Asymptotic behaviour in some eikonal models for production

The high energy behaviour of the total cross section is investigated in the frame of eikonal models for high-energy production processes. The dynamics is introduced by means of phenimenological Hamiltonians; the results are compared with the results of other kinds of dynamics, particularly with some procedure of unitarization of the multiperipheral model.     

Pre-resonant enhancement of the Raman scattering from KI:Tl+

E_g and T_2g spectra at room and nitrogen temperature at several laser wavelengths are reported for KI:Tl⁺. As resonance with Tl⁺ -absorption bands is approached the optic phonon parts of the spectra are enhanced relative to the acoustic phonon parts, and distortions occur within the acoustic and optic parts of the spectra. A model for the scattering is employed which involves electron-phonon coupling to nearest and nn neighbors. Exchange and crystal field contributions to the electron-phonon coupling are distinguished.     

Mixed metal complexes in solution. Part 4. Formation and stability of heterobinuclear complexes of cadmium(II)-citrate with some bivalent metal ions in aqueous solution

Summary The systems, Cd-Ni-citrate, Cd-Mn-citrate and Cd-Zn citrate have been investigated pH-metrically at 25°C and I = 0.1 mol dm^–3 (KNO₃).As previously found for analogous citrate systems (namely for Cu-Ni-, Cu-Zn- and Ni-Zn-citrate) the existence of mixed metal complexes of the type [MM(cit)₂H_–2]^4– has been shown. In addition, the species [MM(cit)₂H_–1]^3– was also found to be present for Cd-Ni- and Cd-Zn-citrate systems The significance of the formation of such species is discussed.The existence of mixed metal complexes is also discussed in connection with the transport and the absorption of metal ions in biological systems.     

The liquid-phase oxidation of 2,4-dimethylpentane

The initiated oxidation of 2, 4-dimethylpentane in the neat liquid phase at 100°C with 760 torr O₂ gives more than 90% of a mixture of 2,4-dihydroperoxy-2,4-dimethylpentane and 2-hydroperoxy-2, 4-dimethylpentane in a ratio of 7:1. The rate of oxidation depends closely on the [initiator]^1/2, consistent with a mechanism in which chain termination occurs mostly by interactions of two 2-hydroperoxy-2, 4-dimethyl-4-pentylperoxy radicals. 2, 4-Dimethylpentane oxidizes only one sixth as fast as isobutane at the same rate of initiation at 100°C. In cooxidations of the same hydrocarbons, it is 0.71 as reactive as isobutane toward any of the peroxy radicals involved. 2, 4-Dimethylpentane oxidizes 7.5 times as fast at 1.25°C as at 50°C for the same rate of initiation, but the ratio of dihydroperoxide to monohydroperoxide increases only from 5 to 7, corresponding to a difference in activation energy between intramolecular and intermolecular abstraction of 1 kcal/mole. The overall activation energy (E_p – E_t/2) is 10.7 kcal/mole, close to the value of 12 kcal/mole found for isobutane. Absolute values for E_p, E_t, k_p, k_r, and k_t were derived. Ring closure of 2-hydroperoxy-2, 4-methyl-4-pentyl radicals to oxetane, not detected during oxidation, was observed when this radical was generated at 100°C in the near-absence of oxygen. The ratio of rate constants for oxetane formation and addition of oxygen to the 2, 4dimethyl-2-hydroperoxy-4-pentyl radical is about 5.4 × 10^?5 M at 100°C. Thus, ring closure to oxetane is too slow to compete with addition of oxygen above ?200 torr. At 100°C, 2, 3-dimethylbutane gave no evidence of any intramolecular abstraction. However, 2, 3-dimethylpentane did give at least 12% 2, 4-glycol or hydroxyketone.     

