132,679 research outputs found
Connectivity effects in the segmental self- and cross-reorientation of unentangled polymer melts
No full textThe segmental (bond) rotational dynamics in a polymer melt of unentangled, linear bead-spring chains is studied by molecular dynamics simulations. To single out the connectivity effects, states with limited deviations from the Gaussian behavior of the linear displacement are considered. Both the self and the cross bond-bond correlations with rank l=1,2 are studied in detail. For l=1 the correlation functions are precisely described by expressions involving the correlation functions of the chain modes. Several approximations concerning both the self- and the cross-correlations with l=1,2 are developed and assessed. It is found that the simplified description of the excluded volume static effects derived elsewhere [D. Molin , J. Phys.: Condens. Matter 18, 7543 (2006)] well accounts for the short time cross-correlations. It also allows a proper modification of the Rouse theory which provides quantitative account of the intermediate and the long time decay of the rotational correlations with l=1
Fe2+-Mg order-disorder in orthopyroxene: equilibrium fractionation between the octahedral sites and thermodynamic analysis
No full textThe equilibrium intracrystalline distribution coefficient, k*(D), of Fe* (i.e. Fe2+ + Mn) and Mg between the M1 and M2 sites of three natural nearly binary Fe2+-Mg orthopyroxene crystals (FS14 FS15 and Fs(49)) were determined by annealing experiments at several temperatures between 550 and 1000 degrees C and single crystal X-ray structure refinements. In addition, the X-ray data of an orthopyroxene crystal (Fs(23)) which were collected earlier by Molin et al. (1991) between 700 and 1000 degrees C, were re-refined. The data were processed through two different refinement programs (SHELXL-93 and RFINE90) using both unit and individual weights and also both ionic and atomic scattering factors. The calculated site occupancies were found to agree within their estimated standard errors. However, the use of ionic scattering factors led to significantly better goodness of fit and agreement index, and smaller standard deviations of the site occupancies than those obtained from the use of atomic scattering factors. Furthermore, the weighted refinements yielded significantly smaller standard deviations of the site occupancies than the unweighted refinements even when the same set of reflections was used in the two procedures. The site occupancy data from this study were combined with selected published data to develop expressions of k*(D) as a function of temperature and composition. Calculation of the excess configurational entropy, Delta S-XS, suggests that orthopyroxene should be treated as a two parameter symmetric solution instead of as a "simple mixture" The calculated Delta S-XS values and the excess Gibbs free energy of mixing suggested by available cation exchange data lead to a slightly negative enthalpy of mixing in the orthopyroxene solid solution
Perforated fairings for landing gear noise control
No full textLanding gears of commercial aircraft make an important contribution to total aircraft noise in the approach configuration. Using fairings to shield components from high speed impingement reduces noise. Furthermore, perforating these fairings has been confirmed by flight tests to further enable noise reduction. Following a more fundamental study on the application of perforated fairings, a study has been performed to investigate and optimize the benefits of bleeding air through landing gear fairings. By means of wind tunnel tests, an aerodynamic and acoustic survey has been performed on a simplified generic main landing gear to explore the influence of (perforated) fairings on the lower part of the gear. The results show that for this specific case, the application of impermeable fairings reduces noise in the mid- and high frequency range by shielding sharp edged components from high velocity impingement. However, below 1 kHz the noise is shown to increase significantly. Application of the perforations is shown to diminish this low frequency increase whilst maintaining the reduction in the mid- and high frequency range. The aerodynamic and acoustic measurements point in the direction of the separated flow of the fairings interacting with the downstream gear components responsible for the low frequency noise increase. Bleeding of the air through the fairings reduces the large scale turbulence in the proximity of these components and hence diminishes the low frequency increase. A sharp noise increase is present in the ground view direction at the 500 Hz centred tertsband for the solid fairing, which is confirmed to originate from the articulation link fairing. The flow around the wheels remains largely unaltered by application of the fairings. Furthermore the application of fairings is shown to suppress vortex shedding from the lower part of the main leg. A study on the effectiveness of perforation location reveals that the stagnation area perforations are responsible for most of the low frequency noise decrease. Both articulation link and undertray fairing contribute to the reduction. Exposing the perforate outside the stagnation area induces perforate self-noise, most significantly on the sides of the lower articulation link fairing. Agreeing with previous studies, the phenomenon scales with the local component of the shearing flow velocity and orifice diameter
Considerations on the assessment of macroseismic intensity
No full textFollowing a brief overview on the main characteristics of MCS (1930; the most employed in Italy), MSK (1981) and EMS (1992) macroseismic scales, considerations on their validity to underline the severity of damage on buildings are reported and discussed. Observations carried out on damage related to recent Italian earthquakes show that the seismic behaviour of buildings does not fit that provided by the scales, particularly with regard to the relationship <<vulnerability class vs. building type>>
FIGURE 9a–d in The South American species of Chalcis Fabricius (Hymenoptera: Chalcididae)
No full textFIGURE 9a–d. Chalcis ornatifrons (Cameron), female: a, protarsal claw [insert: basal denticles]; b, metafemur and metatibia, outer face; c, propodeum; d, gaster, lateral.Published as part of Saguiah, Pâmella Machado, Molin, Ana Dal & Tavares, Marcelo Teixeira, 2020, The South American species of Chalcis Fabricius (Hymenoptera: Chalcididae), pp. 353-383 in Zootaxa 4885 (3) on page 367, DOI: 10.11646/zootaxa.4885.3.3, http://zenodo.org/record/429669
COOLING HISTORY OF A DYKE ON ALICUDI (AEOLIAN ISLANDS) FROM INTRACRYSTALLINE FE-2+-MG EXCHANGE-REACTION IN CLINOPYROXENE
No full textSeven rock samples were collected from the cross-section of a 2.8-m-thick andesitic dyke on Alicudi (Aeolian Islands, Sicily).
