• Smart Structures and Systems
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• v.13 no.2
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• pp.257-280
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• 2014

In this paper, a novel PARAllel FACtor (PARAFAC) decomposition based Blind Source Separation (BSS) algorithm is proposed for modal identification of structures equipped with tuned mass dampers. Tuned mass dampers (TMDs) are extremely effective vibration absorbers in tall flexible structures, but prone to get de-tuned due to accidental changes in structural properties, alteration in operating conditions, and incorrect design forecasts. Presence of closely spaced modes in structures coupled with TMDs renders output-only modal identification difficult. Over the last decade, second-order BSS algorithms have shown significant promise in the area of ambient modal identification. These methods employ joint diagonalization of covariance matrices of measurements to estimate the mixing matrix (mode shape coefficients) and sources (modal responses). Recently, PARAFAC BSS model has evolved as a powerful multi-linear algebra tool for decomposing an $n^{th}$ order tensor into a number of rank-1 tensors. This method is utilized in the context of modal identification in the present study. Covariance matrices of measurements at several lags are used to form a $3^{rd}$ order tensor and then PARAFAC decomposition is employed to obtain the desired number of components, comprising of modal responses and the mixing matrix. The strong uniqueness properties of PARAFAC models enable direct source separation with fine spectral resolution even in cases where the number of sensor observations is less compared to the number of target modes, i.e., the underdetermined case. This capability is exploited to separate closely spaced modes of the TMDs using partial measurements, and subsequently to estimate modal parameters. The proposed method is validated using extensive numerical studies comprising of multi-degree-of-freedom simulation models equipped with TMDs, as well as with an experimental set-up.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.10
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• pp.1327-1336
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• 2001

The structural modes driven by the low wave-number components of smooth elastic wall pressure provide a relatively weak coupling between the flow and the wall motion. If the elastic thin plate has any resonant mode whose wave-number of resonance coincides with $\omega$/U$\sub$c/, the power will be transmitted to those modes of vibration by the flows. We examine the problem in which the elastic thin plate is subject to pressure fluctuations under turbulent boundary layer. Measurements are presented of the frequency spectra of the near- and far-field pressures and radiated sound contributed by the various wave modes of the thin elastic plate. Dispersion equation for wave motions of elastic plate is used to investigate the effect of bending waves of relatively low wave number on radiated sound. The low wave-number motion of elastic plate is observed to have much less influence on the low-frequency energy of wall pressure fluctuations than that of the rediated sound. High amplitude events of the wall pressure are observed to weakly couple with high-frequency energy of radiated sound for case of low tension applied to the plate. The sound source localization is applied to the measurement of radiated sound by using acoustic mirror system.

• Smart Structures and Systems
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• v.15 no.3
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• pp.665-682
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• 2015

Structural identification or St-Id is 'the parametric correlation of structural response characteristics predicted by a mathematical model with analogous characteristics derived from experimental measurements'. This paper describes a St-Id exercise on Humber Bridge that adopted a novel two-stage approach to first calibrate and then validate a mathematical model. This model was then used to predict effects of wind and temperature loads on global static deformation that would be practically impossible to observe. The first stage of the process was an ambient vibration survey in 2008 that used operational modal analysis to estimate a set of modes classified as vertical, torsional or lateral. In the more recent second stage a finite element model (FEM) was developed with an appropriate level of refinement to provide a corresponding set of modal properties. A series of manual adjustments to modal parameters such as cable tension and bearing stiffness resulted in a FEM that produced excellent correspondence for vertical and torsional modes, along with correspondence for the lower frequency lateral modes. In the third stage traffic, wind and temperature data along with deformation measurements from a sparse structural health monitoring system installed in 2011 were compared with equivalent predictions from the partially validated FEM. The match of static response between FEM and SHM data proved good enough for the FEM to be used to predict the un-measurable global deformed shape of the bridge due to vehicle and temperature effects but the FEM had limited capability to reproduce static effects of wind. In addition the FEM was used to show internal forces due to a heavy vehicle to to estimate the worst-case bearing movements under extreme combinations of wind, traffic and temperature loads. The paper shows that in this case, but with limitations, such a two-stage FEM calibration/validation process can be an effective tool for performance prognosis.

