• Structural Engineering and Mechanics
• /
• v.46 no.2
• /
• pp.153-177
• /
• 2013

Pendulums can be used as passive vibration control devices in several structures and machines. In the present work, the nonlinear behavior of a pendulum-tower system is studied. The tower is modeled as a bar with variable cross-section with concentrated masses. First, the vibration modes and frequencies of the tower are obtained analytically. The primary structure and absorber together constitute a coupled system which is discretized as a two degrees of freedom nonlinear system, using the normalized eigenfunctions and the Rayleigh-Ritz method. The analysis shows the influence of the geometric nonlinearity of the pendulum absorber on the response of the tower. A parametric analysis also shows that, with an appropriate choice of the absorber parameters, a pendulum can decrease the vibration amplitudes of the tower in the main resonance region. The results also show that the pendulum nonlinearity cannot be neglected in this type of problem, leading to multiplicity of solutions, dynamic jumps and instability. In order to improve the effectiveness of the control during the transient response, a hybrid control system is suggested. The added control force is implemented as a non-linear variable stiffness device based on position and velocity feedback. The obtained results show that this strategy of nonlinear control is attractive, has a good potential and can be used to minimize the response of slender structures under various types of excitation.

• Journal of the Korean Society for Railway
• /
• v.12 no.6
• /
• pp.989-995
• /
• 2009

Short-time Fourier transforms (STFT) are useful for analyzing signals with harmonics that vary with time. If the variation of the harmonics with time is smooth, such as in kinematic vibrations in vehicles, then it is possible to improve the STFT using order spectra and correlation analysis. In this paper, it is shown how correlation analysis can be performed when the speed signal is noisy or unknown and then it is shown how order spectra become simple to compute after this analysis. The results are illustrated by an analysis of axle and car body vibrations in the prototype high-speed train, HSR-350x.

• Computational Structural Engineering
• /
• v.10 no.3
• /
• pp.217-223
• /
• 1997

This paper deals with the basic concepts of sensitivity analysis in a time domain for the transient response of a lumped mass system. Sensitivity analysis methods in thme domain for determining the effects of parameter changes on the response of a dynamic system by external excitation are presented. The parametric sensitivity of a lumped mass system in time domain can be investigated using different types of sensitivity functions, including first order standard and percentage sensitivity functions. These sensitivity functions are determined as a function of partial derivatives of system variables taken with respect to system parameters. In addition, we compared the results of the analytical method by direct method and those of numerical methods.

• Earthquakes and Structures
• /
• v.7 no.6
• /
• pp.1223-1240
• /
• 2014

Seismic isolation has been established as an effective earthquake-resistant design method and the lead rubber bearings (LRBs) are among the most commonly used seismic isolation systems. In the scientific literature, a sharp bilinear model is often used for capturing the hysteretic behaviour of the LRBs in the analysis of seismically isolated structures, although the actual behaviour of the LRBs can be more accurately represented utilizing smoothed plasticity, as captured by the Bouc-Wen model. Discrepancies between these two models are quantified in terms of the computed peak relative displacements at the isolation level, as well as the peak inter-storey deflections and the absolute top-floor accelerations, for the case of base-isolated buildings modelled as multi degree-of-freedom systems. Numerical simulations under pulse-like ground motions have been performed to assess the effect of non-linear parameters of the seismic isolation system and characteristics of both the superstructure and the earthquake excitation, on the accuracy of the computed peak structural responses. Through parametric analyses, this paper assesses potential inaccuracies of the computed peak seismic response when the sharp bilinear model is employed for modelling the LRBs instead of the more accurate and smoother Bouc-Wen model.

• Proceedings of the KSME Conference
• /
• 2000.11a
• /
• pp.510-515
• /
• 2000

Dynamic stability of a flying structure undertaking constant and pulsating axial forces is investigated in this paper. The equations of motion of the structure, which is idealized as a free-free beam, are derived by using the hybrid variable method and the assumed mode method. The structural system includes a directional control unit to obtain the directional stability. The analysis model presented in this paper considers the nonlinear effect due to rigid body motion of the beam. Dynamic stability of the system is influenced by the nonlinear effect. In order to examine the nonlinear effect, first the unstable regions of the linear system are obtained by using the method based upon Floquet's theory, and dynamic responses of the nonlinear system in the unstable region are obtained by using direct time integration method. Dynamic stability of the nonlinear system is determined by the obtained dynamic responses.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
• /
• 2013.04a
• /
• pp.273-278
• /
• 2013

This paper presents optimal design procedures of mount based on a magnetorheological (MR) fluid to isolate the vibration in heavy diesel engine system. At first, frequency response and force-displacement transmissibility methods are used to get required damping force that is necessary for effective vibration isolation. From this result, a new type of high damping force engine mount is proposed and the governing equation of Bingham plastic behavior of MR fluid in flow path is mathematically derived under cylindrical coordinates. Finally, parametric design optimization featuring finite element is performed using ANSYS software to get the required damping force in MR mount system which can be used to reduce engine vibration. Damping force of the MR mount is then determined as an objective function in this analysis based on ANSYS. Furthermore, Magnetic analysis is then applied in this process.

