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Excitation Frequency (excitation + frequency)

Distribution by Scientific Domains

Engineering	58%
Physics and Astronomy	16%

Selected Abstracts

Numerical simulation of reservoir sediment and effects on hydro-dynamic response of arch dams

EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 12 2001
Chuhan Zhang
Abstract Based on the dynamic theory for saturated porous media by Biot (Journal of the Acoustical Society of America 1956; 28: 168,178), a numerical model is presented to analyse the reflection behaviours of reservoir sediment and compared with those from the visco-elastic model. It is concluded that the two models give very similar results of reflection coefficient , within the frequency range of interest. Then, using the two models, the change of the reflection coefficients , with various sedimentation parameters and excitation frequencies are studied in detail. The results are further used in the analysis of response functions of hydro-dynamic pressures on, and structural displacements of the Xiang Hong Dian arch dam, for which some results from a field vibration test are available. It appears that effects of water compressibility with sediment reflection on hydro-dynamic pressures and structural response are not significant for this specific case. Copyright © 2001 John Wiley & Sons, Ltd. [source]

Fluorescence and Raman spectra on painting materials: reconstruction of spectra with mathematical methods

JOURNAL OF RAMAN SPECTROSCOPY, Issue 10 2006
Iacopo Osticioli
Abstract SERDS (shift excitation difference spectroscopy) and SSRS (subtracted shifted Raman spectroscopy) methods were applied for fluorescence-background rejection in the Raman spectra of colored materials. These techniques are based on the assumption that the fluorescence contribution can be completely eliminated by subtracting two Raman spectra acquired at two shifted laser excitation frequencies. For the SERDS method a micro-Raman experimental apparatus coupled with a tunable diode laser (central emission at 684 nm) was set up. SSRS measurements were made on a commercial micro-Raman instrument; in this case the shifted spectrum was obtained by moving the spectrometer grating. Raman spectra were then reconstructed by applying the difference deconvolution method that automatically converts the difference signals in Raman peaks through a deconvolution operation. These techniques were tested on two reference colors (ultramarine and 6,6,-dibromoindigotine) and two colored samples of unknown composition (a Pompeian pink powder and a blue paint from a XVII century painting). Fluorescence-background subtraction and the following operation of spectra reconstruction took place successfully with no errors in Raman peaks, width and wavenumber position. In addition, even weak spectral details were revealed favoring the comparison with reference data for a molecular identification. Copyright © 2006 John Wiley & Sons, Ltd. [source]

Stimulation of the rat somatosensory cortex at different frequencies and pulse widths

NMR IN BIOMEDICINE, Issue 1 2006
N. Van Camp
Abstract Functional MRI (fMRI) during electrical somatosensory stimulation of the rat forepaw is a widely used model to investigate the functional organization of the somatosensory cortex or to study the underlying mechanisms of the blood oxygen level-dependent (BOLD) response. In reality, somatosensory stimuli have complex timing relationships and are of long duration. However, by default electrical sensory stimulation seems to be performed at an extremely short pulse width (0.3,ms). As the pulse duration may alter the neuronal response, our aim was to investigate the influence of a much longer stimulus pulse width (10,ms) using BOLD fMRI during electrical forepaw stimulation. The optimal neuronal response was investigated by varying the stimulus frequency at a fixed pulse duration (10,ms) and amplitude (1,mA). In a parallel experiment we measured the neuronal response directly by recording the somatosensory evoked potentials (SEPs). Quantification of the BOLD data revealed a shift in the optimal response frequencies to 8,10,Hz compared with 1,Hz at 0.3,ms. The amplitude of the recorded SEPs decreased with increasing stimulation frequency and did not display any correlation with the BOLD data. Nevertheless, the summated SEPs, which are a measure of the integrated neuronal activity as a function of time, displayed a similar response profile, with a similar maximum as observed by relative BOLD changes. This shift in optimal excitation frequencies might be related to the fact that an increased pulse width of an electrical stimulus alters the nature of the stimulation, generating also sensorimotor instead of merely somatosensory input. This may influence or alter the activated pathways, resulting in a shift in the optimal response profile. Copyright © 2006 John Wiley & Sons, Ltd. [source]

A frequency domain technique for maintaining resonance condition during sine dwell vibration testing of packages