• Bulletin of the Korean Chemical Society
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• v.34 no.7
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• pp.2051-2055
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• 2013

Existing theoretical calculations predict that infrared spectra of the two most fundamental reactive carbo-ions, methyl cation $CH{_3}^+$ with $D_{3h}$ symmetry and protonated methyl cation $CH{_5}^+$ with $C_s(I)$, $C_s(II)$, and $C_{2v}$ symmetries, appear together in the 7-${\mu}m$ region corresponding to the C-H bending modes. Vibrational band profiles of $CH{_3}^+$ and $CH{_5}^+$ have been compared by ab initio calculation methods that use the basis sets of MP2/aug-cc-pVTZ and CCSD(T)/cc-pVTZ. Our results indicate that the bands of rotation-vibration transitions of $CH{_3}^+$ and $CH{_5}^+$ should overlap not only in the 3-${\mu}m$ region corresponding to the C-H stretching modes but also in the 7-${\mu}m$ region corresponding to the C-H bending modes. Five band intensities of $CH{_5}^+$ among fifteen vibrational transitions between 6 and 8 ${\mu}m$ region are stronger than those of the ${\nu}_2$ and ${\nu}_4$ bands in $CH{_3}^+$. Ultimate near degeneracy of the two bending vibrations ${\nu}_2$ and ${\nu}_4$ of $CH{_3}^+$along with the stronger intensities of $CH{_5}^+$ in the three hydrogen scrambling structures may cause extreme complications in the analysis of the high-resolution carbo-ion spectra in the 7-${\mu}m$ region.

• Journal of KSNVE
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• v.8 no.6
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• pp.1166-1171
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• 1998

The influences of the coupler consisting of stiffness and mass between neighboring two spans on mode localization are studied theoretically, and the results are confirmed by numerical examples. The mass of the coupler makes a structure sensitive to mode localization especially in higher modes while the stiffness does in all modes. A new type of delocalization phenomenon is observed for the first time in some modes for which mode localization does not occur or is very weak although structural disturbances are severe. A spring-mass system consisting of two substructures and a coupler connecting them is considered in the part of analytical study. As example structures for numerical analysis. simply supported continuous two-span beams with a coupler having a rotational stiffness and a mass moment of inertia on the mid support are considered.

• Smart Structures and Systems
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• v.23 no.6
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• pp.537-551
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• 2019

In this paper, a method for analyzing the damping performance of stay cables incorporating magnetorheological (MR) dampers in the passive control mode is developed taking into account the cable sag and inclination, the damper coefficient, stiffness and mass, and the stiffness of damper support. Both numerical and asymptotic solutions are obtained from complex modal analysis. With the asymptotic solution, analytical formulas that evaluate the equivalent damping ratio of the sagged cable-damper system in consideration of all the above parameters are derived. The main thrust of the present study is to develop an general design formula and a universal curve for the optimal design of MR dampers for adjustable passive control of sagged cables. Two sag-affecting coefficients are derived to reflect the effects of cable sag on the maximum attainable damping ratio and the optimal damper coefficient. For the cable configurations commonly used in cable-stayed bridges, the sag-affecting coefficients are directly expressed in terms of the sag-extensibility parameter to facilitate the control design. A case study on adjustable passive vibration control of the longest cable (536 m) on Stonecutters Bridge is carried out to demonstrate the influence of the sag for the damper design, and to figure out the necessity of adjustability of damper coefficients for achieving maximum damping ratio for different vibration modes.

