559 research outputs found
Nonlinear Hysteretic Torsional Waves
We theoretically study and experimentally report the propagation of nonlinear
hysteretic torsional pulses in a vertical granular chain made of cm-scale,
self-hanged magnetic beads. As predicted by contact mechanics, the torsional
coupling between two beads is found nonlinear hysteretic. This results in a
nonlinear pulse distortion essentially different from the distortion predicted
by classical nonlinearities, and in a complex dynamic response depending on the
history of the wave particle angular velocity. Both are consistent with the
predictions of purely hysteretic nonlinear elasticity and the
Preisach-Mayergoyz hysteresis model, providing the opportunity to study the
phenomenon of nonlinear dynamic hysteresis in the absence of other type of
material nonlinearities. The proposed configuration reveals a plethora of
interesting phenomena including giant amplitude-dependent attenuation, short
term memory as well as dispersive properties. Thus, it could find interesting
applications in nonlinear wave control devices such as strong
amplitude-dependent filters
X-ray Linear Dichroism in cubic compounds: the case of Cr3+ in MgAl2O4
The angular dependence (x-ray linear dichroism) of the Cr K pre-edge in
MgAl2O4:Cr3+ spinel is measured by means of x-ray absorption near edge
structure spectroscopy (XANES) and compared to calculations based on density
functional theory (DFT) and ligand field multiplet theory (LFM). We also
present an efficient method, based on symmetry considerations, to compute the
dichroism of the cubic crystal starting from the dichroism of a single
substitutional site. DFT shows that the electric dipole transitions do not
contribute to the features visible in the pre-edge and provides a clear vision
of the assignment of the 1s-->3d transitions. However, DFT is unable to
reproduce quantitatively the angular dependence of the pre-edge, which is, on
the other side, well reproduced by LFM calculations. The most relevant factors
determining the dichroism of Cr K pre-edge are identified as the site
distortion and 3d-3d electronic repulsion. From this combined DFT, LFM approach
is concluded that when the pre-edge features are more intense than 4 % of the
edge jump, pure quadrupole transitions cannot explain alone the origin of the
pre-edge. Finally, the shape of the dichroic signal is more sensitive than the
isotropic spectrum to the trigonal distortion of the substitutional site. This
suggests the possibility to obtain quantitative information on site distortion
from the x-ray linear dichroism by performing angular dependent measurements on
single crystals
Assessing a treatment on the basis of an individual or a group. An example: the homeopathic treatment of digestive-tract strongyles in sheep
Homeopathic treatments, widely used in organic farming, remain unevaluated. Assessment is difficult since the individuals that respond to treatment are not identified, although it is central to the concept of homeopathic treatment. Classifying lambs into those to be treated (since they have high parasitic infection rate or poor production performances) or that should remain untreated (in other words, even when treated, they will not benefit from treatment) is not simple. The identification of lambs to be treated can be based on parasitological examinations (eggs per gram of faeces), clinical (anaemia or diarrhoea)or production-related (weight gain) results. The classification of lambs was a posteriori and based on dendrograms using UPGMA (unweighted pairwise grouping on arithmetic average) and Gower’s similarity index. Parasitological, clinical and production identifiers were used for assessing the efficacy of Teucrium marum (9 CH) on digestive-tract strongyles. There was no reduction in gastro-intestinal infection in lambs with high infection rates or poor live weight gain
Inventaire des mollusques d'eau douce en Ituri (Haut-Zaïre) : conséquences sanitaires pour l'homme et le bétail
Cent soixante-dix points d'abreuvement pour le bétail ont été prospectés de juin 1986 à août 1988 en Ituri (nord-est du Zaïre) afin de déterminer les principaux mollusques d'eau douce pouvant intervenir dans la transmission des trématodoses animales et humaines. Dix-neuf espèces de gastéropodes ont été rencensées. Quatre gîtes sur dix n'hébergent pas ou très peu de mollusques. Les trois espèces les plus fréquemment rencontrées sont #Lymnaea natalensis, #Biomphalaria pfeifferi (15,3 %). La distribution des deux premières espèces semble très homogène au niveau régional et ne permet pas l'établissement d'une relation avec le risque parasitaire (#Fasciola gigantica et #Schistosoma mansoni). Inversement, la répartition de #Bulinus africanus (et secondairement celle de #B. forskalii) correspond globalement à la distribution de #Schistosoma bovis chez les bovins. L'association de #Lymnaea natalensis et de #Bulinus africanus au sein des gîtes rend compte de l'association des infestations à #Fasciola gigantica et #Schistosoma bovis$ chez les bovins. (Résumé d'auteur
Etude de l'infestation des mollusques terrestres de la région de Rabat (Maroc) par les larves de protostrongylidés dans les conditions naturelles
La dynamique de l'infestation par les protostrongylidés, chez les mollusques terrestres, a été étudiée dans la région de Rabat. L'objectif était d'établir les facteurs de risque concernant l'infestation des mollusques et, par voie de conséquence, celle des ovins. Les facteurs de risque sont les précipitations, l'espèce et l'âge des mollusques. Le degré d'infestation était maximal de décembre à février. La période à haut-risque est en novembre pour les mollusques et s'étend de décembre à février pour les ovin
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