• Structural Engineering and Mechanics
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• 제85권1호
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• pp.81-88
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• 2023

This paper begins by analyzing cable vibrations due to external excitations and their effects on the overall dynamic behavior of cable-stayed bridges. It is concluded that if the natural frequency of a cable approaches any natural frequency of the bridge, the cable loses its rigidity and functionality. The results of this analysis explain the phenomenon that occurred on the Dubrovnik Bridge in Croatia during a storm and measures for its retrofit. A field test was conducted before the bridge was opened to traffic. It was concluded: "The Bridge excited unpleasant transverse superstructure vibration with the frequency of approximately 0.470 Hz. Hence, it seems possible that a pair of stays vibrating in phase may excite deck vibrations". Soon after this Bridge opened, a storm dumped heavy damp snow in the area, causing the six longest cable stay pairs of the main span to undergo large-amplitude vibrations, and the superstructure underwent considerable displacements in combined torsion-sway and bending modes. This necessitated rehabilitation measures for the Bridge including devices to suppress the large-amplitude vibrations of cables. The rehabilitation and monitoring of the Bridge are also presented here.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 한국소음진동공학회 2002년도 추계학술대회논문집
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• pp.1022-1027
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• 2002

This paper describes the fluttering characteristics of the upper pad in a tilting pad journal bearing(6-pad, LOP type) using a steam turbine. In order to investigate the phenomena of the upper pad fluttering experimentally, the absolute vibration of the upper pads the relative vibration between bearing and shaft and the circumferential distribution of the film thickness are measured under the different values of supply oil flow rate, shaft speed and bearing load. It can be known that the fluttering mechanism of the upper pads has a tendency of the self-excited vibration from the study of fluttering frequencies and amplitudes with the change of shaft speed. furthermore, it is observed that the incipient upper pad fluttering velocity is increased by the increase of oil supply flow rate and fluttering amplitude of the upper pads is increased by the decrease of the oil flow rate and by the increase of the bearing load.

• Journal of the Korean Society for Precision Engineering
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• 제21권10호
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• pp.127-136
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• 2004

This paper is the first of two companion papers concerning the analysis and design of a pneumatic vibration isolation system. The design optimization of the pneumatic vibration isolation system is required for the reduction of cost, endeavor and time, and it needs modeling and calculation algorithm. The nonlinear models are devised from the fluid mechanical expression for components of the system and the calculation algorithm is derived from the mathematical relationship between the models. It is shown that the orifice makes the nonlinear property of the transmissibility curve that the resonant frequency changes by the amplitude of excited vibration. Linearization of the nonlinear models is tried to reduce elapsed time and truncation error accumulation and to enable the transmissibility calculation of the system with multi damping chambers. The equivalent mechanical models generated by linearization clarify the function of each component of the system and lead to the linearized transfer function that can give forth to the transmissibility exactly close to that of nonlinear models. The modified successive under-relaxation method is developed to calculate the linearized transfer function.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• 제13권6호
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• pp.467-473
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• 2003

This paper describes the fluttering characteristics of the upper pad in a tilting pad journal bearing(6-pad, LOP type) using a steam turbine. In order to investigate the phenomena of the upper pad fluttering experimentally, the absolute vibration of the upper pads, the relative vibration between bearing and shaft and the circumferential distribution of the film thickness are measured under the different values of supply oil flow rate, shaft speed and bearing load. It can be known that the fluttering mechanism of the upper pads has a tendency of the self-excited vibration from the study of fluttering frequencies and amplitudes with the change of shaft speed. Furthermore, it is observed that the incipient upper pad fluttering velocity is increased by the increase of oil supply flow rate and the fluttering amplitude of the upper pads is increased by the decrease of the ell flow rate and by the increase of the bearing load.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 한국추진공학회 2008년 영문 학술대회
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• pp.810-812
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• 2008

Previously study on structural design of the main wing of the twenty-seat class WIG(Wing in Ground Effect) craft. In the final design, three spars construction was selected for safety in the critical flight load, and the Carbon-Epoxy material was selected for lightness and structural stability. In this study, the forced vibration analysis was performed on the composite main wing structure of the twenty-seat class WIG craft with two-stroke pusher type reciprocating engine. The vibration analysis based on the finite element method was performed using a commercial FEM code, MSC/NASTRAN. Excitations for the frequency response analysis were assumed as the H-mode(horizontal mode), the V-mode(vertical mode) and the X-mode(twisted mode) which are typical main vibration modes of engine. And excitations for the transient response analysis were assumed as the L-mode(longitudinal mode) with the oscillating propeller thrust which occurs in operation. According to the result of forced vibration analysis, structural design was modified to reduce the vibrations.

