• Journal of the Computational Structural Engineering Institute of Korea
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• v.20 no.5
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• pp.521-532
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• 2007

Elastic wave propagations in the semi-infinite transversely isotropic cylinder under various kinds of longitudinal impact loads are analyzed using the axisymmetric finite element method and Houbolt time-integration scheme. For which the finite element program is newly constructed and verified through the comparison of present numerical results with those by other researchers. E-type glass-epoxy composite cylinders with different fiber volume fractions are adopted and studied in detail with dynamic responses of the isotropic cylinder. Three dimensional wave motions are given in graphic form to show the realistic view of the wave propagation. Nondimensionalized dynamic characteristic variables which relate the size of finite element mesh, the time step, and the wave speed are presented for obtaining accurate and stable numerical results.

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.12
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• pp.2328-2339
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• 1992

The propagation of coherent time-harmonic elastic L-and SV-waves is studied in a medium with random distribution of cylindrical inclusions. The purpose of the research is to characterize the dynamic elastic moduli and the attenuation properties of fiber-reinforced composite materials. The cylindes representing the fibers are assumed to be distributed in parallel with each other and the direction of incident waves are normal to the cylinder axes. A multiple scattering formula using the single scattering coefficients in conjunction with the Lax's quasicrystalline approximation is derived from which the dispersion relation for such medium is obtained. In order to formulate the multiple scattering interaction between cylinders, the pair correlation functions are generated by the Monte Carlo simulation technique. From the numerically evaluated complex wavenumbers, the propagation speed of the average wave, the coherent attenuation and the effective elastic moduli are presented as functions of frequency and fiber volume fraction.

• Geophysics and Geophysical Exploration
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• v.12 no.2
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• pp.183-191
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• 2009

Finite difference method using not general SSG (standard staggered grid) but RSG (rotated staggered grid) was applied to simulation of elastic wave propagation. Special free surface boundary condition such as imaging method is needed in finite difference method using SSG in elastic wave propagation. But free surface boundary condition in finite difference method using RSG is easily solved with adding air layer or vacuum layer. Recently PML (Perfectly Matched layer) is widely used to eliminate artificial reflection waves from finite boundary because of its' greate efficiency. Absorbing ability of CPML (convolutional Perfectly Matched Layer) that is more efficient than that of PML and CPML that don't use splitting of wave equation that should be adapted to PML was applied to FDM using RSG in this study. Frequency absorbing characteristic and energy absorbing ability in CPML layer were investigated and CPML eliminated artificial boundary waves very effectively in FDM using RSG in being compared with that of Cerjan's absorbing method. CPML method also diminished amplitude of waves in boundary layer of solid-liquid model very well.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.23 no.2
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• pp.167-175
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• 2013

The wave propagation characteristics of an acoustic metamaterial composed of periodically repeated one-dimensional Helmholtz resonator array was investigated considering the effects of dimensional changes of the resonator geometry on the transmission coefficient and band gap. The effective impedance and transmission coefficient of the acoustic metamaterials are obtained based on the acoustic transmission line method. The designed acoustic metamaterials exhibit band gaps and negative bulk modulus that are non-existent properties in the nature. The band gap of the acoustic metamaterial is strongly dependent on the geometry parameters of Helmholtz resonators and lattice spacing. Also, a new type of metamaterial that is periodically constructed with two different resonators was designed to open the local resonance band gap without change of Bragg scattering.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.28 no.6
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• pp.763-773
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• 2004

This paper suggests the 2 D.O.F mathematical model of the High Frequency Vibro-Hammer (HFVH), introduces an experimental method for measuring of the attenuation of piling vibration and proves what experiments are coincident with the equation of wave propagation. As vibratory installation of piles and casings has many economic merits in the construction field, most of all contractors prefer to vibratory pile driving method than the other. Compared to impact pile driving, vibratory installation has the advantage of reducing vibration or noise pollution and can drive piles under high frequency. Experiments serve estimations of capabilities and limitations of the HFVH's excitation force and finding of sensitivity for important soil resistance parameters. Also, we discuss the HFVH that can drive with three kinds of input waves (triangular, sine and square wave) and propose the design of parameters to improve actuating performance in it.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 1995.10a
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• pp.221-226
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• 1995

