121,421 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
Development of a measuring technique based on JET second D-T campaign (DTE2) experience for assessing fusion power at ITER during D-T operation using the radial gamma-ray spectrometer
No full textThe ITER Radial Gamma-Ray Spectrometer (RGRS) consists of three
gamma-ray detectors observing the plasma through three collimated,
coplanar, radial lines of sight (LoS). The system was initially designed
to monitor the runaway electron emission and the alpha-particle density
profile {[}Nocente et al., Nucl. Fusion 57, 076016 (2017)]. This work
presents a novel technique for measuring the fusion power during D-T
operation using the RGRS. This method is based on the absolute
measurement of the 17 MeV fusion gamma-rays and a semi-analytical
computation of their transport from the plasma source to the detectors.
This approach was initially developed and tested at JET during the
second D-T campaign (DTE2) on a single LoS diagnostic {[}Dal Molin et
al., Phys. Rev. Lett. (submitted) (2024); Rebai et al., Phys. Rev. C
(submitted) (2024); and Marcer et al., Nucl. Fusion (unpublished)
(2024)]. This work exploits the multiple LoS of the RGRS to create a
combined virtual diagnostic whose detected fraction of the total plasma
emission is less affected by variations in the plasma emission profile,
reducing systematic uncertainties on the estimated total emission,
compared to the individual detectors
A new radiative microfurnace for X-ray single-crystal diffractometry
No full textMicrofurnaces available today for X-ray single-crystal investigation (radiative, gas-flame, gas-flow) are generally temperature-monitored by a thermocouple placed in a different site with respect to the crystal. The new F1 microfurnace aims at the following goals: fast crystal mounting; access to a large portion of reciprocal lattice; low absorption of direct beam; efficient in situ temperature measurement; fast power supply regulation; high temperature stability; controlled atmosphere. In the F1, the crystal is directly glued to a thermocouple with refractory cement. The heating body, fixed on the chi and radially shiftable along the phi axis circle, hosts the Pt winding and connections of both gas flow and power supply, and is enclosed in a thin pyrolitic boron nitride (PBN) shield. Temperature stability is within +/- 1 degreesC. Calibration of the F1 was carried out by collecting X-ray data from a single-crystal synthetic periclase at temperatures: of 28, 150, 300, 450, 600, 700, 900 and 1000 degreesC, and yielded the equation: a = 0.0000625 (7) [T(K) - 273] + 4.2083 (4), based on the ratio between cell edge and temperature. Structural refinements gave structural parameters and thermal expansion values at the investigated temperature. The mean coefficient of linear expansion up to 1000 degreesC was 14.3 x 10(-6) degreesC
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
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
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
Theoretical prediction of the Abraham hydrogen bond acidity and basicity factors from a Reaction Field method
No full textA new methodology for the theoretical evaluation of the hydrogen bond acidity Sigma alpha(2)(H) and basicity Sigma beta(2)(H) Abraham descriptors is presented. The first step is a quantum mechanical calculation at the Hartree-Fock level using a moderate basis set, including the solute-solvent interaction through a Reaction Field method, namely the Polarizable Continuum Model (PCM). The density charge on the surface of the cavity surrounding the solute, which contains the signature of the specificity of the molecule, is then translated into effective atomic charges through a suitable algorithm. These atomic charges can be related to the acidity and basicity properties of the molecule by a proper parametrization of empirical atomic factors, which account for the specific H-bonding capabilities of the individual atoms and group of atoms. The Abraham descriptors can be then evaluated with a high degree of accuracy for a large number of classes of molecules. Calculations performed for a set of 55 compounds give a standard deviation of 0.029 and 0.044 for Sigma alpha(2)(H) and Sigma beta(2)(H), respectively. The correlation coefficients are 0.994 and 0.974
CircIMPACT: An R package to explore circular RNA impact on gene expression and pathways
No full textCircular RNAs (circRNAs) are transcripts generated by back-splicing. CircRNAs might regulate cellular processes by different mechanisms, including interaction with miRNAs and RNA-binding proteins. CircRNAs are pleiotropic molecules whose dysregulation has been linked to human diseases and can drive cancer by impacting gene expression and signaling pathways. The detection of circRNAs aberrantly expressed in disease conditions calls for the investigation of their functions. Here, we propose CircIMPACT, a bioinformatics tool for the integrative analysis of circRNA and gene expression data to facilitate the identification and visualization of the genes whose expression varies according to circRNA expression changes. This tool can highlight regulatory axes potentially governed by circRNAs, which can be prioritized for further experimental study. The usefulness of CircIMPACT is exemplified by a case study analysis of bladder cancer RNA-seq data. The link between circHIPK3 and heparanase (HPSE) expression, due to the circHIPK3-miR558-HPSE regulatory axis previously determined by experimental studies on cell lines, was successfully detected
Plio-Quaternary evolution of the central Apennines: the case of the Medium-Upper Aniene River Valley (Latium, central Italy)
No full textCOOLING 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
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