• Proceedings of the KSME Conference
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• 2005.11a
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• pp.53.1-53.1
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• 2005

• Journal of the Korean Society for Nondestructive Testing
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• v.20 no.2
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• pp.138-149
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• 2000

Various modeling techniques for ultrasonic wave propagation and scattering problems in finite solid media are presented. Elastodynamic boundary value problems in inhomogeneous multi-layered plate-like structures are set up for modal analysis of guided wave propagation and numerically solved to obtain dispersion curves which show propagation characteristics of guided waves. As a powerful modeling tool to overcome such numerical difficulties in wave scattering problems as the geometrical complexity and mode conversion, the Boundary Element Method(BEM) is introduced and is combined with the normal mode expansion technique to develop the hybrid BEM, an efficient technique for modeling multi mode conversion of guided wave scattering problems. Time dependent wave forms are obtained through the inverse Fourier transformation of the numerical solutions in the frequency domain. 3D BEM program development is underway to model more practical ultrasonic wave signals. Some encouraging numerical results have recently been obtained in comparison with the analytical solutions for wave propagation in a bar subjected to time harmonic longitudinal excitation. It is expected that the presented modeling techniques for elastic wave propagation and scattering can be applied to establish quantitative nondestructive evaluation techniques in various ways.

• ETRI Journal
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• v.11 no.1
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• pp.3-19
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• 1989

반도체 재료의 결정구조를 분석하기 위하여 사용되는 전자현미경의 전자선 회절패턴 형성에 대하여 연구하였다. 결정질 재료의 회절현상을 논의하기 위해 역격자이론에 대하여 간단히 살펴보고 X선과 전자선 회절패턴의 차이점 등을 고찰하였다. 파동역학에 기초한 회절물리학의 운동학 이론(kinematic theory)을 기본으로 하여 실리콘 결정에 대한 구조인자(structure factor) 및 형태인자(shape factor)를 계산하고 회절패턴으로부터 면지수를 결정하는 과정을 논의하였다. Basic program을 구성하여 전자현미경의 입력 data로부터 해당 회절패턴을 형성하도록 하였으며 이 결과는 실제의 패턴과 일치하였다. 이 program은 추후 전자현미경의 컴퓨터 시뮬레이션을 위하여 중요하게 이용될 수 있을 것으로 판단되었다.

• Journal of the KSME
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• v.34 no.10
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• pp.796-807
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• 1994

이 글에서는 고속철도 터널에서 열차에 의해 발생하는 압축파의 특성과 그의 전파거동에 관하여 기술하였다. 또 터널내에서 발생한 압축파가 터널 출구에서 충격음(미기압파)으로 방출되어 주 변의 환경에 큰 영향을 미치고 있다는 문제점을 지적하였으며, 이러한 소음의 발생 메카니즘과 소음의 저감 및 제어방법에 대하여 개괄적으로 기술하였다.

• Geophysics and Geophysical Exploration
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• v.26 no.3
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• pp.114-125
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• 2023

The physics-informed neural network (PINN) has been proposed to overcome the limitations of various numerical methods used to solve partial differential equations (PDEs) and the drawbacks of purely data-driven machine learning. The PINN directly applies PDEs to the construction of the loss function, introducing physical constraints to machine learning training. This technique can also be applied to wave equation modeling. However, to solve the wave equation using the PINN, second-order differentiations with respect to input data must be performed during neural network training, and the resulting wavefields contain complex dynamical phenomena, requiring careful strategies. This tutorial elucidates the fundamental concepts of the PINN and discusses considerations for wave equation modeling using the PINN approach. These considerations include spatial coordinate normalization, the selection of activation functions, and strategies for incorporating physics loss. Our experimental results demonstrated that normalizing the spatial coordinates of the training data leads to a more accurate reflection of initial conditions in neural network training for wave equation modeling. Furthermore, the characteristics of various functions were compared to select an appropriate activation function for wavefield prediction using neural networks. These comparisons focused on their differentiation with respect to input data and their convergence properties. Finally, the results of two scenarios for incorporating physics loss into the loss function during neural network training were compared. Through numerical experiments, a curriculum-based learning strategy, applying physics loss after the initial training steps, was more effective than utilizing physics loss from the early training steps. In addition, the effectiveness of the PINN technique was confirmed by comparing these results with those of training without any use of physics loss.