• Smart Structures and Systems
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• 제19권6호
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• pp.665-678
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• 2017

In this paper the tuned mass-damper-inerter (TMDI) is considered for passive vibration control and energy harvesting in harmonically excited structures. The TMDI couples the classical tuned mass-damper (TMD) with a grounded inerter: a two-terminal linear device resisting the relative acceleration of its terminals by a constant of proportionality termed inertance. In this manner, the TMD is endowed with additional inertia, beyond the one offered by the attached mass, without any substantial increase to the overall weight. Closed-form analytical expressions for optimal TMDI parameters, stiffness and damping, given attached mass and inertance are derived by application of Den Hartog's tuning approach to suppress the response amplitude of force and base-acceleration excited single-degree-of-freedom structures. It is analytically shown that the TMDI is more effective from a same mass/weight TMD to suppress vibrations close to the natural frequency of the uncontrolled structure, while it is more robust to detuning effects. Moreover, it is shown that the mass amplification effect of the inerter achieves significant weight reduction for a target/predefined level of vibration suppression in a performance-based oriented design approach compared to the classical TMD. Lastly, the potential of using the TMDI for energy harvesting is explored by substituting the dissipative damper with an electromagnetic motor and assuming that the inertance can vary through the use of a flywheel-based inerter device. It is analytically shown that by reducing the inertance, treated as a mass/inertia-related design parameter not considered in conventional TMD-based energy harvesters, the available power for electric generation increases for fixed attached mass/weight, electromechanical damping, and stiffness properties.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 한국소음진동공학회 1998년도 춘계학술대회논문집; 용평리조트 타워콘도, 21-22 May 1998
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• pp.478-485
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• 1998

The interior of open cavities exposed to a grazing flow is known to experience, strong periodic pressure, oscillations sustained for a wide range of flow velocities. In this study, an original approach was followed to develop a describing function model for the flow-excitation mechanism, governed by the shedding of discrete vortices within the shear layer over the orifice. A feedback loop analysis was performed to predict the frequency and the amplitude of the interior pressure fluctuations. Furthermore, a limit cycle stability analysis based on the extended Nyquist Stability criterion allowed the predictions of the onset and termination velocities for various modes. The analytical model was verified experimentally.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 한국소음진동공학회 1998년도 춘계학술대회논문집; 용평리조트 타워콘도, 21-22 May 1998
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• pp.471-477
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• 1998

A constant flow-rate control valve provides effective distributions and controls of running water in a pipe system. The noise characteristics were measured to reveal the noise sources depending on pressure differences across a valve. Valve noise is mostly dominated by bubble dynamics under cavitating conditions. In this study, the sound powers from a flow control valve of constant flow rates are effectively normalized. Flow-excited dynamic systems for which there is no strong coupling between the flow and the system response can be described using a linear source-filter model. On this assumption, the normalized sound powers can be decomposed of noise source function and a response function. The source spectra in, terms of cavitation frequency show cavitation events occurring at narrow banded frequencies greater than 10 kHz. There also possibly exist two kinds of cavitating modes based on our experimental data.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 한국공작기계학회 2003년도 춘계학술대회 논문집
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• pp.382-387
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• 2003

Many type of smart materials and control laws are available to actively adjust the structure from various external disturbances. Usually, a certain type of control law to activate a specific smart material is tell established, but the effectiveness of the control scheme is limited by the choice of the smart materials and the responses of the structure. ER fluid is adequate to provide small but arbitrary control forces at any point along the structure. It was found that active vibration control of the structure embedded with ER fluids fluidly to suppress the vibration excited with broad band frequency due to the limited change of the structure characteristics. To compensate this limited effect of the control scheme with ER fluid alone, PPF control using PZT as an actuator is added to construct a hybrid controller.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 한국소음진동공학회 2014년도 추계학술대회 논문집
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• pp.355-357
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• 2014

The usefulness of unsteady combustion was experimentally investigated using confined premixed flames stabilized by a rearward-facing step. For this purpose, apparatus of forced pulsating mixture supply, which could be modulated its amplitude and frequency, was designed. The unsteady combustion used in this experiment plays an important role in controlling self-excited combustion oscillations and furthermore it exhibits desirable performance, from a practical point of view, such as high load combustion and reduction of pollutant emission like nitric oxide.