터빈 익렬, 펌프 익차, 원형 냉각탑, 치차 등과 같이 동일한 형상이 원주 방향으로 반복되어 있는 순환 대칭 구조물의 진동특성을 유한 요소법을 사용하여 해석하는 경우에 전체구조를 모델링하는 대신에 구조물을 동일한 형상의 부분구조로 분할하여 부분구조 한개만을 모델링하고 분할된 경계에서 적절한 경계조건을 부과하여 진동해석을 수행함으로서 컴퓨터 기억용량을 절감시키고 계산시간을 단축할 수 있는 방법이 널리 사용되고 있다. Orris and Petyt[1]는 부분구조의 양쪽 분할 경계면, 즉 연결 경계상에 있는 절점변위의 상관관계를 복소파동전파식을 이용해서 구하여 부분구조의 감소된 복소강성행렬 및 질량행렬을 만들고 실수부와 허수부를 분리하여 유한요소해석을 수행하는 방법을 제안하였다. 유한요소 프로그램 ANSYS[2]에서는 이와 같은 방법을 사용하고 있다. Thomas[3]는 순회 정규모드를 이용하였고, 참고문헌[4]에서는 순회행렬을 이용하였다. 또한 유한요소 프로그램 MSC/NASTRAN[5]에서는 푸리에 급수를 이용하고 유한요소 절점의 위치 및 변위를 원통 좌표계를 표현하여 순환대칭구조물의 유한요소해석을 수행할 수 있도록 되어있다. 본 논문에서는 순환 대칭구조물의 형상의 주기성과 순환성을 고려하여 이산퓨리에 변환을 이용함으로써 순환대칭구조물의 유한요소진동해석을 체계적으로 저용량의 컴퓨터에서 신속하고 정확하게 수행할 수 있는 방법을 제안하고자 한다.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.25 no.2
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• pp.214-219
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• 2001

In this paper, the nonlinear behavior of ultrasonic wave in partially degraded material is considered. For this aim, FDM(finite difference method) model for the nonlinear wave equation was developed with the restriction to the 1-D longitudinal wave motion and how the partial degradation in material contributes to the detected nonlinear parameter was analyzed quantitatively. In order to verify the rightness of this simulation method, the relation between the detected nonlinear parameter and the continuous distribution of degradation obtained from simulation was compared with experiment results and the simulation and experiment results showed similar tendency. It can be known from simulation result that the degree of degradation, the range of degradation and the continuous distribution of degradation have strong correlation with the detected nonlinear parameter. As it was possible in these simulations that only special part is assumed as degraded one, the quantitative evaluation of partially degraded material may be obtained by using this method.

• Journal of the Korean Geotechnical Society
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• v.20 no.1
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• pp.61-73
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• 2004

Rayleigh wave and Love wave are the major elastic waves belonging to the category of the surface wave. The fact that Love wave is not contaminated by P-wave makes Love wave superior to Rayleish wave and other body waves. Therefore, the information that Love wave carries is more distinct and clearer than the information of Rayleigh wave. In this study, for the purpose of employing Love wave in the SASW method, the dispersion characteristics of the Love wave were extensively investigated by the theoretical, numerical and experimental approaches. The 2-D and 3-D finite element analyses for the half space and two-layer systems were performed to determine the phase velocities from Love wave as well as from both the vertical and the horizontal components of Rayleigh wave. Also, the SASW measurements were performed at the geotechnical sites to verify the results obtained by the numerical analysis. The results of the numerical analysis and the field testing indicated that the dispersion characteristics of Love wave can be an extended information to make better evaluation of the subsurface stiffness structure by SASW method.

• Journal of the Korean Society for Railway
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• v.16 no.2
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• pp.85-92
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• 2013

An important source of noise from railways is rolling noise caused by wheel and rail vibrations induced by acoustic roughness at the wheel-rail contact. The main contributors to rolling noise are the sleepers, rail, and wheels. In order to analyze and predict rolling noise, it is necessary to understand the vibrating behaviors of railway tracks, as well as of the wheels. In the present paper, theoretical modeling methods for railway track are reviewed in terms of rolling noise; these methods are applied for the three representative types of domestic railway tracks operated: the conventional ballasted track, KTX ballasted track and KTX concrete track. The characteristics of waves propagating along rails are investigated and compared among the types of tracks. The tracks are modeled as discretely supported Timoshenko beams and are compared in terms of the averaged squared amplitude of velocity, which is directly related to the sound radiation from the rails.

• Geophysics and Geophysical Exploration
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• v.3 no.1
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• pp.19-24
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• 2000

Seismic reflection data was obtained by using 28 and 100 Hz geophones at the ground subsidence sites in an old coal mine area. Frequency spectrum of the geophone analyzed with offset revealed that 1) In the near offset ($1\~10m$), the signals in the 100 Hz geophone data contains higher frequency components (up to 300 Hz) than that of the 28 Hz (<200 Hz), 2) In the intermediate offset ($11\~39m$), although the 28 Hz geophone data showed very similar frequency characteristics as the near offset data, the 100Hz geophone data seemed to be contaminated by noise at high frequency zone (>200 Hz). In the far offset ($\geq40\;m$), the signals in both the 28 and 100 Hz geophone data are attenuated to noise level at high frequency Bone more than 150 Hz.