• Journal of Korean Society of Coastal and Ocean Engineers
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• v.14 no.1
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• pp.76-85
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• 2002

The floating breakwater generally has an excellent water exchanging capability, but with some lowering harbor tranquility due to the wave transmission underneath floating body. In the initial stage of design, it is thus required to investigate several alternatives of breakwater layout for their performance of harbor tranquility. The present study aims to formulate a sort of reflection-transmission boundary condition of floating breakwater so that the existing numerical method using time dependent mild slope equation can still be applied to the case of floating breakwaters. The two and three dimensional tests were each performed to demonstrate the performance of the boundary condition. It was found that the reflection and transmission characteristics around the breakwater were well reproduced by the boundary condition. Finally, the reflection-transmission boundary condition were applied to a floating breakwater installed in an imaginary harbor with an irregular shape and bottom topography. The results surely showed that the present numerical method can effectively used in practical works related to the real sea construction of floating breakwaters.

• Proceedings of the Korea Water Resources Association Conference
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• 2022.05a
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• pp.205-205
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• 2022

본 연구에서는 심해 선형파 조건에서 수중 투과성 방파제 주변의 유속장에 대해 nonhomogeneous Riemann-Hilbert problem을 이용한 해석해 및 수치해를 도출하고, 이를 반사계수와 투과계수를 산정하는 데에 활용한다. 여러 개의 얇은 투과성 판이 일렬로 배열되어 수중에 고정되어있고 규칙파가 작용하는 경우, Riemann-Hilbert problem을 정의할 수 있다. 본 연구에서는 얇은 판으로 이루어진 수중 방파제에 대한 homogeneous Riemann-Hilbert problem을 푸는 것을 넘어, 투과성 판으로 이루어진 수중 방파제에 대해 nonhomogeneous Riemann-Hilbert problem을 정의하고, 이에 대해 무한경계조건과 판 근처에서의 유속장 경계조건을 이용해 해석해를 유도하였다. 투과성 방파제의 경우 permeable boundary를 가지므로 제시한 상황은 기하학적 비선형성을 지닌다. 이에 대해 투수성을 기초로 미소 매개변수를 정의하고, 섭동법(perturbation method)을 이용해 유속장에 대한 leading order solution과 first order solution을 도출하였다. Leading order solution은 Evans (1970) 등의 선행연구에서 제시한 해와의 비교를 통해 그 타당성을 검증하였고, First order solution을 이용해 반사계수와 투과계수를 산정하여 방파제의 투수성이 유속장에 미치는 영향을 고려하였다. 아울러 수치해를 도출하여 해석해의 결과와 비교 및 분석하였다. 본 연구에서 제시한 해석해는 방파제에 가해지는 힘을 산정하는 등 다양한 방향으로 활용 가능하며, 향후 수치해나 실험값을 비교, 검증하기 위한 기초 자료로써 활용될 수 있다.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.19 no.2
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• pp.170-178
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• 2007

Most of parabolic approximation models employ a relatively limited open boundary condition in which there is no depth variation in the longshore direction outside of the computation domain so that Snell's law may be presumed to hold. Existing Kirby's condition belongs to this category and in the paper both modified Kirby's method and Dirichlet boundary condition are presented in detail and numerical results of three methods were shown. Judging from computation to wave propagations over a circular shoal in a constant depth, the method based on present Dirichlet boundary condition with fictitious numerical adjusting regions in both sides of the computation domain gives the least distorted amplitude ratio distribution.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.15 no.2
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• pp.116-126
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• 2003

In this study, a method that leads to make a simple decision on important parameters in analysis of wave field in permeable rubble-mound, block-mound breakwater, such as penetration velocity of incident waves and resistance coefficient, is introduced. A model that could analyze wave field of permeable breakwater in harbor, by applying these methods and arbitrary transmission coefficient boundary condition to a time-dependent mild-slope equation, was introduced. The verification of the model was done by carrying out 2-D physical model test on permeable breakwater, measuring the change in water surface elevation, comparing the computation result with time series, and comparing the result gained from the 3-D physical model test on permeable block-mound breakwater in an field harbor with the computation result in terms of regional wave height ratio in a harbor.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.28 no.5B
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• pp.591-594
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• 2008

Present study aims at the development of a reasonable boundary condition for a structure over inclined seabed in case of the numerical model with quadrilateral mesh system. The technique for the inclined impermeable/permeable boundary in the quadrilateral mesh is newly proposed. The new technique and LES-WASS-3D model (Hur and Lee, 2007) have been used for the investigation of the dynamics of fluid field, and validated through the comparison with a typical stair-type boundary condition. 3-Dimensional numerical model with Large Eddy Simulation is called LES-WASS-3D, and is able to simulate directly interaction of WAve Structure Sea bed/Sandy beach.

