2,199 research outputs found
Laser induced Zero-Group Velocity resonances in Transversely Isotropic cylinder
The transient response of an elastic cylinder to a laser impact is studied.
When the laser source is a line perpendicular to the cylinder axis, modes
guided along the cylinder are generated. For a millimetric steel cylinder up to
ten narrow resonances can be locally detected by laser interferometry below 8
MHz. Most of these resonances correspond to Zero-Group Velocity guided modes
while a few others can be ascribed to thickness modes. We observe that the
theory describing the propagation of elastic waves in an isotropic cylinder is
not sufficient to precisely predict the resonance spectrum. In fact, the
texture of such elongated structure manifest as elastic anisotropy. Thus, a
transverse isotropic (TI) model is used to calculate the dispersion curves and
compare them with the measured one, obtained by moving the source along the
cylinder. The five elastic constants of a TI cylinder are adjusted leading to a
good agreement between measured and theoretical dispersion curves. Then, all
the resonance frequencies are satisfactorily identified.Comment: 23 pages, 7 figures, submitted to the JAS
Laser beam shaping for enhanced Zero-Group Velocity Lamb modes generation
Optimization of Lamb modes induced by laser can be achieved by adjusting the
spatial source distribution to the mode wavelength (). The
excitability of Zero-Group Velocity (ZGV) resonances in isotropic plates is
investigated both theoretically and experimentally for axially symmetric
sources. Optimal parameters and amplitude gains are derived analytically for
spot and annular sources of either Gaussian or rectangular energy profiles. For
a Gaussian spot source, the optimal radius is found to be .
Annular sources increase the amplitude by at least a factor of 3 compared to
the optimal Gaussian source. Rectangular energy profiles provide higher gain
than Gaussian ones. These predictions are confirmed by semi-analytical
simulation of the thermoelastic generation of Lamb waves, including the effect
of material attenuation. Experimentally, Gaussian ring sources of controlled
width and radius are produced with an axicon-lens system. Measured optimal
geometric parameters obtained for Gaussian and annular beams are in good
agreement with theoretical predictions. A ZGV resonance amplification factor of
2.1 is obtained with the Gaussian ring. Such source should facilitate the
inspection of highly attenuating plates made of low ablation threshold
materials like composites.Comment: 11 pages, 12 figure
Non-adiabatic oscillations of fast-rotating stars: the example of Rasalhague
Early-type stars generally tend to be fast rotators. In these stars, mode
identification is very challenging as the effects of rotation are not well
known. We consider here the example of Ophiuchi, for which dozens of
oscillation frequencies have been measured. We model the star using the
two-dimensional structure code ESTER, and we compute both adiabatic and
non-adiabatic oscillations using the TOP code. Both calculations yield very
complex spectra, and we used various diagnostic tools to try and identify the
observed pulsations. While we have not reached a satisfactory mode-to-mode
identification, this paper presents promising early results.Comment: 4 pages, 3 figures. SF2A 2017 proceeding
Characterization of a calcium phospho-silicated apatite with iron oxide inclusions
An iron oxide containing calcium phosphate–silicate hydroxyapatite was synthesized by calcination at 900 °C of a sample obtained by precipitation in basic aqueous solution of Ca, P, Si, Fe and Mg containing acidic solution made from dissolution of natural minerals. XRD and FTIR were used for crystallographic characterization of the main apatitic phase. Its composition was determined using ICP-AES. EDX coupled with SEM and TEM evidenced the heterogeneity of this compound and the existence of iron–magnesium oxide. Magnetic analyses highlighted that this phase was non-stoichiometric magnesioferrite (Mg1.2Fe1.8O3.9) spherical nanoparticles. Those analyses also put into evidence the role of calcination in synthesis. Carbonates detected by FTIR and estimated by SEM-EDX in non-calcinated sample were removed from apatitic structure, and crystallization of apatite was enhanced during heating. Moreover, there was phase segregation that led to magnesioferrite formation
Système de prédiction des pays vulnérables aux conséquences de la fluctuation de la production du maïs américain dû à un aléa climatique
Au travers de cette étude, nous proposons de mettre en place un système d’information géographique (SIG) prédictif afin d’obtenir le classement des pays vulnérables à la brusque variation des prix du maïs. L'élément déclencheur de cette variation sera un aléa climatique sur la zone de culture américaine, premier pays producteur. Afin d'analyser les conséquences d'une catastrophe naturelle, il fut nécessaire de construire un indice final de vulnérabilité par le regroupement de plusieurs thématiques (agricole, économique et socio politique). Les données des différentes thématiques ainsi que les résultats sont intégrés dans une base de données à référence spatiale pour permettre la restitution des résultats dans un SIG web
Blind identification of Ambisonic reduced room impulse response
Recently proposed Generalized Time-domain Velocity Vector (GTVV) is a
generalization of relative room impulse response in spherical harmonic (aka
Ambisonic) domain that allows for blind estimation of early-echo parameters:
the directions and relative delays of individual reflections. However, the
derived closed-form expression of GTVV mandates few assumptions to hold, most
important being that the impulse response of the reference signal needs to be a
minimum-phase filter. In practice, the reference is obtained by spatial
filtering towards the Direction-of-Arrival of the source, and the
aforementioned condition is bounded by the performance of the applied
beamformer (and thus, by the Ambisonic array order). In the present work, we
suggest to circumvent this problem by properly modelling the GTVV time series,
which permits not only to relax the initial assumptions, but also to extract
the information therein is a more consistent and efficient manner, entering the
realm of blind system identification. Experiments using measured room impulse
responses confirm the effectiveness of the proposed approach.Comment: Submitte
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