Thin section microprobe analysis of phenocrysts (pyroxenes, olivine and plagioclase) and whole-rock X-ray fluorescence show that samples at the chilled margins are sharply distinct, with the highest Mg, Cr and Ni contents.
Seven single crystals of clinopyroxene were analysed by microprobe techniques, while their structures were refined using the X-ray diffraction method; different Mg-Fe2+ ordering in the M2 and M1 sites of clinopyroxene were observed across the dyke section.
The equilibration temperature, evaluated by applying the geothermometer proposed by Molin & Zanazzi (1991), gives the highest values at the dyke margins (850-degrees-C) and the lowest values close to the middle (630-degrees-C).
By applying the theoretical model of Jaeger (1968) for the cooling of volcanic bodies, as well as the Kudo & Weill (1970) plagioclase-liquid geothermometer (modified by Mathez, 1973), it was possible to calculate the cooling rates across the dyke section: from 8 and 34-degrees-C/h near the boundaries to 1-degrees-C/h in the middle
FIGURE 10a–d in The South American species of Chalcis Fabricius (Hymenoptera: Chalcididae)
No full textFIGURE 10a–d. Chalcis ornatifrons (Cameron), male: a, habitus, lateral; b, antennal scape, outer face; c, protarsal claw [arrow: bifid apex]; d, hypopygium, ventral.Published as part of Saguiah, Pâmella Machado, Molin, Ana Dal & Tavares, Marcelo Teixeira, 2020, The South American species of Chalcis Fabricius (Hymenoptera: Chalcididae), pp. 353-383 in Zootaxa 4885 (3) on page 368, DOI: 10.11646/zootaxa.4885.3.3, http://zenodo.org/record/429669
Authigenic mica in Early Miocene volcaniclastic rocks of the Macelj area, Hrvatsko Zagorje, Croatia
No full textIn the Early Miocene zeolitized volcanicalstic rocks from NW part of Hrvatsko zagorje green authigenic mica occurs in thin veins and coatings of glass shard vesicles. X-ray powder pattern with widened but still relatively sharp reflections is characteristic for 1M micas, with Fe-rich octahedral sheet. The observed value d(060) is 1.508 Å. IR spectrum is characterized by sharp absorption bands in the OH stretching region, with two strongest bands at 3580 cm-1 and 3600 cm-1 ascribed to Al-Fe3+ and Al-Mg cationic environment of the OH groups. Microprobe analyses revealed that this is an interlayer-deficient dioctahedral mica, with Al as dominant cation in octahedral sheet, viM3+>1.2, and low tetrahedral substitution, with quite peculiar chemical composition that does not correspond ideally to any member of the mica group
Considerations on the assessment of macroseismic intensity
No full textFollowing a brief overview on the main characteristics of MCS (1930; the most employed in Italy), MSK
(1981) and EMS (1992) macroseismic scales, considerations on their validity to underline the severity of damage
on buildings are reported and discussed. Observations carried out on damage related to recent Italian
earthquakes show that the seismic behaviour of buildings does not fit that provided by the scales, particularly
with regard to the relationship >.JCR Journalope
Accurate excluded-volume corrections to the single-chain static properties of a melt of unentangled polymers
No full textActual polymer chains cannot cross themselves and each other. However, the popular Rouse model for unentangled polymers considers the chains as being like 'phantoms'. It is shown that excluded volume effects on single-chain statics may be introduced by analytic corrections to the Rouse results. The final expressions do not depend on free parameters. They exhibit excellent agreement with the molecular-dynamics simulations of polymer melts with chain lengths in the range 3 <= M <= 30. Preliminary results for entangled polymer melts are presented
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