new effective iterative scheme with block inverse matrix preconditioner, which is named by present authors, is presented and its results are compared with the one using Jacobi preconditioner. PVM and MPI are used for message passing and synchronization between processors. The performance and efficiency of each algorithm is discussed and comparisons are made among different algorithms.

• Journal of the Korean Society for Industrial and Applied Mathematics
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• 제3권2호
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• pp.161-171
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• 1999

Aerodynamic sensitivity analysis codes are developed via the hand-differentiation using a direct differentiation method and an adjoint method respectively from discrete two-dimensional compressible Navier-Stokes equations. Unlike previous other researches, Baldwin-Lomax algebraic turbulence model is also differentiated by hand to obtain design sensitivities with respect to design variables of interest in turbulent flows. Discrete direct sensitivity equations and adjoint equations are efficiently solved by the same time integration scheme adopted in the flow solver routine. The required memory for the adjoint sensitivity code is greatly reduced at the cost of the computational time by allowing the large banded flux jacobian matrix unassembled. Direct sensitivity code results are found to be exactly coincident with sensitivity derivatives obtained by the finite difference. Adjoint code results of a turbulent flow case show slight deviations from the exact results due to the limitation of the algebraic turbulence model in implementing the adjoint formulation. However, current adjoint sensitivity code yields much more accurate sensitivity derivatives than the adjoint code with the turbulence eddy viscosity being kept constant, which is a usual assumption for the prior researches.

• 대한조선학회지
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• 제25권3호
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• pp.3-12
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• 1988

A natural or an artificial harbor can exhibit frequency(or period) dependent water surface oscillations when excited by incident waves. Such oscillations in harbors can cause significant damage to moored ships and adjacent structures. This can also induce undesirable current in harbors. Many previous investigators have studied various aspects of harbor resonance problem. In the percent paper, both a localizes finite element method(LFEM) which is based on the functional constructed by Chen & Mei(1974) and Bai & Yeung(1974) and an integral equation method which was used by Lee(1969) are applied to harbor resonance problem. The present method(LFEM) shows computationally more efficient than the integral equation method. Our test results shows good agreement compared with other results. This enhanced computational efficiency is due to the fact that the present method gives a banded symmetric coefficients matrix and requires much less computational time in the calculation of the influence coefficients matrix than the integral equation method involved with Green's function. To test the present numerical scheme, two models are treated here. The present method(LFEM) can be extended to a fully three dimensional harbor problem with the similar computational advantage.

• 한국토양환경학회지
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• 제1권1호
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• pp.89-102
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• 1996

지하의 유체 유동 및 물질 변환을 해석하기 위하여 다중다상이론을 이용한 통합 모형을 개발하였다 종합적 지배식은 4개의 상내의 화합물들의 물질 및 힘평형 관계를 고려하여 유도되었다. 복합한 이동 및 변환 현상을 설명하고, 공간적 차원을 변동적으로 나타내기 위하여 관계된 모든 변수 및 식들을 함축적이면서 조직적으로 표현하였다. 도출된 비선형시스템은 다차원 유한요소프로_I램으로서 해를 구하였다. 본 개발된 프로그램은 역동적으로 메모리 용량을 조절하여 일이삼차원 문제를 PC부터 SP2슈퍼컴퓨터까지 여러 종류의 기종에서 해석할 수 있다. 계산시간과 저장용량을 줄이기 위하여 시스템식을 분리시키고, 슈퍼컴의 벡터 및 병렬처리를 이용하여 띠행렬의 해를 구하였다. 유속이 우세한 경우의 수치해석상의 불안정한 문제를 해결하기 위하여 상류가중, 질량묶음, 요소별 파라미터 평가법 등을 적용하였다. 일차원 이동문제에 대하여 유한요소법과 유한차분법의 수치해의 안정성 조건을 검토하였다. 구체적인 지하수 유동 및 오염문제에 대한 모델링 예는 본 논문집의 연계 논문에 수록하였다.