PACKAGING TECHNOLOGY AND SCIENCE, Issue 6 2000
Vincent Rouillard
Abstract When a package undergoes progressive damage during a sine dwell vertical vibration test, its dynamic characteristics, such as stiffness and damping, will change. This will result in variation of its resonance frequency during a test. This paper demonstrates that tracking the resonance by feedback control of the excitation frequency during resonance dwell tests is essential. Furthermore, this paper shows the differences in the results obtained from dwell tests with uncontrolled and controlled excitation frequencies. A computer-based resonance tracking system was developed and used together with a commercial vibration table to experimentally verify the benefits of such a technique. Copyright,©,2000 John Wiley & Sons, Ltd. [source]

TCOs for nip thin film silicon solar cells

PROGRESS IN PHOTOVOLTAICS: RESEARCH & APPLICATIONS, Issue 3 2009
T. Söderström
Abstract Substrate configuration allows for the deposition of thin film silicon (Si) solar cells on non-transparent substrates such as plastic sheets or metallic foils. In this work, we develop processes compatible with low Tg plastics. The amorphous Si (a-Si:H) and microcrystalline Si (µc-Si:H) films are deposited by plasma enhanced chemical vapour deposition, at very high excitation frequencies (VHF-PECVD). We investigate the optical behaviour of single and triple junction devices prepared with different back and front contacts. The back contact consists either of a 2D periodic grid with moderate slope, or of low pressure CVD (LP-CVD) ZnO with random pyramids of various sizes. The front contacts are either a 70,nm thick, nominally flat ITO or a rough 2,µm thick LP-CVD ZnO. We observe that, for a-Si:H, the cell performance depends critically on the combination of thin flat or thick rough front TCOs and the back contact. Indeed, for a-Si:H, a thick LP-CVD ZnO front contact provides more light trapping on the 2D periodic substrate. Then, we investigate the influence of the thick and thin TCOs in conjunction with thick absorbers (µc-Si:H). Because of the different nature of the optical systems (thick against thin absorber layer), the antireflection effect of ITO becomes more effective and the structure with the flat TCO provides as much light trapping as the rough LP-CVD ZnO. Finally, the conformality of the layers is investigated and guidelines are given to understand the effectiveness of the light trapping in devices deposited on periodic gratings. Copyright © 2008 John Wiley & Sons, Ltd. [source]

Rotational and Vibrational Temperature Measurements in a High-Pressure Cylindrical Dielectric Barrier Discharge (C-DBD)

CONTRIBUTIONS TO PLASMA PHYSICS, Issue 1 2005
N. Masoud
Abstract The rotational (TR) and vibrational (Tv) temperatures of N2 molecules were measured in a high-pressure cylindrical dielectric barrier discharge (C-DBD) source in Ne with trace amounts (0.02 %) of N2 and dry air excited by radio-frequency (rf) power. Both TR and Tv of the N2 molecules in the C 3,u state were determined from an emission spectroscopic analysis the 2nd positive system (C 3,u , B3,g). Gas temperatures were inferred from the measured rotational temperatures. As a function of pressure, the rotational temperature is essentially constant at about 360 K in the range from 200 Torr to 600 Torr (at 30W rf power) and increases slightly with increasing rf power at constant pressure. As one would expect, vibrational temperature measurements revealed significantly higher temperatures. The vibrational temperature decreases with pressure from 3030 K at 200 Torr to 2270 K at 600 Torr (at 30 W rf power). As a function of rf power, the vibrational temperature increases from 2520 K at 20 W to 2940 K at 60 W (at 400 Torr). Both TR and Tv also show a dependence on the excitation frequency at the two frequencies that we studied, 400 kHz and 13.56 MHz. Adding trace amounts of air instead of N2 to the Ne in the discharge resulted in higher TR and Tv values and in a different pressure dependence of the rotational and vibrational temperatures. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Experimental and theoretical simulations of seismic poundings between two adjacent structures

EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 4 2003
K. T. Chau
Abstract Shaking table tests have been carried out to investigate the pounding phenomenon between two steel towers of different natural frequencies and damping ratios, subject to different combinations of stand-off distance and seismic excitations. Both harmonic waves and ground motions of the 1940 El Centro earthquake are used as input. Subjected to sinusoidal excitations, poundings between the two towers could appear as either periodic or chaotic. For periodic poundings, impact normally occurs once within each excitation cycle or within every other excitation cycle. A type of periodic group poundings was also observed for the first time (i.e. a group of non-periodic poundings repeating themselves periodically). Chaotic motions develop when the difference of the natural frequency of the two towers become larger. Under sinusoidal excitations, the maximum relative impact velocity always develops at an excitation frequency between the natural frequencies of the two towers. Both analytical and numerical predictions of the relative impact velocity, the maximum stand-off distance, and the excitation frequency range for pounding occurrences were made and found to be comparable with the experimental observations in most of the cases. The stand-off distance attains a maximum when the excitation frequency is close to that of the more flexible tower. Pounding appears to amplify the response of the stiffer structure but suppress that of the more flexible structure; and this agrees qualitatively with previous shaking table tests and theoretical studies. Copyright © 2003 John Wiley & Sons, Ltd. [source]

Soil,pile,structure interaction under SH wave excitation

EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 3 2003
K. K. Koo
Abstract A continuum model for the interaction analysis of a fully coupled soil,pile,structure system under seismic excitation is presented in this paper. Only horizontal shaking induced by harmonic SH waves is considered so that the soil,pile,structure system is under anti-plane deformation. The soil mass, pile and superstructure were all considered as elastic with hysteretic damping, while geometrically both pile and structures were simplified as a beam model. Buildings of various heights in Hong Kong designed to resist wind load were analysed using the present model. It was discovered that the acceleration of the piled-structures at ground level can, in general, be larger than that of a free-field shaking of the soil site, depending on the excitation frequency. For typical piled-structures in Hong Kong, the amplification factor of shaking at the ground level does not show simple trends with the number of storeys of the superstructure, the thickness and the stiffness of soil, and the stiffness of the superstructure if number of storeys is fixed. The effect of pile stiffness on the amplification factor of shaking is, however, insignificant. Thus, simply increasing the pile size or the superstructure stiffness does not necessarily improve the seismic resistance of the soil,pile,structure system; on the contrary, it may lead to excessive amplification of shaking for the whole system. Copyright © 2003 John Wiley & Sons, Ltd. [source]

The correct form of the Raman scattering tensor

JOURNAL OF RAMAN SPECTROSCOPY, Issue 1 2004
Dr S. Hassing
Abstract In the literature about Raman scattering, an unsatisfactory ambiguity exists in the basic Kramers,Heisenberg formula. The sign in front of the damping constant ,, appears to be arbitrary. In the present paper we show that the correct form is the one where ,, has the same sign as the excitation frequency ,. The argument is based on the causality principle, leading to the requirement that the real and imaginary parts of the scattering tensor constitute a pair of Hilbert transforms. Copyright © 2004 John Wiley & Sons, Ltd. [source]

A frequency domain technique for maintaining resonance condition during sine dwell vibration testing of packages

Highly (001)-textured WS2,x films prepared by reactive radio frequency magnetron sputtering

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 14 2004
K. Ellmer
Abstract Highly (001)-oriented WS2,x films were grown onto oxidized silicon substrates by reactive magnetron sputtering from a metallic tungsten target in argon-hydrogen sulfide mixtures. The best films with respect to the van-der-Waals orientation, i.e. with the (001) planes parallel to the substrate surface, were grown by excitation of the plasma with radio frequency of 27.12 MHz. These films exhibit the largest grains and the lowest film strain. It is shown that this effect is not due to the lower deposition rate at this high excitation frequency. Instead it was found that the lower DC voltage at the sputtering target is advantageous for the film growth since the bombardment of the growing film by highly energetic particles is avoided by this type of plasma excitation. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

A Note on a Nonlinear Model of a Piezoelectric Rod

PROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2003
R. Gausmann Dipl.-Ing.
If piezoceramics are excited by weak electric fields a nonlinear behavior can be observed, if the excitation frequency is close to a resonance frequency of the system. To derive a theoretical model nonlinear constitutive equations are used, to describe the longitudinal oscillations of a slender piezoceramic rod near the first resonance frequency. Hamilton's principle is used to receive a variational principle for the piezoelectric rod. Introducing a Rayleigh Ritz ansatz with the eigenfunctions of the linearized system to approximate the exact solution leads to nonlinear ordinary differential equations. These equations are approximated with the method of harmonic balance. Finally it is possible to calculate the amplitudes of the displacements numerically. As a result it is shown, that the Duffing type nonlinearities found in measurements can be described with this model. [source]