• The Journal of the Acoustical Society of Korea
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• v.27 no.3
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• pp.129-139
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• 2008

The purpose of the present work is to derive the reflection and transmission coefficients of $S_0\;and\;A_0$ mode Lamb waves in relation to the geometry of a rectangular notch when the waves propagate across the notch in an elastic plate. Firstly, the excitable modes of the Lamb wave were analyzed with respect to the plate thickness. The scattering phenomena were divided into three independent processes according to the boundary shape of the notch and the direction of the wave propagation. Linear equations for each process were derived with corresponding free or continuous boundary conditions to analyze the scattered waves. By the rule of linear superposition, the waves scattered at each process were summed for each mode. Then the steady-state reflection and transmission coefficients of the scattered waves were determined so that the difference of energy flux between the incident and the scattered waves would remain within 4%.

• Journal of the Korean Society for Railway
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• v.16 no.1
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• pp.20-25
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• 2013

The frame elements of modern high speed trains are typically fabricated with extruded aluminum panels. However, the sound transmission loss (STL) of extruded aluminum panels is less satisfactory than flat panels with the same surface density. This study proposes a method for prediction of the sound transmission loss of extruded aluminum panels using finite element analysis. The panel is modeled by finite element analysis, and the STL is calculated using a measure of Sommerfeld radiation at the specimen surface, boundary conditions, and the internal loss factor of the material. In order to verify the validity of the predicted value, intensity transmission loss was measured on the aluminum specimen according to ASTM E2249-02. The proposed method of analysis will be utilized to predict the sound insulation performance of extruded aluminum panels for railway vehicles in the design stage, and to establish measures for their improvement.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 2011.04a
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• pp.693-696
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• 2011

본 논문에서는 레벨셋 방법을 이용하여, 소음을 차단하기 위한 음향 구조물의 형상 최적 설계를 수행하였다. 음향 결정 구조에서는 음향이 흩어져 있는 결정 구조에 의해서 굴절되기 때문에 결정 모양을 조정함으로써, 음향 거동을 제어 할 수 있다. 형상 최적 설계의 목적은 특정한 각도와 각속도로 입사되는 입사파에 대해서 음향 투과율(acoustic transmittance)이 최소가 되도록 음향 결정의 형상(inclusion shape)을 결정하는 것이다. 음향 압력(acoustic pressure)은 주기성을 갖는 음향 결정에 대해서 헬몰츠(Helmoltz)형태의 지배 방정식을 풀어서 얻을 수 있다. 본 연구에서는 음향 구조물로 결정이 수평 방향으로는 주기적으로 무한히 분포하고 수직방향으로는 유한한 층간 구조를 가지고 있는 소음 방어벽 (Noise barrier)을 고려한다. 결정의 위치는 고정되어 있고, 결정의 형상을 설계 변수로서 음파의 거동을 제어할 수 있도록 하였다. 주기적 구조물을 고려하기 때문에 결정의 좌와 우에 Bloch 이론을 적용해 주기적 경계조건을 부과하였고, 소음 방어벽 위와 아래에는 임피던스 행렬(impedance matrix)를 이용하여, 무한 균질 영역과 소음 방어벽사이의 음파 투과를 모사하였다. 복잡한 형상 변화를 표현하기 위해 임시적 경계를 이용한 레벨셋 방법을 사용하였다. 설계 민감도 해석을 통해 목적함수가 감소하는 방향으로 경계에서의 수직 벡터를 계산하고, 이를 헤밀턴-자코비(Hamilton-Jacob) 방정식에 대입하여, 최적의 형상을 나타내는 레벨셋 함수를 구하였다.

• The Journal of the Convergence on Culture Technology
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• v.9 no.3
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• pp.641-646
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• 2023

In this paper, TM(transverse magnetic) scattering problems by a resistive strip grating between a double dielectric layer are analyzed by using the FGMM(fourier galerkin moment method) known as a numerical method of electromagnetic fileld. The boundary conditions are applied to obtain the unknown field coefficients, the scattered electromagnetic fields are expanded in a series of Floquet mode functions, and the resistive boundary condition is applied to analysis of the resistive strip. Overall, as the uniform resistivity of the resistive strip increased, the size of the current density induced in the resistance band decreased, the reflected power decreased, and the transmitted power increased. In addition, As the thickness of the dielectric layer increased, the reflected power increased and the transmitted power relatively decreased. The numerical results of the structure proposed in this paper are shown in good agreement compared to the results of PMM, a numerical analysis method of the existing paper.

• Proceedings of the Acoustical Society of Korea Conference
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• 1984.12a
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• pp.52-56
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• 1984

파동 이론에 의한 수중에서의 저주파 음파전달은 음향학적 경계 조건에 의해 결정되는 Normal Mode로 특징지어 진다. Normal Mode는 수층(Water Layer)뿐만 아니라 수직적으로 층상 구조인 해저 퇴적층(Subatrate)의 음향 특성을 포함하여 결정되는 파동 방정식의 해로서 이에 의해 수층 및 해저 퇴적층에서의 음압 분포와 감쇠를 계산할 수 있다. 본 논문은 저주파 음파 전달에 관한 Normal Mode 이론에 의하여 음원의 주파수와 해저 퇴적층에서의 음속 분포등에 따른 각 Mode의 음압 분포, 감쇠등에 관한 음향학적 해석으로 원거리까지 진행하는 수중 음파의 해저 퇴적층 투과 심도를 추출하였다.