• Bulletin of the Korean Chemical Society
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• v.7 no.1
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• pp.59-62
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• 1986

Folded Box coherent anti-Stokes Raman spectroscopy (FBCARS) spectra of the fundamental symmetric stretch(213 $cm^{-1}$) and the first overtone(421 $cm^{-1}$) of $I_2$ have been recorded over the pump wavelength range from 495 nm to 580 nm. The lineshapes were fitted using the least squares method and the A and C term of CARS were calculated. From the fitting of A and C terms, the real(R) and imaginary(I) term intensity of CARS were calculated using harmonic oscillator program. The calculated CARS excitation profile of $I_2$ with R and I term data was about 270 $cm^{-1}$ red shifted from the absorption spectrum.

• Journal of Aerospace System Engineering
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• v.13 no.5
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• pp.9-15
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• 2019

In this paper, dynamic analysis of a nonlinear active vibration absorber is conducted with a time delay acceleration feedback to suppress the vibration of a nonlinear single degree of freedom primary system. The primary system consisting of linear and nonlinear cubic springs, mass, and damper is subjected to the multi-harmonic hard excitation with a parametric excitation. It is proposed to reduce the vibration of the primary system and the absorber by using a lead zirconate titanate (PZT) stack actuator in series with a spring in the absorber which configures as an active vibration absorber. The method of multiple scales (MMS) is used to obtain the approximate solution of the system under the internal resonance, subharmonic, superharmonic, and principal parametric resonance conditions simultaneously. Frequency and time responses of the system are investigated considering a delay in the feedback for the various parameters of the absorber configuration and controlling force.

• Structural Engineering and Mechanics
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• v.68 no.3
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• pp.359-368
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• 2018

This paper deals with the dynamic stability of nanocomposite pipes conveying pulsating ferrofluid. The pipe is reinforced by carbon nanotubes (CNTs) where the agglomeration of CNTs are considered based on Mori-Tanaka model. Due to the existence of CNTs and ferrofluid flow, the structure and fluid are subjected to axial magnetic field. Based on Navier-Stokes equation and considering the body forced induced by magnetic field, the external force of fluid to the pipe is derived. For mathematical modeling of the pipe, the first order shear deformation theory (FSDT) is used where the energy method and Hamilton's principle are used for obtaining the motion equations. Using harmonic differential quadrature method (HDQM) and Bolotin's method, the motion equations are solved for calculating the excitation frequency and dynamic instability region (DIR) of the structure. The influences of different parameters such as volume fraction and agglomeration of CNTs, magnetic field, structural damping, viscoelastic medium, fluid velocity and boundary conditions are shown on the DIR of the structure. Results show that with considering agglomeration of CNTs, the DIR shifts to the lower excitation frequencies. In addition, the DIR of the structure will be happened at higher excitation frequencies with increasing the magnetic field.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.24 no.8 s.179
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• pp.1931-1939
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• 2000

Operating excitation forces of the linear vibratory system are normally determined by direct measurement techniques using load cells, strain gauges, etc. But, hydraulic forces of the rotating turbomachinery such as centrifugal pumps are exerted on an impeller due to asymmety of the flow by the interaction between pump impeller and volute. So, investigations of wide range of hydraulic designs and geometric deviations are difficult by direct method. This paper presents a hybrid approach for fourier transformed operational excitation forces, which uses pseudo-inverse matrix of the transfer matrix for the system and the measured vibrational data with standard installed pump. The determination of the transfer function matrix is based on a linear rotor/stationary system and steady state harmonic response in finite element analysis. And, vibrational data is collected in both vertical and horizontal directions at inboard and outboard bearing housings. The results of the process may be enhanced by making acceleration measurements at many more locations than there are forces to be determined.

• Structural Engineering and Mechanics
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• v.66 no.2
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• pp.161-172
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• 2018

The passive control of structures using a pendulum tuned mass damper has been extensively studied in the technical literature. As the frequency of the pendulum depends only on its length and the acceleration of gravity, to tune the frequency of the pendulum with that of the structure, the pendulum length is the only design variable. However, in many cases, the required length and the space necessary for its installation are not compatible with the design. In these cases, one can replace the classical pendulum by a virtual pendulum which consists of a mass moving over a curved surface, allowing thus for a greater flexibility in the absorber design, since the length of the pendulum becomes irrelevant and the shape of the curved surface can be optimized. A mathematical model for a building with a pendular tuned mass damper and a detailed parametric analysis is conducted to study the influence of this device on the nonlinear oscillations and stability of the main system under harmonic and seismic base excitation. In addition to the circular profiles, different curved surfaces with softening and hardening characteristics are analyzed. Also, the influence of impact on energy dissipation is considered. A detailed parametric analysis is presented showing that the proposed damper can not only reduce sharply the displacements, and consequently the internal forces in the main structure, but also the accelerations, increasing user comfort. A review of the relevant aspects is also presented.

