• 대한수학회보
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• 제38권3호
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• pp.517-526
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• 2001

In this paper, we study the continuous wavelet transform on the Heisenberg group H$^n$, and describe the related continuous multiscale analysis. By using the wavelet packet transform we obtain a reconstruction formula on L$^2$(H$^n$).

• Interaction and multiscale mechanics
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• 제1권4호
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• pp.467-476
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• 2008

In this work, a nano-size rocksalt crystal (magnesium oxide) is considered as a continuous collection of unit cells, while each unit cell consists of discrete atoms; and modeled by a multiscale concurrent atomic/continuum field theory. The response of the crystal to an electromagnetic (EM) wave is studied. Finite element analysis is performed by solving the governing equations of the multiscale theory. Due to the applied EM field, the inhomogeneous motions of discrete atoms in the polarizable crystal give rise to the change of microstructure and the polarization wave. The relation between the natural frequency of this system and the driving frequency of the applied EM field is found and discussed.

• Journal of Mechanical Science and Technology
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• 제20권12호
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• pp.2147-2158
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• 2006

The continuous wavelet transform (CWT) has a frequency-adaptive time-frequency tiling property, which makes it popular for the analysis of dispersive elastic wave signals. However, because the time-frequency tiling of CWT is not signal-dependent, it still has some limitations in the analysis of elastic waves with spectral components that are dispersed rapidly in time. The objective of this paper is to introduce an advanced time-frequency analysis method, called the dispersion-based continuous wavelet transform (D-CWT) whose time-frequency tiling is adaptively varied according to the dispersion relation of the waves to be analyzed. In the D-CWT method, time-frequency tiling can have frequency-adaptive characteristics like CWT and adaptively rotate in the time-frequency plane depending on the local wave dispersion. Therefore, D-CWT provides higher time-frequency localization than the conventional CWT. In this work, D-CWT method is applied to the analysis of dispersive elastic waves measured in waveguide experiments and an efficient procedure to extract information on the dispersion relation hidden in a wave signal is presented. In addition, the ridge property of the present transform is investigated theoretically to show its effectiveness in analyzing highly time-varying signals. Numerical simulations and experimental results are presented to show the effectiveness of the present method.

• Interaction and multiscale mechanics
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• 제5권1호
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• pp.1-12
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• 2012

Considerable longitudinal rail forces and displacements may develop in continuous welded rail (CWR) track on long-span bridges due to temperature variations. The track stability may be disturbed due to excessive relative displacements between the sleepers and ballast bed and the accompanied reduction in frictional resistance. For high-speed tracks, however, solving these problems by installing rail expansion devices in the track is not an attractive solution as these devices may cause a local disturbance of the vertical track stiffness and track geometry which will require intensive maintenance. With reference to temperature, two actions are considered by the bridge loading standards, the uniform variation in the rail and deck temperature and the temperature gradient in deck. Generally, the effect of temperature gradient has been disregarded in the interaction analysis. This paper mainly deals with the effect of temperature gradient on the track-bridge interaction with respect to the support reaction, rail stresses and stability. The study presented in this paper was not mentioned in the related codes so far.

• Interaction and multiscale mechanics
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• 제3권4호
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• pp.321-331
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• 2010

A new technique is proposed for bridge structural damage detection based on spatial wavelet analysis of the time history obtained from vehicle body moving over the bridge, which is different from traditional detection techniques based on the bridge response. A simply-supported Bernoulli-Euler beam subjected to a moving spring-mass unit is established, with the crack in the beam simulated by modeling the cracked section as a rotational spring connecting two undamaged beam segments, and the equations of motion for the system is derived. By using the transfer matrix method, the natural frequencies and mode shapes of the cracked beam are determined. The responses of the beam and the moving spring-mass unit are obtained by modal decomposition theory. The continuous wavelet transform is calculated on the displacement time histories of the sprung-mass. The case study result shows that the damage location can be accurately determined and the method is effective.

• Composites Research
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• 제33권6호
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• pp.415-420
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• 2020

탄소섬유강화 플라스틱(CFRP)의 계면 특성은 복합재료의 전체적인 기계적 특성을 제어하므로 매우 중요하다. 이에 따라, 탄소나노튜브(CNT)로 탄소섬유(CF) 표면을 개질하는 것이 계면을 강화하기 위해 활발히 연구되고 있다. 그러나 대부분의 표면 개질 방법은 자체적으로 한계가 있다. 예를 들어, CVD 성장에서 탄소섬유의 CNT를 성장시키기 위해 600~1000℃ 범위의 고온을 적용해야 하며, 이는 탄소섬유 자체에 손상을 줄 수 있으므로 물성이 저하될 수 있다. 한편, 본 연구에서는, 폴리아마이드(PA) 610/CF/CNT 복합재가 PA610의 계면중합을 통해 제조되었으며, 유기계와 수계 사이의 계면에서 PA610/CNT 중합이 일어난다. 탄소섬유는 CNT가 균일하게 분산된 PA610으로 코팅되었다. 복합재 내에서의 CNT 분산상태는 주사전자현미경으로 관찰되었으며, 열중량 분석을 통해 복합재의 열안정성을 분석하였다. 그리고 섬유 뽑힘 시험을 통해 섬유와 기지 간의 계면 결합력을 측정하였다.

• 한국정보기술학회논문지
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• 제17권6호
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• pp.103-114
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• 2019

주파수변조연속파형(FMCW) 레이더 시스템을 사용하는 이동 객체탐지가 최근 각광을 받고 있다. 레이더 객체탐지는 탐지범위 내 존재하는 고정된 객체 및 클러터들로부터 반사되는 잡음신호로 인해 매우 도전적인 문제이다. 본 논문에서는 FCMW 레이다를 이용하여 잡음배경하 이동객체탐지를 위해 강인한 주성분분석법(RPCA)을 이용한다. 먼저 원 레이더 입력신호에 보상과 보정을 적용한다. 다음 경사하강법을 사용하는 RPCA가 저계수의 성질을 갖는 잡음배경 모델을 구하기 위해 사용된다. 본 논문에서는 RPCA 계산을 위해 소요계산량이 적은 새로운 업데이트 알고리즘을 제안한다. 마지막으로 이동객체는 자동 다중스케일에 기반한 피크 탐지법에 의해 정위한다. 모든 단계는 슬라이딩 윈도우 방법 기반하여 처리된다. 제안된 방법을 타 RPCA 기반의 방법들과 다양한 실험 시나리오 상에서 비교했을 때, 처리 속도와 정확도 척도에서 우수한 결과를 보였다.