• Structural Engineering and Mechanics
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• v.70 no.5
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• pp.623-637
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• 2019

This paper analyzes the non-stationary vibration and super-harmonic resonances in nonlinear dynamic motion of viscoelastic nano-resonators. For this purpose, a new coupled size-dependent model is developed for a plate-shape nano-resonator made of nonlinear viscoelastic material based on modified coupled stress theory. The virtual work induced by viscous forces obtained in the framework of the Leaderman integral for the size-independent and size-dependent stress tensors. With incorporating the size-dependent potential energy, kinetic energy, and an external excitation force work based on Hamilton's principle, the viscous work equation is balanced. The resulting size-dependent viscoelastically coupled equations are solved using the expansion theory, Galerkin method and the fourth-order Runge-Kutta technique. The Hilbert-Huang transform is performed to examine the effects of the viscoelastic parameter and initial excitation values on the nanosystem free vibration. Furthermore, the secondary resonance due to the super-harmonic motions are examined in the form of frequency response, force response, Poincare map, phase portrait and fast Fourier transforms. The results show that the vibration of viscoelastic nanosystem is non-stationary at higher excitation values unlike the elastic ones. In addition, ignoring the small-size effects shifts the secondary resonance, significantly.

• International Journal of Aeronautical and Space Sciences
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• v.13 no.3
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• pp.332-348
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• 2012

The present study deals with the parametric resonance behaviour of laminated composite curved shell panels in a hygrothermal environment using Bolotin's approach. A simple laminated model is developed using first order shear deformation theory (FSDT) for the vibration and dynamic stability analysis of laminated composite shells subjected to hygrothermal conditions. A computer program based on the finite element method (FEM) in a MATLAB environment is developed to perform all necessary computations. Quantitative results are presented to show the effects of curvature, ply-orientations, degree of orthotropy and geometry of laminates on the parametric instability of composite curved shell panels for different temperature and moisture concentrations. The excitation frequencies of laminated composite panels decrease with the increase of temperature and moisture due to reduction of stiffness for all laminates.

• Journal of Power Electronics
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• v.18 no.4
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• pp.1165-1177
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• 2018

This paper investigates the active damping of grid-connected LCL filter resonance using high-pass filter (HPF) of the grid current. An expression for such HPF is derived in terms of the filter components. This expression facilitates a general study of the actively damped filter behavior in the discrete time domain. Limits for the HPF parameters are derived to avoid the excitation of unstable open loop poles since such excitation can reduce both the damping performance and the system robustness. Based on this study, straightforward co-design steps for the active damping loop along with the fundamental current regulator are proposed. A numerical example along with simulation and experimental results are presented to verify the theoretical analyses.

• Journal of KSNVE
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• v.8 no.4
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• pp.715-722
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• 1998

An investigation into the stochastic bifurcation and response statistics of an autoparameteric system under broad-band random excitation is made. The specific system examined is a spring-pendulum system with internal resonance, which is known to be a good model for a variety of engineering systems, including ship motions with nonlinear coupling between pitching and rolling motions. The Fokker-Planck equations is used to genrage a general first-order differential equation in the dynamic moment of response coordinates. By means of the Gaussian and non-Gaussian closure methods the dynamic moment equations for the random responses of the system are reduced to a system of autonomous ordinanary differential equations. In view of equilibrium solutions of this system and their stability we examine the stochastic bifurcation and response statistics. The analytical results are compared with results obtained by Monte Carlo simulation.

• Structural Engineering and Mechanics
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• v.75 no.1
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• pp.87-100
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• 2020

The present paper investigates the combination resonance behavior of imperfect spiral stiffened functionally graded (SSFG) cylindrical shells with internal and external functionally graded stiffeners under two-term large amplitude excitations. The structure is embedded within a generalized nonlinear viscoelastic foundation, which is composed of a two-parameter Winkler-Pasternak foundation augmented by a Kelvin-Voigt viscoelastic model with a nonlinear cubic stiffness, to account for the vibration hardening/softening phenomena and damping considerations. With regard to classical plate theory of shells, von-Kármán equation and Hook law, the relations of stress-strain are derived for shell and stiffeners. The spiral stiffeners of the cylindrical shell are modeled according to the smeared stiffener technique. According to the Galerkin method, the discretized motion equation is obtained. The combination resonance is obtained by using the multiple scales method. Finally, the influences of the stiffeners angles, foundation type, the nonlinear elastic foundation coefficients, material distribution, and excitation amplitude on the system resonances are investigated comprehensively.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2000.11a
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• pp.612-617
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• 2000

This paper describes the experience gained from the treatments for prevention of blade failure occurred in the low-pressure turbine. Some cracks due to high cycle fatigue were found at the blades in low-pressure turbines after long time operation. Such failure was mainly caused by the resonance of the blade with the vane passing frequency excitation. If a natural frequency of the blade exists near the excitation frequency, a resonant vibration can occur and leads to a large amount of stress which may cause fatigue failures in turbine blades. To avoid the resonance of the blade, some modifications have been performed and full-scaled mockup testing has been done to confirm the verification for modification. Test result shows that enlarging the span cover is very useful to change the natural frequency of the grouped blades effectively.

