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Results derived from exact linear homogeneous elastodynamic theory are used for two-dimensional unloaded plates in order to understand certain features generated by proper symmetric Lamb modes. It is shown that S1 modes for all elastic materials have a phase velocity defined below the usual critical frequency and which initially exhibits anomalous dispersion (has a negative slope with respect to frequency). Over a certain range, it has a phase velocity that is double valued. In addition, there are an infinite number of proper symmetric Lamb modes that have this characteristic for materials with a Poisson ratio equal to 1/3. It also appears that all A3n modes are anomalous when V(L) < or = 2 V(T). The cause and implication of these effects are examined, including an associated negative group velocity over a small frequency zone for these modes. Further, it is noted that all proper symmetric Lamb modes have a plateau region in phase velocity with respect to wave number. It is shown that this always occurs for a phase velocity corresponding to the longitudinal bulk velocity of the elastic material. These issues are examined along with how one may obtain material parameters and possibly plate thickness from their dispersion curves. 相似文献
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Experiments by Gittings, Bandyopadhyay and Durian (Europhys. Lett. 65, 414 (2004)) demonstrate that light possesses a higher probability to propagate in the liquid phase of a foam due to total
reflection. The authors term this observation photon channelling which we investigate in this article theoretically. We first
derive a central relation in the work of Gitting et al. without any free parameters. It links the photon's path-length fraction f in the liquid phase to the liquid fraction ɛ. We then construct two-dimensional Voronoi foams, replace the cell edges by
channels to represent the liquid films and simulate photon paths according to the laws of ray optics using transmission and
reflection coefficients from Fresnel's formulas. In an exact honeycomb foam, the photons show superdiffusive behavior. It
becomes diffusive as soon as disorder is introduced into the foams. The dependence of the diffusion constant on channel width
and refractive index is explained by a one-dimensional random-walk model. It contains a photon channelling state that is crucial
for the understanding of the numerical results. At the end, we shortly comment on the observation that photon channelling
only occurs in a finite range of ɛ. 相似文献
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MF Miller IA Franchi AS Sexton CT Pillinger 《Rapid communications in mass spectrometry : RCM》1999,13(13):1211-1217
The use of infrared laser-assisted fluorination to release oxygen from milligram quantities of silicates or other oxide mineral grains is a well-established technique. However, relatively few studies have reported the optimisation of this procedure for oxygen-17 isotope measurements. We describe here details of an analytical system using infrared (10 μm) laser-assisted fluorination, in conjunction with a dual inlet mass spectrometer of high resolving power ( approximately 250) to provide (17)O and (18)O oxygen isotope measurements from 0.5-2 mg of silicates or other oxide mineral grains. Respective precisions (1) of typically 0.08 and 0.04 per thousand are obtained for the complete analytical procedure. Departures from the mass-dependent oxygen isotope fractionation line are quantified by Delta(17)O; our precision (1) of such measurements on individual samples is shown to be +/-0.024 per thousand. In turn, this permits the offset between parallel, mass-dependent fractionation lines to be characterised to substantially greater precision than has been possible hitherto. Application of this system to investigate the (17)O versus (18)O relationship for numerous terrestrial whole-rock and mineral samples, of diverse geological origins and age, indicates that the complete data set may be described by a single, mass-dependent fractionation line of slope 0.5244+/- 0.00038 (standard error). Copyright 1999 John Wiley & Sons, Ltd. 相似文献
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CM Silva MF Duarte ML Mira MH Florêncio K Versluis AJ Heck 《Rapid communications in mass spectrometry : RCM》1999,13(12):1098-1103
Fast atom bombardment, combined with high-energy collision-induced tandem mass spectrometry, has been used to investigate gas-phase metal-ion interactions with captopril, enalaprilat and lisinopril, all angiotensin-converting enzyme inhibitors.Suggestions for the location of metal-binding sites are presented. For captopril, metal binding occurs most likely at both the sulphur and the nitrogen atom. For enalaprilat and lisinopril, binding preferably occurs at the amine nitrogen. Copyright 1999 John Wiley & Sons, Ltd. 相似文献
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The effects on the radiation response of Harwell polymethylmethacrylate (PMMA) dosimeters of dose-rate, radiation type, temperature during irradiation and post-irradiation storage, and post-irradiation stability, are of importance to the operators of commercial irradiation facilities.
This paper describes recent studies of the effects of some of these parameters on the radiation response of Harwell Red 4034, Amber 3042, and Gammachrome YR Perspex dosimeters, and provides data on batch to batch variation and shelf-life. 相似文献
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The reaction 208Pb(p,t)206Pb leading to the unnatural parity 3+ state of 206Pb is analyzed in the framework of finite range DWBA formalism. The main aspect of the present analysis is the use of a realistic triton wavefunction with S, S′ and D state components which permit this transition in DWBA. Excellent agreement between theoretical prediction and experiment is obtained casting serious doubt on the treatment of this reaction as a sequential transfer process. 相似文献
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Angular momentum selection rules for (p, t) reactions within the framework of DWBA formalism are given for the particular case where the triton wavefunction is classified according to its orbital permutation symmetry. The selection rules demonstrate that certain previous calculations of unnatural parity transitions should exactly vanish, and suggest the important result that “weak” unnatural parity transitions can occur as direct one step reactions via the mixed symmetry S and D state components of the triton wavefunction. 相似文献
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The Barshay-Temmer theorem predicts that the angular distribution of the products of the reaction 2H(α, 3He)3H will be symmetric about 90° in the c.m. system. Recently, asymmetries have been reported in the angular distribution in excess of 10 % at some angles. These asymmetries clearly violate the theorem. In an effort to explain the asymmetry as well as describe the angular distribution an exact finite-range multi-interaction DWBA calculation, which includes all appropriate one-particle transfer reaction mechanisms, was employed. Semi-realistic three-nucleon wave functions were utilized in one case in deriving the form factors appropriate for two-particle clustering of the tri-nucleon systems and various Coulomb effects associated with the interactions were included. It was found that both the asymmetry and angular distribution are well described within the present formalism. 相似文献
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