• Wind and Structures
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• v.16 no.2
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• pp.157-178
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• 2013

Output-only modal parameter identification is based on the assumption that external forces on a linear structure are white noise. However, harmonic excitations are also often present in real structural vibrations. In particular, it has been realized that the use of forced acceleration responses without knowledge of external forces can pose a problem in the modal parameter identification, because an external force is imparted to its impulse acceleration response function. This paper provides a three-stage identification procedure as a solution to the problem of harmonic and white noise excitations in the acceleration responses of a linear dynamic system. This procedure combines the uses of the mode indicator function, the complex mode indication function, the enhanced frequency response function, an iterative rational fraction polynomial method and mode shape inspection for the correlation-related functions of the force-embedded acceleration responses. The procedure is verified via numerical simulation of a five-floor shear building and a two-dimensional frame and also applied to ambient vibration data of a large-span roof structure. Results show that the modal parameters of these dynamic systems can be satisfactorily identified under the requirement of wide separation between vibration modes and harmonic excitations.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2006.11a
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• pp.703-708
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• 2006

The cantilever beam with nonlinearity has many dynamic characteristics of nonlinear vibration. Nonlinear terms of a flexible cantilever beam include inertia, spring, damping, and warping. When the beam is given basic harmonic excitation, it shows planar and nonplanar vibrations due to one-to-one resonance. And when the one-to-one resonance occurs, the flexible beam shows different behaviors in those vibrations. For the one-to-one resonance occurring in each mode, the phase value of the planar vibration is different from that of the nonlinear vibration. This paper investigates the phase change and the phase difference between such planar and nonplanar vibrations which are caused by one-to-one resonance.

• Journal of KSNVE
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• v.6 no.1
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• pp.27-34
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• 1996

In this paper, design sensitivity of vibration displacement and acceleration is computed and design sensitivity, the derivative information of responses with respect to design perameters, is used as a design guidance tool to reduce the vibration. First, the harmonic vibration analysis of deck and simplified ship structures is performed by finite element method and secondly continuum disign sensityivity for excessive dynamic response is computed by continuum method. Both the direct and modal frequency response methods for the finite element analysis are adopted. Sensitivities of structural components such as upper plate, side wall, bilge, bottom plate are compared and the reductionof vibration is obtained by the proper increase of thickness of each component.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.17 no.1 s.118
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• pp.48-54
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• 2007

The cantilever beam with nonlinearity has many dynamic characteristics of nonlinear vibration. Nonlinear terms of a flexible cantilever beam include inertia, spring, damping, and warping. When the beam is given basic harmonic excitation, it shows planar and nonplanar vibrations due to one-to-one resonance. And when the one-to-one resonance occurs, the flexible beam shows different behaviors in those vibrations. For the one-to-one resonance occurring in each mode, the phase value of the planar vibration is different from that of the nonlinear vibration. This paper investigates the phase change and the phase difference between such planar and nonplanar vibrations which are caused by one-to-one resonance.

• Journal of Ocean Engineering and Technology
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• v.2 no.1
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• pp.170-175
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• 1988

A study on the dynamic vibration absorber with coil spring and oil damper was carried out both theoretically and experimentally. A main mass is attached to a foundation using coil spring and oil damper. A harmonic motion was applied to the foundation. The effects of the dynamic vibration sbsorber are theoretically summarized in graphs, and tested on a vibratory model for the isolation of actual mechanical vibration. As a result, the first resonance amplitude ratio increased and the second resonance amplitude ratio decreased as the absorber spring constant increased. When the absorber mass increase, the first resonance amplitude ratio is decreased and the second resonace amplitude ratio is increased.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 1997.10a
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• pp.76-81
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• 1997

In this paper, the combustion characteristics of marine diesel engines are investigated. Also, the pv diagrams of diesel engine and the torque harmonic coefficients are calculated. Their reliability are verified by comparing the calculated values with those of engine maker. The calculated results of torsional vibration with these theoretical harmonic coefficients show very good agreement with those of engine maker's results. The results of this study may be useful for the calculation of torsional vibration for diesel engine propulsion shafting, especially for 4-stroke engine which is not easy to get harmonics.

