• Smart Structures and Systems
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• 제33권3호
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• pp.243-251
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• 2024

Modeling of vibrations of a rotating pendulum with working shape memory alloy rod has been performed in the frames of a microstructural model taking into account the latent heat release, absorption and the heat exchange during direct and reverse martensitic transformation. It has been shown that the influence of the latent heat, the rate of preliminary deviation of the pendulum from the equilibrium, the rate of heating and cooling can have a significant impact on the vibrations and damping characteristics of the system.

• 대한기계학회논문집A
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• 제32권7호
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• pp.541-548
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• 2008

To improve the operational stability of the 100 Watts class Micro Gas Turbine, the air foil bearing with additional damping material has been investigated. The key of structure is that a viscoelastic material is coated under the top foil. The compliant foil journal bearing and thrust bearing are designed to withstand high load of vibrations at the operational speed 870,000 rpm. Test is executed in room temperature. Rotor has stably operated above 480,000 rpm. It is over 55% of the designed speed 870,000 rpm. Synchronous and subsynchronous vibrations are both well controlled. Vibration amplitude diminished over 50%. With the help of increased damping resulting from the viscoelasticity, the rotor stability of Micro turbocharger has been improved.

• Structural Engineering and Mechanics
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• 제48권5호
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• pp.711-736
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• 2013

In this paper new implicit time integration called N-IHOA is presented for dynamic analysis of high damping systems. Here, current displacement and velocity are assumed to be functions of the velocities and accelerations of several previous time steps, respectively. This definition causes that only one set of weighted factors is calculated from the Taylor series expansion which leads to a simple approach and reduce the computational efforts. Moreover a comprehensive study on stability of the proposed method i.e., N-IHOA compared with IHOA integration which is performed based on amplification matrices proves the ability of the N-IHOA in high damping vibrations such as control systems. Also, wide range of numerical examples which contains single/multi degrees of freedom, damped/un-damped, free/forced vibrations from finite element/finite difference demonstrate that the accuracy and efficiency of the proposed time integration is more than the common approaches such as the IHOA, the Wilson-${\theta}$ and the Newmark-${\beta}$.

• 한국해양공학회지
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• 제7권2호
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• pp.37-43
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• 1993

The simplest vibrtion absorber is the impact damper which consists of a small mass allowed to impact between two gaps sp that energy is dissipated by conversion into noise and heat. Impact damping is a passive vibration control technique to attenuate the vibrations of lightly damped. It has been investigated to reduce the excessive vibrations of turbin blades, radar antennas, machine tools and tall light poles. In this paper, the efficiency of impact vibration absorber was investigated. A steady state vibration of two equispaced impacts per cycle was assumed. The analysis based on the assumption has been considered and the theory is examined experimentally.

• Smart Structures and Systems
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• 제3권4호
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• pp.405-422
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• 2007

A new hybrid damping technique for vibration reduction in flexible structures, wherein a combination of layers of hard passive damping alloys and active (smart) magnetostrictive material is used to reduce vibrations, is proposed. While most conventional vibration control treatments are based exclusively on either passive or active based systems, this technique aims to combine the advantages of these systems and simultaneously, to overcome the inherent disadvantages in the individual systems. Two types of combined damping systems are idealized and studied here, viz., the Noninteractive system and the Interactive system. Frequency domain studies are carried out to investigate their performance. Finite element simulations using previously developed smart beam elements are carried out on typical metallic and laminated composite cantilever beams treated with hybrid damping. The influence of various parameters like excitation levels, frequency (mode) and control gain on the damping performance is investigated. It is shown that the proposed system could be used effectively to dampen the structural vibration over a wide frequency range. The interaction between the active and passive damping layers is brought out by a comparative study of the combined systems. Illustrative comparisons with 'only passive' and 'only active' damping schemes are also made. The influence and the mode dependence of control gain in a hybrid system is clearly illustrated. This study also demonstrates the significance and the exploitation of strain dependency of passive damping on the overall damping of the hybrid system. Further, the influence of the depthwise location of damping layers in laminated structures is also investigated.

