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Harmonic Excitation (harmonic + excitation)

Distribution by Scientific Domains
Engineering75%

Selected Abstracts

Three-dimensional models of reservoir sediment and effects on the seismic response of arch dams

EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 10 2004
O. Maeso
Abstract The important effects of bottom sediments on the seismic response of arch dams are studied in this paper. To do so, a three-dimensional boundary element model is used. It includes the water reservoir as a compressible fluid, the dam and unbounded foundation rock as viscoelastic solids, and the bottom sediment as a two-phase poroelastic domain with dynamic behaviour described by Biot's equations. Dynamic interaction among all those regions, local topography and travelling wave effects are taken into account. The results obtained show the important influence of sediment compressibility and permeability on the seismic response. The former is associated with a general change of the system response whereas the permeability has a significant influence on damping at resonance peaks. The analysis is carried out in the frequency domain considering time harmonic excitation due to P and S plane waves. The time-domain results obtained by using the Fourier transform for a given earthquake accelerogram are also shown. The possibility of using simplified models to represent the bottom sediment effects is discussed in the paper. Two alternative models for porous sediment are tested. Simplified models are shown to be able to reproduce the effects of porous sediments except for very high permeability values. Copyright © 2004 John Wiley & Sons, Ltd. [source]

Fatigue life expenditure assessment and countermeasure for turbine blades due to harmonic excitations of slip energy recovery drives

EUROPEAN TRANSACTIONS ON ELECTRICAL POWER, Issue 8 2009
Jong-Ian Tsai
Abstract The long-term effect of noncharacteristic harmonic currents arising from a slip energy recovery drive (SERD) on the fatigue life expenditure in turbine-generator blades is presented in this paper. Since the SERD converter can be rated at a fraction of a motor due to its static converter cascade with the wound-rotor and with high efficiency characteristics, still the feedwater pumps (FPs) in a few power plants are driven by such an induction motor (IM) drive. However, because the frequencies of the three main harmonic terms of the recovery currents are subsynchronous and offer a probability distribution due to the adjustable speed operation, a systematic fatigue estimation approach was devised by the author to investigate the long-term impact for the low-pressure (LP) turbine blades. From the simulation results, it was found that such a long-term harmonic excitation becomes a cause of turbine blade failure for single generator connected to the SERD system, even though the amplitude of these harmonic currents is normal. By the effect analysis of uncertainty, the countermeasure for the turbine integrity was then found. Copyright © 2008 John Wiley & Sons, Ltd. [source]

Wave propagation in nonlinear one-dimensional soil model

INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 4 2009
J. Ahn
Abstract The objective of the research conducted by the authors is to explore the feasibility of determining reliable in situ values of shear modulus as a function of strain. In this paper the meaning of the material stiffness obtained from impact and harmonic excitation tests on a surface slab is discussed. A one-dimensional discrete model with the nonlinear material stiffness is used for this purpose. When a static load is applied followed by an impact excitation, if the amplitude of the impact is very small, the measured wave velocity using the cross-correlation indicates the wave velocity calculated from the tangent modulus corresponding to the state of stress caused by the applied static load. The duration of the impact affects the magnitude of the displacement and the particle velocity but has very little effect on the estimation of the wave velocity for the magnitudes considered herein. When a harmonic excitation is applied, the cross-correlation of the time histories at different depths estimates a wave velocity close to the one calculated from the secant modulus in the stress,strain loop under steady-state condition. Copyright © 2008 John Wiley & Sons, Ltd. [source]

A finite element solution of acoustic propagation in rigid porous media

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 10 2005
A. Bermúdez
Abstract This paper deals with the acoustical behaviour of a rigid porous material. A finite element method to compute both the response to an harmonic excitation and the free vibrations of a three-dimensional finite multilayer system consisting of a free fluid and a rigid porous material is considered. The finite element used is the lowest order face element introduced by Raviart and Thomas, that eliminates the spurious or circulation modes with no physical meaning. For the porous medium a Darcy's like model and the Allard,Champoux model are taken into account. The numerical results show that the finite element method allows us to compute the response curve for the coupled system and the complex eigenfrequencies. Some of them have a small imaginary part but there are also overdamped modes. Copyright © 2004 John Wiley & Sons, Ltd. [source]

Torsional Vibration Damping Through Frictional Torsion Damper with Structural Friction and Slide Taken into Consideration

PROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2005
Zbigniew Skup Ph. DSC
The paper is concerned with a non-linear discrete stationary mechanical system containing a frictional torsion damper. Proper effect of vibration damping in a two-degree-of freedom system can be reached by the right selection of geometrical parameters for given loads, as pre-determined by a mathematical model. Structural friction was considered, as well as small relative sliding of damper's discs cooperating with a plunger. The system vibrates under harmonic excitation. The problem was considered on the assumption of uniform unit pressure distribution between the contacting surfaces of friction discs and the plunger. When the discs are sliding, the friction coefficient varies, depending on relative angular velocity. Friction characteristics were assumed on the basis of the author's own research and experimental testing by other authors. Properties of the material were assumed to be in accordance with classical theory of elasticity. The author analysed the influence of parameters of the dynamic system upon amplitude and frequency characteristics as well as on phase and frequency characteristics. The equation of motion was solved by means of the slowly-varying-parameters method and, in order to compare the results, by means of numerical simulation. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Fatigue life expenditure assessment and countermeasure for turbine blades due to harmonic excitations of slip energy recovery drives

EUROPEAN TRANSACTIONS ON ELECTRICAL POWER, Issue 8 2009
Jong-Ian Tsai
Abstract The long-term effect of noncharacteristic harmonic currents arising from a slip energy recovery drive (SERD) on the fatigue life expenditure in turbine-generator blades is presented in this paper. Since the SERD converter can be rated at a fraction of a motor due to its static converter cascade with the wound-rotor and with high efficiency characteristics, still the feedwater pumps (FPs) in a few power plants are driven by such an induction motor (IM) drive. However, because the frequencies of the three main harmonic terms of the recovery currents are subsynchronous and offer a probability distribution due to the adjustable speed operation, a systematic fatigue estimation approach was devised by the author to investigate the long-term impact for the low-pressure (LP) turbine blades. From the simulation results, it was found that such a long-term harmonic excitation becomes a cause of turbine blade failure for single generator connected to the SERD system, even though the amplitude of these harmonic currents is normal. By the effect analysis of uncertainty, the countermeasure for the turbine integrity was then found. Copyright © 2008 John Wiley & Sons, Ltd. [source]

Quasilocal vibrations and the Boson peak in glasses

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 11 2004
D. A. ParshinArticle first published online: 16 NOV 200
Abstract The problem of the low-frequency harmonic excitations and of the Boson peak in glasses is reviewed in the scope of recent theoretical developments. It is shown that the Boson peak inevitably appears in the reduced density of states g(,)/,2 of quasilocal vibrations in glasses which are additional to phonons harmonic excitations. We show that the same physical mechanism is fundamental for such seemingly different phenomenon as formation of the two-level systems in glasses. The fundamental reason for the Boson peak and two-level system formation is an instability of the spectrum of quasilocal harmonic modes weakly interacting with the high frequency surrounding and with each other. The instability controlled by the anharmonicity creates a new stable universal spectrum of harmonic vibrations with a Boson peak feature. We show that under pressure the Boson peak in glasses is always shifted to higher frequencies. For high enough pressures P the Boson peak frequency ,b , P1/3. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]