• Wind and Structures
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• v.28 no.6
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• pp.347-354
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• 2019

This study provides an approximate analytical solution to the fractional vibration problem of thin plate governing anomalous motion of plate subjected to in-plane loads. The method of variable separable is employed to transform the fractional partial differential equations under consideration into a fractional ordinary differential equation in temporal variable and a bi-harmonic plate equation in spatial variable. The technique of conformable fractional derivative is utilized to solve the resulting fractional differential equation and the approach of finite sine integral transform method is used to solve the accompanying bi-harmonic plate equation. The deflection field which measures the transverse displacement of the plate is expressed in terms of product of Bessel and trigonometric functions via the temporal and spatial variables respectively. The obtained solution reduces to the solution of the free vibration problem of thin plate in literature. This work shows that conformable fractional derivative is an efficient mathematical tool for tracking analytical solution of fractional partial differential equation governing anomalous vibration of thin plates.

• Proceedings of the KWS Conference
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• 2002.10a
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• pp.408-411
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• 2002

Effect of in-situ vibration on the properties of A-grade steel SMA weldment has been investigated. Welding was performed on the steel fixed at the experimental jig under the mechanical vibration of a given frequency. The applied frequency varied from 39 to 43.5 Hz (harmonic frequency). For weldments formed under the vibration with a sub-harmonic frequency, both the columnar width of the weld metal and the prior austenite grain size of the HAZ near the fusion line clearly decreased. This indicates that the vibration increase the cooling rate after welding. Vibration effect was also found at the weld metal formed at the center region of the weldment. The weld metal showed liner microstructure both in columnar zone and in equiaxed zone with thinner grain boundary ferrite. However mechanical properties of the weld metal did not exactly follow the microstructural changes developed under the vibration. The weld metal formed under the vibration revealed higher yield and tensile strength but lower ductility and impact toughness, compared with the conventional weld metal.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.26 no.4
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• pp.391-397
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• 2016

Vibration disturbances induced by the reaction wheels can severely degrade the performance of high precision payloads on board satellites with high pointing stability requirements. The unwanted disturbances produced by the reaction wheels are composed of fundamental harmonic disturbances due to the flywheel imbalance and sub/higher harmonic disturbances due to bearing irregularities, motor imperfections and so on. Because the wheel speed is constantly changed during the operation of a reaction wheel, the vibration disturbance induced by the reaction wheels can magnify the satellite vibration when the rotating frequency of wheel meets the natural frequency of satellite structure. In order to provide an effective isolation of the reaction wheel disturbances, isolation performance of a hybrid vibration isolator is investigated. In this paper, hybrid vibration isolator that combines passive and active components is developed and its hybrid isolation performance using notch filter control is evaluated in single-axis. The hybrid isolation performance using notch filter control show additional performance improvement compared to the results using only passive components.

• Structural Monitoring and Maintenance
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• v.4 no.3
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• pp.221-236
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• 2017

In this study, the feasibility of vibration-based damage alarming algorithms are numerically evaluated for wind turbine tower structures which are subjected to harmonic force excitation. Firstly, the algorithm of vibration-based damage alarming for the wind turbine tower (WTT) is visited. The natural frequency change, modal assurance criterion (MAC) and frequency-response-ratio assurance criterion (FRRAC) are utilized to recognize changes in dynamic characteristics due to a structural damage. Secondly, a finite element model based on a real wind turbine tower is established in a structural analysis program, Midas FEA. The harmonic force is applied at the rotor level as presence of excitation. Several structural damage scenarios are numerically simulated in segmental joints of the wind turbine model. Finally, the natural frequency change, MAC and FRRAC algorithm are employed to identify the structural damage occurred in the finite element model. The results show that these criteria could be used as promising damage existence indicators for the damage alarming in wind turbine supporting structures.