• Structural Engineering and Mechanics
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• v.87 no.2
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• pp.117-127
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• 2023

To efficiently attenuate seismic responses of a structure, a novel pneumatic-driven active dynamic vibration absorber (PD-ADVA) is proposed in this study. PD-ADVA aims to realize closed-loop control using a simple and intuitive control algorithm, which takes the structure velocity response as the input signal and then outputs an inverse control force to primary structure. The corresponding active control theory and phase control mechanism of the system are studied by numerical and theoretical methods, the system's control performance and amplitude-frequency characteristics under seismic excitations are explored. The capability of the proposed active control system to cope with frequency-varying random excitation is evaluated by comparing with the optimum tuning TMD. The analysis results show that the control algorithm of PD-ADVA ensures the control force always output to the structure in the opposite direction of the velocity response, indicating that the presented system does not produce a negative effect. The phase difference between the response of uncontrolled and controlled structures is zero, while the phase difference between the control force and the harmonic excitation is π, the theoretical and numerical results demonstrate that PD-ADVA always generates beneficial control effects. The PD-ADVA can effectively mitigate the structural seismic responses, and its control performance is insensitive to amplitude. Compared with the optimum tuning TMD, PD-ADVA has better control performance and higher system stability, and will not have negative effects under seismic wave excitations.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2005.05a
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• pp.424-429
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• 2005

In this paper, the response characteristics of one to one resonance on the rectangular cantilever beam in which basic harmonic excitations are applied by nonlinear coupled differential integral equations are studied. This equations have 3-dimensional non-linearity of nonlinear inertia and nonlinear curvature. Galerkin and multi scale methods are used for theoretical approach to one to one internal resonance. Nonlinear response characteristics of 1st, 2nd, 3rd modes are measured from the experiment for basic harmonic excitation. From the experimental result, geometrical terms of nonlinearity display light spring effect and these terms play an important role in the response characteristics of low frequency modes. Dynamic behaviors in the out of plane are also studied.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.9 no.2
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• pp.20-25
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• 2010

A camera module is supported by a gimbal structure in collection equipment of image information. During flight, the gimbal system undergoes serious accelerations with wide frequencies. To get the correct images, the camera module must be stably vibrated under these conditions. If natural frequency unfortunately exists in a exciting frequency range, resonance occurs there. Hence, harmonic responses analysis is needed to know correct vibration characteristic of the gimbal system. Finite element analysis was performed to get an acceleration of the gimbal system by mode superposition after extracting mode shapes and natural frequencies. Considering damping ratio of 2%, the reponses of gimbal structure were calculated from excitations with a design frequency band. As results, a maximum acceleration transmissibility, which is the ratio of response to excitation, was obtained and it can be used to design the gimbal structure effectively.

• Coupled systems mechanics
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• v.7 no.3
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• pp.353-371
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• 2018

The Wavenumber or more accurately Wavenumber-FD approach was initially introduced for two-dimensional dynamic analysis of concrete gravity dam-reservoir systems. The technique was formulated in the context of pure finite element programming in frequency domain. Later on, a variation of the method was proposed which was referred to as Wavenumber-TD approach suitable for time domain type of analysis. Recently, it is also shown that Wavenumber-FD approach may be applied for three-dimensional dynamic analysis of concrete arch dam-reservoir systems. In the present study, application of its variation (i.e., Wavenumber-TD approach) is investigated for three-dimensional problems. The method is initially described. Subsequently, the response of idealized Morrow Point arch dam-reservoir system is obtained by this method and its special cases (i.e., two other well-known absorbing conditions) for time harmonic excitation in stream direction. All results for various considered cases are compared against the exact response for models with different values of normalized reservoir length and reservoir base/sidewalls absorptive conditions.

• 한국공간정보시스템학회:학술대회논문집
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• 2004.05a
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• pp.161-168
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• 2004

The dynamic instability of snapping phenomena has been studied by many researchers. Few papers deal with dynamic buckling under loads with periodic characteristics, and the behavior under periodic excitations is expected to be different from behavior under STEP excitations. We investigate the fundamental mechanisms of the dynamic instability when the sinusoidally shaped arch structures are subjected to sinusoidally distributed excitations with pin-ends. The mechanisms of dynamic indirect snapping of shallow arches are especially investigated under not only STEP function excitations but also under sinusoidal harmonic excitations, applied in the up-and-down direction. The dynamic nonlinear responses are obtained by the numerical integration of the geometrically nonlinear equation of motion, and examined by Fourier spectral analysis in order to get the frequency-dependent characteristics of the dynamic instability for various load levels.