• 한국공작기계학회:학술대회논문집
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• 한국공작기계학회 2003년도 춘계학술대회 논문집
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• pp.79-85
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• 2003

One of unique characteristics of guided waves is a dispersive behavior that guided wave velocity changes with an excitation frequency and mode. In practical applications of guided wave techniques, it is very important to identify propagating modes in a time-domain waveform for determination of defect location and size. Mode identification can be done by measurement of group velocity in a time-domain waveform. Thus, it is preferred to generate a single or less dispersive mode But in many cases, it is difficult to distinguish a mode clearly in a time-domain waveform because of superposition of multi modes and mode conversion phenomena. Time-frequency analysis is used as efficient methods to identify modes by presenting wave energy distribution in a time-frequency. In this study, experimental guided wave mode identification is carried out in a steel plate using time-frequency analysis methods such as wavelet transform. The results are compared with theoretically calculated group velocity dispersion curves. The results are in good agreement with analytical predictions and show the effectiveness of using the wavelet transform method to identify and measure the amplitudes of individual guided wave modes.

• 한국공작기계학회논문집
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• 제12권5호
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• pp.94-100
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• 2003

One of unique characteristics of guided waves is a dispersive behavior that guided wave velocity changes with an excitation frequency and mode. In practical applications of guided wave techniques, it is very important to identify propagating modes in a time-domain waveform for determination of detect location and size. Mode identification can be done by measurement of group velocity in a time-domain waveform. Thus, it is preferred to generate a single or less dispersive mode But, in many cases, it is difficult to distinguish a mode clearly in a time-domain waveform because of superposition of multi modes and mode conversion phenomena. Time-frequency analysis is used as efficient methods to identify modes by presenting wave energy distribution in a time-frequency. In this study, experimental guided wave mode identification is carried out in a steel plate using time-frequency analysis methods such as wavelet transform. The results are compared with theoretically calculated group velocity dispersion own. The results are in good agreement with analytical predictions and show the effectiveness of using the wavelet transform method to identify and measure the amplitudes of individual guided wave modes.

• 한국전자파학회지:전자파기술
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• 제6권2호
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• pp.11-18
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• 1995

본 연구는 평판형, 그리드형, 특수형상 등의 페라이트 전파흡스체의 흡수능 측정시스템을 구축하는 데에 목적이 있다. 그러나 30MHz와 같이 낮은 주파수에서 흡수능을 측정하느 것은 대단히 곤란 . 이와같은 문제를 해결하기 위하여 평행평판 스 라인을 사용하여 정재파측정법 및 시간영역법을 제안하였으며, 실제 측정시스템을 구성하요 제안한 측정볍으로 그리드형 페라이트 전자파흡수체의 특성을 측정. 평가하였다.특성을 측정. 평가하였다.

• 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.

• 한국전자파학회논문지
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• 제11권2호
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• pp.186-196
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• 2000

본 논문에서는 이중통신용 안테나 시스템을 위한 변형된 사각 루프 주파수 선택 반사기를 제안하고, 그 특성 을 해석하였다 변형된 사각 루프 주파수 선택 반사기를 해석함에 있어서 유전체의 영향과 주기적 배열구조임을 고려하기 워하여 파수 영역 방법(Spectral Domain Method)과 Floquet 이론을 도입하였으며, 수치해석 방법으로 는 rooftop basis function을 이용한 모멘트법을 적용하였다 해석결과로는 기존의 사각 루프와 변형된 사각 루프 의 전송 특성플 비교하였으며. 변형된 사각 루프의 경우 입사각과 편파 변화에 따른 전송 특성도 제시하였다. 수 치해석 방법의 타당성을 검증하기 위해 변형된 사각 루프 주파수 선택 반사기를 실제 제작 및 측정하여 수치해 석 결파와 비교하였다. 측정결과는 수치해석 결과와 비교적 잘 일치함을 보였다

• Nuclear Engineering and Technology
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• 제55권3호
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• pp.968-980
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• 2023

