Criterion of stability and implementation issues of hybrid frequency-time-domain procedure for non-linear dynamic analysis

A criterion of stability pertaining to the hybrid frequency-time-domain procedure is derived. It stems from the initial-value theorem on one side and from the condition of convergence which applies to the harmonic counterpart of the procedure on the other side. The criterion depends on the dynamic stiffnesses of both the original non-linear system and the pseudo-linear system evaluated at the Nyquist frequency. Since at this frequency both stiffnesses are usually controlle by the mass, the criterion of stability is satisfied for most structural systems. The procedure must, however, be applie sequentially to individual time segments. The validity of this criterion is confirmed by a numerical investigation performe for a SDOF system. The solution of non-linear dynamic problems requires consideration of implementation issues which are also discusse in the paper. The analysis of a non-linear soil-structure-interaction system in which the soil's stiffness coefficients an directly defined in the frequency domain also demonstrates the accuracy of the hybrid frequency-time-domain procedure.     

Carboxylic monoacids in the air of mayombe forest (Congo): Role of the forest as a source or sink

In the tropical rain forests of the Congo during the dry season, from June to September 1987, carboxylic acid partial pressures (P _gas) in the air above the canopy, at ground level, and at the boundary layer, were estimated from water samples such as fog and rainwater. The concentrations of these acids were also measured in the sap of tree leaves. Tree leaves act as a sink or as a source if the acid P _gas is greater of lower than the acid concentrations in molecular form in sap. For each of these soluble gases, there is a value of P _gas where the exchange is nul. This is called the compensation point. Values of the compensation point for some tree leaves were evaluated according to Henry's law. Henry's law coefficients at ppm levels were redetermined for formic (HCOOH), acetic (CH₃COOH), propionic (CH₃CH₂COOH), and isobutyric (CH₃CH(CH₃)COOH) acids.By comparison of P _gas and compensation points, it is concluded that the forest was a potential source for these acids. The soil-or the litter-acts as a significant source of a carboxylic acid of C₃ or C₄ atoms in the aliphatic chain. This carboxylic acid, not yet fully characterized, could play an important role in the rain acidity in forested zones of the equatorial areas.The direct emission of these carboxylic acids by vegetation was the main source in the boundary layer above the forest. The average emissions were 3.1×10⁹, 7.8×10⁹, and 8.4×10⁹ molecules cm^-2 s^-1 for HCOOH, CH₃COOH, and CH₃CH₂COOH, respectively. The savanna is an exogenous source of HCOOH and CH₃CH₂COOH during moderately rainy days for 30% of the time. The ozonolysis of isoprene seems to be a small source of HCOOH.     

Nonlinear least-squares method via an isomorphic geometrical setup

The resolution of a nonlinear parametric adjustment model is addressed through an isomorphic geometrical setup with tensor structure and notation, represented by a u-dimensional “model surface” embedded in a flat n-dimensional “observational space”. Then observations correspond to the observational-space coordinates of the pointQ, theu initial parameters correspond to the model-surface coordinates of the “initial” pointP, and theu adjusted parameters correspond to the model-surface coordinates of the “least-squares” point . The least-squares criterion results in a minimum-distance property implying that the vector Q must be orthogonal to the model surface. The geometrical setup leads to the solution of modified normal equations, characterized by a positive-definite matrix. The latter contains second-order and, optionally, thirdorder partial derivatives of the observables with respect to the parameters. This approach significantly shortens the convergence process as compared to the standard (linearized) method.     

Determination of Fluorine in Eighty International Geochemical Reference Samples by Proton Induced Gamma Ray Emission Spectrometry (PIGE)

Fluorine content has been determined non destructively in eighty international geochemical reference samples (GRS) including rocks, soils, sediments, minerals and ores, by proton induced gamma ray emission spectrometry (PIGE) using the nuclear reaction ¹⁹F(p,αγ) ¹⁶O. Under our operating conditions, the detection limit is around 25 ppm. Our analytical results are compared with the presently preferred values when available or with other published data. The general agreement between this work and reported values is reasonably good. The present paper includes results on several GRS for which existing data are rather scarce or even inexistant.     