• Advances in aircraft and spacecraft science
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• 제3권3호
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• pp.331-349
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• 2016

Stiff and light fibre reinforced composites as used in air- and space-craft applications tend to high sound emission. Therefore, the damping properties are essential for the entire structural and acoustic engineering. Viscous damping is an established and reasonably linear model of the dissipation behaviour. Commonly, it is assumed to be isotropic and constant over all modes. For anisotropic materials it depends on the fibre orientation as well as the elastic and thermal material properties. To portray the orthogonal anisotropic behaviour, a model for unidirectional fibre reinforced plastics (frp) has been developed based on the classical laminate theory by ADAMS and BACON starting in 1973. Their approach includes three damping coefficients - for longitudinal damping in fibre direction, damping transversal to the fibres and shear based dissipation. The damping of a laminate is then accumulated layer wise including the anisotropic stiffness. So far, the model has been applied mainly to thermoset matrix materials. In this study, an experimental parameter estimation for different thermoplastic frp with angle ply and cross ply layups was carried out by measuring free vibrations of cantilever beams. The results show potential and limits of the ADAMS/BACON damping criterion. In addition, a possibility of modelling the anisotropic damping is shown. The implementation in standard FEA software is used to study the influence of boundary conditions on the damping properties and numerically estimate the radiated sound power of thin-walled frp parts.

• 한국공간구조학회논문집
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• 제18권1호
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• pp.77-84
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• 2018

The identification of damping ratios in buildings is a well-known problem and appears to be of important and crucial interest in the safety and serviceability design. When compared to an estimation of the stiffness, i.e. natural frequency, and mass, the damping ratio is the most difficult quantity to determine. Many previous studies have examined the characteristics of damping ratios from ambient vibration, but the measurement time is roughly within 2 hours. In this paper, characteristics of damping ratios and natural frequencies of 4 story RC building were investigated using long-term ambient vibration. Free vibrations were obtained using random decrement technique, and damping ratios were evaluated by the envelop function, continuous wavelet transform, and logarithmic decrement. It was found that although the natural frequencies show little variations with time, the damping ratios show some variations with time and the largest variations found in the damping ratios obtained from the continuous wavelet transform. The damping ratios from the envelop function showed the smallest mean and standard deviation. And the probability distribution of damping ratios seems to follow the logarithmic normal distribution.

• Wind and Structures
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• 제22권1호
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• pp.107-131
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• 2016

The need for a more affordable, reliable, clean and secure energy has led to explorations in non-traditional sources, particularly renewable energies. Wind is one of the cleanest energy sources that plays a significant role in augmenting sustainability. Wind turbines, as energy convertors, are usually tall and slender structures, and depending on their location (inland or offshore), they can be subject to high wind and/or strong wave loadings. These loads can cause severe vibrations with detrimental effects on energy production, structural lifecycle and initial cost. A dissipativity analysis study was carried out to know whether wind turbine towers require damping enhancement or rigidity modifications for vibration suppression. The results suggest that wind turbines are lightly damped structures and damping enhancement is a potential solution for vibration lessening. Accordingly, the paper investigates different damping enhancement techniques for vibration mitigation. The efficacy of tuned mass damper (TMD), tuned liquid column damper (TLCD), tuned sloshing damper (TSD), and viscous damper (VD) to reduce vibrations is investigated. A comparison among these devices, in terms of robustness and effectiveness, is conducted. The VD can reduce both displacement and acceleration responses of the tower, better than other types of dampers, for the same control effort, followed by TMD, TSD, and finally TLCD. Nevertheless, the use of VDs raises concerns about where they should be located in the structure, and their application may require additional design considerations.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2012년도 춘계학술대회 논문집
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• pp.701-706
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• 2012

This research is concerned with the derivation of equations of motion for a slewing beam and the application of input shaper to the bang-bang control to achieve vibration suppression. When a uniform beam with a tip mass rotates about the axis perpendicular to the undeformed beam's longitudinal axis, it experiences inertial loading. Hence, the beam vibrates. In this paper, we used the input shaper for the maneuvering control to suppress vibrations. The maneuvering control which can achieve a minimum-time control is a bang-bang control. The input-shaped bang-bang maneuvering is used to suppress vibrations both theoretically and experimentally. The slewing beam experiment is not an easy subject because of the inherent damping existing inside the rotor. We propose the use of a negative damping to eliminate the rotor damping. Numerical and experimental results show that the input-shaper can be effectively used for the vibration suppression of a slewing beam.

• 한국소음진동공학회논문집
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• 제22권6호
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• pp.542-549
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• 2012

This research is concerned with the derivation of equations of motion for a slewing beam and the application of input shaper to the bang-bang control to achieve vibration suppression. When a uniform beam with a tip mass rotates about the axis perpendicular to the undeformed beam's longitudinal axis, it experiences inertial loading. Hence, the beam vibrates. In this paper, we used the input shaper for the maneuvering control to suppress vibrations. The maneuvering control which can achieve a minimum-time control is a bang-bang control. The input-shaped bang-bang maneuvering is used to suppress vibrations both theoretically and experimentally. The slewing beam experiment is not an easy subject because of the inherent damping existing inside the rotor. We propose the use of a negative damping to eliminate the rotor damping. Numerical and experimental results show that the input-shaper can be effectively used for the vibration suppression of a slewing beam.