• Structural Engineering and Mechanics
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• v.75 no.4
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• pp.435-444
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• 2020

Ultrasonic guided wave testing is a very promising non-destructive testing method for rails, which is of great significance for ensuring the safe operation of railways. On the basis of the semi-analytical finite element (SAFE) method, a analytical model of 59R2 grooved rail was proposed, which is commonly used in the ballastless track of modern tram. The dispersion curves of ultrasonic guided waves in free rail and supported rail were obtained. Sensitivity analysis was then undertaken to evaluate the effect of rail elastic modulus on the phase velocity and group velocity dispersion curves of ultrasonic guided waves. The optimal guided wave mode, optimal excitation point and excitation direction suitable for detecting rail integrity were identified by analyzing the frequency, number of modes, and mode shapes. A sinusoidal signal modulated by a Hanning window with a center frequency of 25 kHz was used as the excitation source, and the propagation characteristics of high-frequency ultrasonic guided waves in the rail were obtained. The results show that the rail pad has a relatively little influence on the dispersion curves of ultrasonic guided waves in the high frequency band, and has a relatively large influence on the dispersion curves of ultrasonic guided waves in the low frequency band below 4 kHz. The rail elastic modulus has significant influence on the phase velocity in the high frequency band, while the group velocity is greatly affected by the rail elastic modulus in the low frequency band.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2001.05a
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• pp.478-483
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• 2001

We experimentally attempted to understand the vibration characteristics of a flexible pipe excited by vortex shedding. This has been extensively studied in the past decades (For example, see ［2-9］). However, there are still areas that need more study. One of them is to study the relation between spatial characteristics of a flow induced vibrating pipe, such as its length, the distribution of wave number, and frequency responses. A non-linear mechanism between the responses of in-line and cross-flow directions is also an area of interests, if the pipe is relatively long so that structural modal density is reasonably high. In order to investigate such areas, two kinds of instrumented pipe were designed. The instrumented pipes, of which the lengths are equally 6m, are wound with rubber and silicon tape in different ways, having different vortex shedding conditions. One has uniform cross-section of diameter of 26. 7mm, and the other has equally spaced by 4 sub-sections, which are composed of different diameters of 75.9, 61.1, 45.6 and 26.7mm. Both pipes are towed in a water tank (200m ${\times}$ 16m ${\times}$ 7m) so that they experienced different vortex shedding excitations. The towing pipe experiments exhibit several valuable features. One of them is that the natural frequencies and their corresponding strain mode shapes dominate the strain response of the uniform pipe. However, for those of non-uniform pipe, the responses are more likely local and many modes participate in it.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.23 no.3
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• pp.209-217
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• 2013

This paper presents an optimal design of a magnetorheological(MR) fluid-based mount(MR mount) that can be used for to vibration control in diesel engines of ships. In this work, a mount that uses mixed-modes(squeeze mode, flow mode, and shear mode) is proposed and designed. To determine the actuating damping force of the MR mount required for efficient vibration control, the excitation force from a diesel engine is analyzed. In this analysis, a model of a V-type engine is considered. The relationship between the velocity and pressure of gas in terms of the torque acting on the piston is derived. Subsequently, by integrating the field-dependent rheological properties of commercially available MR fluid with the excitation force, the appropriate size of the MR mount is designed. In addition, to achieve the maximum actuating force under geometric constraints, design optimization is undertaken using the ANSYS parametric design language software. Through magnetic density analysis, optimal design parameters such as the bottom gap and radius of coil are determined.

• Journal of the Society of Disaster Information
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• v.8 no.2
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• pp.188-196
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• 2012

Dynamic parameters such as natural frequencies can provide global stiffness information of a structure, and thus be utilized in monitoring structural integrity of large structures such as a containment. To identify the dynamic parameters without interrupting normal operation, a modal analysis method based on ambient vibration measurements should be applied. In this study, dynamic parameters of the containment of Wolsong Unit 2 are identified using ambient vibration measurement data. The feasibility of the study is verified using a numerical model for the containment. From the modal analysis, dynamic parameters of the containment with acceptable correlation to analytical modes can be estimated.