• Journal of the Society of Naval Architects of Korea
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• v.59 no.3
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• pp.134-140
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• 2022

Ship local structure sometimes experiences severe vibration due to the resonance with an excitation force generated by the propulsion system. In that case, the installation of dynamic vibration absorber such as Tuned Mass Damper (TMD) on the structure can be considered as an effective alternative countermeasure to reduce the troublesome vibration if structural modification or change of excitation frequencies is difficult. Meanwhile, the conventional optimal design method of TMD premises the target structure exposed on an excitation force without the constraint of its magnitude and frequency range. However, the frequencies of major ship excitation forces due to propulsion system are normally bounded and its magnitude is varied according to its operation speed. Hence, the optimal design of TMD to reduce the resonant vibration of ship local structure should be differently approached compared with the conventional ones. For the purpose, this paper proposes an optimal design method of TMD considering maximum frequency and magnitude variation of a target harmonic excitation component. It is done by both lowering the resonant response at the 1st natural frequency and locating the 2nd natural frequency over maximum excitation frequency for the idealized 2 degree of freedom system consisted of the structure and the TMD. For the validation of the proposed method, a numerical design case of TMD for a ship local structure exposed on resonant vibration due to a propeller excitation force is introduced and its performance is compared with the conventionally designed one.

• Proceedings of the KSME Conference
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• 2001.06d
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• pp.878-883
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• 2001

The effects of external excitation with various frequencies and amplitudes on the flame behavior and pollution emission characteristics from a laminar jet flame are experimentally investigated. Measurements of $NO_{x}$ emission indices($EINO_{x}$), performed in unconformed and vertical lifted flame at resonance frequency by strong excitation, have been conducted experimentally. It was also conducted to investigate the effects of excited frequency on $NO_{x}$ emissions with a various frequency ranged 0Hz to 2kHz. From the vertical lifted flame like turbulent of the excited jet with resonance frequency was shown that the dependence of $NO_{x}$ emission could be categorized into three groups: Group 1 of intermediate flame length and relative narrow flame volume yielding low $NO_{x}$ emission, Group 2 of short flame length but large flame volume yielding high $NO_{x}$ emission and Group 3 of long flame length with low temperature contours yielding high $NO_{x}$ emission.

• The Journal of the Acoustical Society of Korea
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• v.38 no.3
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• pp.340-343
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• 2019

In this paper, a problem of mechanical resonance between traction motor's rigid body mode and traction motor's excitation force is introduced, and a bogie design variable affecting the control of resonance response is reviewed numerically. To solve the resonance problem in rotating machinery with variable rotational speeds, resonance frequency should be out of rotational machine's operation range or dynamic stiffness of structures should be increased for resonance response enough to be low. In general, operation range of a traction motor is from 0 r/min to 4800 r/min. It is not possible that all bogie modes are more than 80 Hz. Therefore, it is very important to find design factor affecting resonance response of traction motor's rigid body modes. It is found that key design variable is the gab between transom pipes from finite element analysis. The larger gab is, the higher resonance response when resonance between traction motor's excitation force and traction motor's rigid body mode is happened.

• Korean Journal of Materials Research
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• v.8 no.9
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• pp.843-848
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• 1998

The Usages of composite materials have been steadily on the rise in the industries of automobiles, air crafts, shipbuilding and other structures for transportations. Commonly required in those industries are light weight and high strength of the structures. Consequently, serious efforts in research have been focused on searching for light materials and on developments and characterizations of advanced substitutes including various kinds of composite materials. In this investigation, transversely isotropic composite materials are chosen and formed into two kinds of beams; Euler-Bernoulli beam(thin team) and Timoshenko beam(thick beam) for determinations of elastic constants. As an experimental technique Impulse Excitation Method is utilized to measure resonance frequencies of the beams of the composite materials in vibration tests. Elastic constants are evaluated from measured resonance frequencies for the two types of beams to observe and establish possible existence of effects of rotary inertia and shear deformations.