• Transactions of the Korean Society for Noise and Vibration Engineering
• /
• v.23 no.5
• /
• pp.472-478
• /
• 2013

This paper presents optimal design procedures of mount based on a magnetorheological(MR) fluid to isolate the vibration in heavy diesel engine system. At first, frequency response and force-displacement transmissibility methods are used to get required damping force that is necessary for effective vibration isolation. From this result, a new type of high damping force engine mount is proposed and the governing equation of Bingham plastic behavior of MR fluid in flow path is mathematically derived under cylindrical coordinates. Finally, parametric design optimization featuring finite element is performed using ANSYS software to get the required damping force in MR mount system which can be used to reduce engine vibration. Damping force of the MR mount is then determined as an objective function in this analysis based on ANSYS. Furthermore, Magnetic analysis is then applied in this process.

• Transactions of the Korean Society of Automotive Engineers
• /
• v.9 no.4
• /
• pp.148-156
• /
• 2001

Brake judder, one of low Sequency vibrations in brake system is determined by the excitation of Brake Torque Variation (BTV). The largest contributor to BTV is disc thickness variation. In this study, the static loads of brake torque at Suspension Mounting Points (SW) are obtained by the quasi-static analysis using DADS. The dynamic loads with frequency of BTV at SW are derived from correlation between forced vibration analysis with static loads and brake test results. And the accelerations at steering wheel were analyzed by forced vibration analysis with dynamic loads using commercial finite element program MSC/NASTRAN so that vibration characteristics of vehicle due to brake judder were investigated. Reliability of analysis results was verified through comparing the brake test results. Also, a parametric study with natural frequencies of frame, such as the 1st torsional mode and 1st bending mode, was conducted to reduce vibration amplitudes. As a result we could detect frame natural frequency conditions to improve vibration characteristics and obtained the frame model to reduce vibration amplitude.

• Journal of the Korean Society of Visualization
• /
• v.11 no.2
• /
• pp.41-45
• /
• 2013

Two-photon microscopy (TPM) is minimally-invasive 3D fluorescence microscopy based on nonlinear excitation, and TPM can visualize cellular structures based on auto-fluorescence. Line-scanning TPM is one of high-speed TPM methods without sacrificing the image resolution by using spatial and temporal focusing. In this paper, we developed line-scanning TPM based on spatial and temporal focusing for auto-fluorescence imaging by exciting the tryptophan. Laser source for this system was an optical parametric oscillator (OPO) and it made near 570 nm femtosecond pulse laser. It had 200fs pulse width and 1.72 nm bandwidth, so that the achievable depth resolution was 2.41um and field of view (FOV) is 10.8um. From the characterization, our system has 3.0 um depth resolution and 12.3 um FOV. We visualized fixed leukocyte cell sample and compared with point scanning system.

• Geomechanics and Engineering
• /
• v.20 no.2
• /
• pp.103-112
• /
• 2020

The dynamic response of crossing tunnels under heavy-haul train loads is still not fully understood. In this study, based on the case of a high-speed tunnel underneath an existing heavy-haul railway tunnel, a model experiment was performed to research the dynamic response characteristics of crossing tunnels. It is found that the under-crossing changes the dynamic response of the existing tunnel and surrounding rock. The acceleration response of the existing tunnel enhances, and the dynamic stress of rock mass between crossing tunnels decreases after the excavation. Both tunneling and the excitation of heavy-haul train loads stretch the tunnel base, and the maximum tensile strain is 18.35 µε in this model test. Then, the measured results were validated by numerical simulation. Also, a parametric study was performed to discuss the influence of the relative position between crossing tunnels and the advanced support on the dynamic behavior of the existing tunnel, where an amplifying coefficient of tunnel vibration was introduced to describe the change in acceleration due to tunneling. These results reveal the dynamic amplifying phenomenon of the existing tunnel during the new tunnel construction, which can be referred in the dynamic design of crossing tunnels.