Sites occupancy of Fe3+ in Garfield Nontronite: A spectroscopic study

New data on the structure of Garfield nontronite have been produced by the use of different spectroscopic techniques: Mössbauer spectroscopy, optical spectroscopy, X-ray absorption edges and EXAFS and NMR. The tetrahedral iron content is found to be no higher than 1 percent. All iron atoms belong to the octahedral sheet excluding the possibility of the presence of non crystallized phases. Some ambiguities remain about the coherence of the octahedral sheet because of the presence of two doublets in the Mössbauer spectrum and at least two lineshapes in NMR spectra of OH which correspond to different environments.     

Archosaurier-Fährtenfaunen der Trias: Die Bedeutung ihrer Ähnlichkeiten und ihrer Verschiedenheiten; ihre mögliche Verwendung im Rahmen der Stratigraphie der Trias

Zusammenfassung Die wichtigsten Merkmale der Ichnofaunen des mittleren und oberen Buntsandsteins, der Mittleren Trias (wesentlich der Fährtensandstein du Lyonnais), des Oberen Trias werden hier betrachtet. Die Verschiedenheiten beziehen sich auf ihre Zusammensetzung, die Morphologie der Eindrücke und die Häufigkeiten der Arten oder Gattungen. Für die Vergleiche wurden nicht nur charakteristische Artenverbindungen sondern auch spezielle lokale Arten benutzt. Unter der Bedingung, die Ichnofaunen sind repräsentativ, erlauben die vorhergehenden Punkte und die Kenntnis der Archosaurierextremitäten Entwicklung während der Trias, die bezügliche stratigraphische Lagen der verschiedenen Fundschichten zu präzisieren.

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The main characters of the Ichnofaunas of 1) the middle and upper Buntsandstein, 2) the Middle Triassic (essentially the Fährtensandstein du Lyonnais) and 3) the upper Triassic are here considered. The difference between these Ichnofaunas relate to their composition, the morphology of the footprints and the frequencies of the ichnogenera or the ichnospecies. For the comparisons are utilized not only ichnospecies associations but also the local or peculiar ichnospecies.On condition that these Ichnofaunas are representative, the results of the studies described above and the knowledge of the evolution of the Archosaur appendicular skeletons during the Triassic, allow to settle the relative stratigraphical positions of the different layers.

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Résumé Les caractères principaux des Ichnofaunes du Buntsandstein moyen et supérieur, du Trias moyen (essentiellement le Fährtensandstein du Lyonnais) et du Trias supérieur sont mis en évidence. Les différences entre ces Ichnofaunes portent sur leur composition, la morphologie des empreintes et les fréquences des ichnogenres ou des ichnoespèces. Pour les comparaisons sont utilisées non seulement les associations d'ichnoespèces mais également les ichnoespèces locales ou particulières.Sous la condition que ces ichnofaunes soient représentatives, les points évoqués cidessus et la connaissance de l'évolution des autopodes des Archosauriens pendant le Trias permettent d'établir les positions stratigraphiques relatives des différents gisements.

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Natural and anthropogenic C2 to C6 hydrocarbons in the central-eastern Venezuelan atmosphere during the rainy season

The levels of low molecular weight hydrocarbons were measured at pristine sites and rural locations affected by hydrocarbon emissions from oil and gas producing fields in Venezuela. At the clean sites, lower concentrations of C₂ to C₆ alkanes were observed, whereas, in comparison with remotes sites, very much higher levels were measured at the polluted sites. Alkenes present relatively high concentrations, with isoprene being the most abundant, all over the study region. The main sources of alkenes are likely to be natural, mainly from vegetation. The levels of alkanes recorded at the clean sites and the alkene levels found everywhere in the region are in agreement with the values reported for other clean sites in the tropics. The increase of ozone production capacity due to the anthropogenic emissions of alkanes from oil and gas fields was estimated. Due to the presence in the atmosphere of important amounts of naturally emitted isoprene, ethene and propene, which makes a substantial contribution to the reactivity of the hydrocarbon mixture, a small increase (<5%) was estimated to occur in the capacity of the ozone production at a regional scale during the rainy season.