• Journal of KSNVE
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• v.6 no.5
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• pp.617-624
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• 1996

Finite Element Method(FEM) is one of the most popularly used method in analyzing the dynamic behaviors of structures. But unless the number of finite elements is large enough, the results from FEM are somewhat different form exact analytical solutions, especially at high frequency range. On the other hand, as the Spectral Element Method(SEM) deals directly with the governing equations of structures, the results from this method cannot but be exact regardless of any frequency range. However, despite two dimensional structures are more general, the SEM has been applied only to the analysis of one dimensional structures so far. In this paper, therefore, new methodologies are introduced to analyze the two dimensional plate structure using SEM. The results from this new method are compared with the exact analytical solutions by letting the two dimensional plate structure be one dimensional and showed the dynamic responses of two dimensional plate by including various waves propagated into x-direction.

• Proceedings of the KSME Conference
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• 2002.08a
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• pp.193-196
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• 2002

The relativistic theory has not been properly taken up by the marine hydrodynamicists. To take on a relativistic view, we confine ourselves to a simple vector case of a wave train in spacetime, to be shown to represent a sound wave or a surface wave, and bring in an observer who is travelling on another platform. We are interested in relative position of each event on these two worldlines. It, then, will be shown that the velocity, the acceleration, the encounter frequency, the group velocity, and the time and the space distance between the wave and the observer on the worldlines should all be derivable in principle. This is interpreted to mean that we really have the relativistic events taking place with different values of time dilation in the sense of 'spacetime', and that the well-known ${\lceil}special Theory of Relativity{\rfloor}$ applies just as well in hydrodynamic waves.

• Journal of KSNVE
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• v.6 no.1
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• pp.83-88
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• 1996

The significance of spectral element method is that it can treat the mass and stiffness distribution exactly in contrast to the conventional finite element method, and therefore the dynamic behaviors within each spectral element can be obtained exactly. The present study provides the derivation of the spectral element of a curved beam, while the previous ones presented that of a straight structure. Further, in order to verify the derived spectral element, the natural frequencies of a ring by the spectral element method are compared with those by the analytical method and those by the FEM. From the verification, derived spectral element is admissible. And the dynamic behaviors of curved beam are simulated by using the derived spectral element of a curved beam.

• Proceeding of EDISON Challenge
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• 2013.04a
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• pp.365-370
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• 2013

현대에 이르러 초음속 운영 영역에서의 항공기에 대한 많은 연구가 진행되고 있으나, 음속 폭음 현상과 충격파 현상에서 야기된 높은 항력 및 연료 효율성 저하로 인하여 그 한계에 부딪치고 있다. Busemann 복엽 익형은 이와 같은 문제를 해결하기 위한 형상이며, 상하 형상에 의한 파동 상쇄효과 및 파동 감소효과를 통해 충격파의 강도와 음속 폭음 효과를 감소시키는 형상이다. 하지만 본 익형은 탈설계 조건에서 항력 계수가 급격하게 증가하는 등의 문제점을 가지고 있다. 본 연구에서는 EDISON_CFD를 이용하여 Busemann 복엽 익형의 주변 유동 특성에 대하여 면밀한 고찰을 수행하였다. 우선 Busemann 복엽 익형의 초기 형상에 대한 유동 조건별 해석을 통하여 탈설계 조건에서의 항력 성능 저하 문제에 대한 고찰을 하였다. 이후 3개의 형상 변수에 대한 매개 변수 연구를 통하여 익형의 각 형상 변수가 탈설계 영역 및 해당 영역에서의 최대 항력 계수에 미치는 영향에 대한 고찰을 수행하였으며, 이를 통하여 기존 형상보다 좁은 탈설계 구간을 가지고, 최대 항력 계수가 약 34.8% 감소한 부스만 복엽 익형을 설계하였다.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.38 no.5
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• pp.373-380
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• 2014

Film flows applied to shell-and-tube heat exchangers in various industrial fields have been studied for a long time. One boundary of the film flow interfaces with a fixed wall, and the other boundary interfaces with a gaseous region. Thus, the flows become so unstable that wavy behaviors are generated on free surfaces as the film Reynolds number increases. First, high-amplitude solitary waves are detected in a low Reynolds number laminar region; then, the waves transit to a low-amplitude, high frequency ripple in a turbulent region. Film thickness is the most significant factor governing heat transfer. Since the wave accompanied in the film flow results in temporal and spatial variations in film thickness, it can be of importance for numerically predicting the film's wavy behavior. In this study, various turbulent models are applied for predicting low-amplitude ripple flows in turbulent regions. The results are compared with existing experimental results, and finally, the applied turbulent models are appraised in from the viewpoint of wavy behaviors.