The dynamic structural responses are sensitive to the time-frequency content of seismic waves, and seismic input motions in time-history analysis are usually required to be compatible with design response spectra according to nuclear codes. In order to generate spectra-compatible input motions while maintaining the intrinsic non-stationarity of seismic waves, an improved time-domain approach is proposed in this paper. To maintain the nonstationary characteristics of the given seismic waves, a new time-frequency envelope function is constructed using the Hilbert amplitude spectrum. Based on the intrinsic mode functions (IMFs) obtained from given seismic waves through variational mode decomposition, a new corrective time history is constructed to locally modify the given seismic waves. The proposed corrective time history and time-frequency envelope function are unique for each earthquake records as they are extracted from the given seismic waves. In addition, a dimension reduction iterative technique is presented herein to simultaneously superimpose corrective time histories of all the damping ratios at a specific frequency in the time domain according to optimal weights, which are found by the genetic algorithm (GA). Examples are presented to show the capability of the proposed approach in generating spectra-compatible time histories, especially in maintaining the nonstationary characteristics of seismic records. And numerical results reveal that the modified time histories generated by the proposed method can obtain similar dynamic behaviors of AP1000 nuclear power plant with the natural seismic records. Thus, the proposed method can be efficiently used in the design practices.

• 한국소음진동공학회논문집
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• 제15권3호
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• pp.266-271
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• 2005

Although time-frequency analysis is useful for dispersive wave analysis, conventional methods such as the short-time Fourier transform do not take the dispersion phenomenon into consideration in the tiling of the time-frequency domain. The objective of this paper is to develop an adaptive time-frequency analysis method whose time-frequency tiling is determined with the consideration of signal dispersion characteristics. To achieve the adaptive time-frequency tiling, each of time-frequency atoms is rotated in the time-frequency plane depending on the local wave dispersion. To carry out this adaptive time-frequency transform, dispersion characteristics hidden in a signal are first estimated by an iterative scheme. To examine the effectiveness of the present method, the flexural wave signals measured in a plate were analyzed.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2004년도 추계학술대회논문집
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• pp.606-610
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• 2004

Although time-frequency analysis is useful for dispersive wave analysis, conventional methods such as the short-time Fourier transform do not take the dispersion phenomenon into consideration in the tiling of the time-frequency domain. The objective of this paper is to develop an adaptive time-frequency analysis method whose time-frequency tiling is determined with the consideration of signal dispersion characteristics. To achieve the adaptive time-frequency tiling, each of time-frequency atoms is rotated in the time-frequency plane depending on the local wave dispersion. To carry out this adaptive time-frequency transform, dispersion characteristics hidden in a signal are first estimated by an iterative scheme. To examine the effectiveness of the proposed method, the flexural wave signals measured in a plate were analyzed.

• 한국지진공학회:학술대회논문집
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• 한국지진공학회 2000년도 추계 학술발표회 논문집 Proceedings of EESK Conference-Fall 2000
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• pp.143-150
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• 2000

A physical lumped parameter model is proposed for the time domain analysis of dam-reservoir system. The exact solution of transmitting boundary is derived for a semi-infinite 2-D reservoir of constant depth. The characteristics of the solution are examined in both frequency and the domains. Mass and damping coefficient are obtained from asymptotic behavior of the frequency domain solution. Further refinement to the lumped model is made by approximating the kernel function of the convolution integral in the exact solution. Finally a new physical lumped parameter model is proposed that consists of two masses, a spring and two dampers for each mode. It is demonstrated that new lumped parameter model of transmitting boundary can give excellent results.

• 대한전기학회:학술대회논문집
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• 대한전기학회 1991년도 하계학술대회 논문집
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• pp.281-285
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• 1991

In this paper, response characteristics by applying impulse into transformer windings in order to precaution diagonosis of power electric instrument are studied. A sample oilless transformer, an aging transformer and a good transformer are slected for models. The experimented results show that for testing the windings distortion and impedanced change of transformer the LVI method are suitable. Also, variation characteristic of output waveforms at time and frequency domain in order to efficient analysis is discussed. The results response that waveforms at frequency domain is very higher resolution at time domain.