• Title/Summary/Keyword: multi modal dispersion

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Experimental and numerical validation of guided wave based on time-reversal for evaluating grouting defects of multi-interface sleeve

  • Jiahe Liu;Li Tang;Dongsheng Li;Wei Shen
    • Smart Structures and Systems
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    • v.33 no.1
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    • pp.41-53
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    • 2024
  • Grouting sleeves are an essential connecting component of prefabricated components, and the quality of grouting has a significant influence on structural integrity and seismic performance. The embedded grouting sleeve (EGS)'s grouting defects are highly undetectable and random, and no effective monitoring method exists. This paper proposes an ultrasonic guided wave method and provides a set of guidelines for selecting the optimal frequency and suitable period for the EGS. The optimal frequency was determined by considering the group velocity, wave structure, and wave attenuation of the selected mode. Guided waves are prone to multi-modality, modal conversion, energy leakage, and dispersion in the EGS, which is a multi-layer structure. Therefore, a time-reversal (TR)-based multi-mode focusing and dispersion automatic compensation technology is introduced to eliminate the multi-mode phase difference in the EGS. First, the influence of defects on guided waves is analyzed according to the TR coefficient. Second, two major types of damage indicators, namely, the time domain and the wavelet packet energy, are constructed according to the influence method. The constructed wavelet packet energy indicator is more sensitive to the changes of defecting than the conventional time-domain similarity indicator. Both numerical and experimental results show that the proposed method is feasible and beneficial for the detection and quantitative estimation of the grouting defects of the EGS.

Approaches of the Computaional Mechanics on the Stress Wave Analysis (응력파동해석에 대한 전산역학적 접근방법)

  • 조윤호;정현규;김승호
    • Proceedings of the Computational Structural Engineering Institute Conference
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    • 2002.04a
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    • pp.415-429
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    • 2002
  • 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.

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Diffraction Analysis of Multi-layered Grating Structures using Rigorous Equivalent Transmission-Line Theory (정확한 등가 전송선로 이론을 사용한 다층 격자 구조의 회절특성 분석)

  • Ho, Kwang-Chun
    • The Journal of the Institute of Internet, Broadcasting and Communication
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    • v.15 no.1
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    • pp.261-267
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    • 2015
  • The eigenvalue problems involving the diffraction of waves by multi-layered grating configurations can be explained by rigorous modal expansion terms. Such a modal solution can be represented by equivalent transmission-line networks, which are generalized forms of simple conventional circuits. This approach brings considerable physical insight into the grating diffraction process of the fields everywhere. In particular, the transmission-line representation can serve as a template for computational algorithms that systematically evaluate dispersion properties, radiation effects and other optical characteristics that are not readily obtained by other methods. To illustrate the validity of the present rigorous approach, the previous research works are numerically confirmed and the results agree well each other.

Multimode-fiber Speckle Image Reconstruction Based on Multiscale Convolution and a Multidimensional Attention Mechanism

  • Kai Liu;Leihong Zhang;Runchu Xu;Dawei Zhang;Haima Yang;Quan Sun
    • Current Optics and Photonics
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    • v.8 no.5
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    • pp.463-471
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    • 2024
  • Multimode fibers (MMFs) possess high information throughput and small core diameter, making them highly promising for applications such as endoscopy and communication. However, modal dispersion hinders the direct use of MMFs for image transmission. By training neural networks on time-series waveforms collected from MMFs it is possible to reconstruct images, transforming blurred speckle patterns into recognizable images. This paper proposes a fully convolutional neural-network model, MSMDFNet, for image restoration in MMFs. The network employs an encoder-decoder architecture, integrating multiscale convolutional modules in the decoding layers to enhance the receptive field for feature extraction. Additionally, attention mechanisms are incorporated from both spatial and channel dimensions, to improve the network's feature-perception capabilities. The algorithm demonstrates excellent performance on MNIST and Fashion-MNIST datasets collected through MMFs, showing significant improvements in various metrics such as SSIM.

Effectiveness of multi-mode surface wave inversion in shallow engineering site investigations (토목관련 천부층 조사에서 다중 모드 표면파 역산의 효과)

  • Feng Shaokong;Sugiyama Takeshi;Yamanaka Hiroaki
    • Geophysics and Geophysical Exploration
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    • v.8 no.1
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    • pp.26-33
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    • 2005
  • Inversion of multi-mode surface-wave phase velocity for shallow engineering site investigation has received much attention in recent years. A sensitivity analysis and inversion of both synthetic and field data demonstrates the greater effectiveness of this method over employing the fundamental mode alone. Perturbation of thickness and shear-wave velocity parameters in multi-modal Rayleigh wave phase velocities revealed that the sensitivities of higher modes: (a) concentrate in different frequency bands, and (b) are greater than the fundamental mode for deeper parameters. These observations suggest that multi-mode phase velocity inversion can provide better parameter discrimination and imaging of deep structure, especially with a velocity reversal, than can inversion of fundamental mode data alone. An inversion of the theoretical phase velocities in a model with a low velocity layer at 20 m depth can only image the soft layer when the first higher mode is incorporated. This is especially important when the lowest measurable frequency is only 6 Hz. Field tests were conducted at sites surveyed by borehole and PS logging. At the first site, an array microtremor survey, often used for deep geological surveying in Japan, was used to survey the soil down to 35 m depth. At the second site, linear multichannel spreads with a sledgehammer source were recorded, for an investigation down to 12 m depth. The f-k power spectrum method was applied for dispersion analysis, and velocities up to the second higher mode were observed in each test. The multi-mode inversion results agree well with PS logs, but models estimated from the fundamental mode alone show f large underestimation of the depth to shallow soft layers below artificial fill.

Range Estimating Performance Evaluation of the Underwater Broadband Source by Array Invariant (Array Invariant를 이용한 수중 광대역 음원의 거리 추정성능 분석)

  • Kim Se-Young;Chun Seung-Yong;Kim Boo-Il;Kim Ki-Man
    • The Journal of the Acoustical Society of Korea
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    • v.25 no.6
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    • pp.305-311
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    • 2006
  • In this paper the performance of a array invariant method is evaluated for source-range estimation in horizontally stratified shallow water ocean waveguide. The method has advantage of little computationally effort over existing source-localization methods. such as matched field processing or the waveguide invariant and array gain is fully exploited. And. no knowledge of the environment is required except that the received field should not be dominated by purely interference This simple and instantaneous method is applied to simulated acoustic propagation filed for testing range estimation performance. The result of range estimation according to the SNR for the underwater impulsive source with broadband spectrum is demonstrated. The spatial smoothing method is applied to suppress the effect of mutipath propagation by high frequency signal. The result of performance test for range estimation shows that the error rate is within 20% at the SNR above 10dB.

Modeling of Elastodynamic Problems in Finite Solid Media (유한 고체내 탄성동역학 문제의 모델링)

  • Cho, Youn-Ho
    • 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.

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