Chiral recognition ability of alpha-cyclodextrin with regard to some monoterpenoids under gas-liquid chromatographic conditions

Gas-liquid chromatography was applied to investigate the mechanism of alpha-cyclodextrin (alpha-CD) complexation processes with some chiral monoterpenoids differing from each other in chemical properties and structure. They were chosen from hydrocarbons, alcohols, aldehydes and ketones of acyclic, monocyclic and bicyclic structure. The relationships between the retention factor, k, of a guest solute (G) and alpha-CD concentration were studied. The obtained data enabled the stoichiometry, the stability of individual complexes and the separation factor of enantiomers to be determined. It was found that almost all the investigated monoterpenoids, apart from the acyclic ones, form inclusion complexes with alpha-CD. Straight-line relations (r vs. [alpha-CD]) were observed for monocyclic alcohols and pulegone, without any trace of enantioselectivity. This behaviour indicates that the 1:1 stoichiometry of the G-CD complexes does not lead to chiral recognition. Parabolic relations arising from 1:2 stoichiometry were found for limonene, alpha-phellandrene, some monocycylic ketones and all the investigated bicyclic terpenoids. It appeared that only the second step of complexation displayed marked enantioselectivity. However, a loss of efficiency resulting from slower equilibration is then noticeable. Attempts are made to rationalize the chromatographic results with respect to the structure of the investigated compounds.     

On the forces transmitted to a vibrating cylinder by a blowing fluid

Summary The measuring apparatus used for experimental wind tunnel studies is described together with the test procedure and salient results. After an analysis of the results, a mechanical model is developed to explain the nature of the phenomenon.

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Sommario Si descrive l'apparato di misura utilizzato per le ricerche sperimentali eseguite in camera a vento al fine di determinare le forze esercitate da una vena fluida su un cilindro libero di vibrare. Si espongono le modalità di prova ed i risultati più salienti. Dopo un'analisi dei risultati si elabora un modello meccanico per interpretare la natura del fenomeno.

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This study forms part of a project that has been carried out jointly by A. Salvi & C. SpA and the Institute of Applied Mechanics, Milan Polytechnic. The Institute of Applied Mechanics received a CNR subsidy to take part in this project.     

Block triangular preconditioners for linearization schemes of the Rayleigh–Bénard convection problem

In this paper, we compare two block triangular preconditioners for different linearizations of the Rayleigh–Bénard convection problem discretized with finite element methods. The two preconditioners differ in the nested or nonnested use of a certain approximation of the Schur complement associated to the Navier–Stokes block. First, bounds on the generalized eigenvalues are obtained for the preconditioned systems linearized with both Picard and Newton methods. Then, the performance of the proposed preconditioners is studied in terms of computational time. This investigation reveals some inconsistencies in the literature that are hereby discussed. We observe that the nonnested preconditioner works best both for the Picard and for the Newton cases. Therefore, we further investigate its performance by extending its application to a mixed Picard–Newton scheme. Numerical results of two‐ and three‐dimensional cases show that the convergence is robust with respect to the mesh size. We also give a characterization of the performance of the various preconditioned linearization schemes in terms of the Rayleigh number.     

On linear versus nonlinear flow rules in strain localization analysis

Summary This note contains some remarks on the analysis of bifurcation phenomena, specifically strain localization (onset of a strain rate discontinuity), in small-deformation elastoplasticity. Nonassociative flow rules are allowed for to cover constitutive models frequently adopted for frictional (and softening) materials such as concrete. The conventional derivation of the localization criterion resting on an incrementally linear comparison material is critically reviewed and compared to the criterion resulting from actual nonlinear plastic flow laws.

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Sommario Si svolgono alcune considerazioni sui fenomeni di biforcazione in solidi elastoplastici in regime di piccole deformazioni (di linearità geometrica) e precisamente sul manifestarsi di localizzazioni intese come discontinuità nel campo delle deformazioni incrementali. Si considerano leggi nonassociate. Vengono così inclusi nella trattazione modelli costitutivi frequentemente adottati per descrivere il comportamento di materiali ad attrito interno e soggetti a danneggiamento (nel senso di degrado di rigidezze elastiche in seguito a deformazioni anelastiche), oltre che soggetti a manifestazioni di instabilità per incrudimento negativo (softening). Si esamina criticamente il criterio, frequentemente adottato in letteratura, di localizzazione fondato su materiale di confronto incrementalmente lineare e si fornisce, corredato da osservazioni comparative, il criterio che risulta dall'assunzione del modello di materiale incrementale non lineare