3,958 research outputs found
Hygrothermal damage mechanisms in graphite-epoxy composites
T300/5209 and T300/5208 graphite epoxy laminates were studied experimentally and analytically in order to: (1) determine the coupling between applied stress, internal residual stress, and moisture sorption kinetics; (2) examine the microscopic damage mechanisms due to hygrothermal cycling; (3) evaluate the effect of absorbed moisture and hygrothermal cycling on inplane shear response; (4) determine the permanent loss of interfacial bond strength after moisture absorption and drying; and (5) evaluate the three dimensional stress state in laminates under a combination of hygroscopic, thermal, and mechanical loads. Specimens were conditioned to equilibrium moisture content under steady exposure to 55% or 95% RH at 70 C or 93 C. Some specimens were tested subsequent to moisture conditioning and 100 cycles between -54 C and either 70 C or 93 C
Effect of dimensionality on the charge-density-wave in few-layers 2H-NbSe
We investigate the charge density wave (CDW) instability in single and double
layers, as well as in the bulk 2H-NbSe. We demonstrate that the density
functional theory correctly describes the metallic CDW state in the bulk
2H-NbSe. We predict that both mono- and bilayer NbSe undergo a CDW
instability. However, while in the bulk the instability occurs at a momentum
, in free-standing layers it
occurs at . Furthermore, while
in the bulk the CDW leads to a metallic state, in a monolayer the ground state
becomes semimetallic, in agreement with recent experimental data. We elucidate
the key role that an enhancement of the electron-phonon matrix element at
plays in forming the CDW ground state.Comment: 4 pages 5 figure
Adiabatic and non-adiabatic phonon dispersion in a Wannier function approach
We develop a first-principles scheme to calculate adiabatic and non-adiabatic
phonon frequencies in the full Brillouin zone. The method relies on the
variational properties of a force-constants functional with respect to the
first-order perturbation of the electronic charge density and on the
localization of the deformation potential in the Wannier function basis. This
allows for calculation of phonon dispersion curves free from convergence issues
related to Brillouin zone sampling. In addition our approach justify the use of
the static screened potential in the calculation of the phonon linewidth due to
decay in electron-hole pairs. We apply the method to the calculation of the
phonon dispersion and electron-phonon coupling in MgB and CaC. In both
compounds we demonstrate the occurrence of several Kohn anomalies, absent in
previous calculations, that are manifest only after careful electron and phonon
momentum integration. In MgB, the presence of Kohn anomalies on the
E branches improves the agreement with measured phonon spectra and
affects the position of the main peak in the Eliashberg function. In CaC we
show that the non-adiabatic effects on in-plane carbon vibrations are not
localized at zone center but are sizable throughout the full Brillouin zone.
Our method opens new perspectives in large-scale first-principles calculations
of dynamical properties and electron-phonon interaction.Comment: 18 pages, 8 figure
First-principle study of excitonic self-trapping in diamond
We present a first-principles study of excitonic self-trapping in diamond.
Our calculation provides evidence for self-trapping of the 1s core exciton and
gives a coherent interpretation of recent experimental X-ray absorption and
emission data. Self-trapping does not occur in the case of a single valence
exciton. We predict, however, that self-trapping should occur in the case of a
valence biexciton. This process is accompanied by a large local relaxation of
the lattice which could be observed experimentally.Comment: 12 pages, RevTex file, 3 Postscript figure
Effect of stratification on the mixing and reaction yield in a T-shaped micro-mixer
The effect of a small density difference, i.e., lower than 12%, between the two miscible liquid streams fed to a T-shaped junction is investigated experimentally and through numerical simulations. Micron-resolution particle image velocimetry (micro-PIV) experiments provided detailed support to the numerical analysis of how stratification influences flow features in different flow regimes. From dimensional analysis, we find that gravitational and inertial fluxes balance each other at a distance L=d/Ri from the confluence along the mixing channel, where d is the hydraulic diameter and Ri is the Richardson number. In general, at distances |y|≪L, the influence of gravity can be neglected, while at |y|≫L the two fluids are fully segregated; in particular, at the confluence, the flow field is the same as the one that we obtain assuming that the two inlet fluids are identical. Thus, in the segregated regime, the contact region separating the two fluids of the inlet streams remains vertical at distances |y|≪L along the mixing channel while it becomes progressively horizontal at |y|≈L. In the vortex regime as well, near the confluence the flow field presents a mirror symmetry, with a very small resulting degree of mixing; however, as we move down the mixing channel, when |y|>L, gravity becomes relevant, leading to a symmetry breaking that promotes convection and enhances mixing. When we further increase the Reynolds number, in the engulfment regime, the degree of mixing becomes much larger due to the mixing induced by the flow instability at the confluence and thus the successive stratification appears to have a small effect on the flow topology, with a degree of mixing that continues to grow very slowly in the mixing channel, similar to what happens in the case of identical inlet fluids. As expected, the onsets of the vortex and engulfment regimes occur at values of the Reynolds number Re that hardly depend on the density difference between the two inlet fluids, provided that Re is defined in terms of the fluid properties of a homogeneous fluid mixture. Finally, the reaction yield along the mixing channel is computed both from numerical and experimental data. In agreement with theoretical predictions, we found that the reaction yield depends on the Damköhler number and the kinetic constant, while it is independent of the density ratio, at least within the range of the investigated conditions
Delivery in pregnant women infected with SARS-CoV-2: A fast review
Background: Few case reports and clinical series exist on pregnant women infected with SARS-CoV-2 who delivered. Objective: To review the available information on mode of delivery, vertical/peripartum transmission, and neonatal outcome in pregnant women infected with SARS-CoV-2. Search strategy: Combination of the following key words: COVID-19, SARS-CoV-2, and pregnancy in Embase and PubMed databases. Selection criteria: Papers reporting cases of women infected with SARS-CoV-2 who delivered. Data collection and analysis: The following was extracted: author; country; number of women; study design; gestational age at delivery; selected clinical maternal data; mode of delivery; selected neonatal outcomes. Main results: In the 13 studies included, vaginal delivery was reported in 6 cases (9.4%; 95% CI, 3.5\u201319.3). Indication for cesarean delivery was worsening of maternal conditions in 31 cases (48.4%; 95% CI, 35.8\u201361.3). Two newborns testing positive for SARS-CoV-2 by real-time RT-PCR assay were reported. In three neonates, SARS-CoV-2 IgG and IgM levels were elevated but the RT-PCR test was negative. Conclusions: The rate of vertical or peripartum transmission of SARS-CoV-2 is low, if any, for cesarean delivery; no data are available for vaginal delivery. Low frequency of spontaneous preterm birth and general favorable immediate neonatal outcome are reassuring
An Overview of Flow Features and Mixing in Micro T and Arrow Mixers
An overview of the mixing performances of micro T mixers operating with a single fluid is presented. The focus is on the relationship between the flow features and mixing. Indeed, T mixers are characterized by a variety of regimes for increasing Reynolds numbers; they are briefly described, in particular in terms of the three-dimensional vorticity field, which can explain the different mixing performances. The effects of changes in the aspect ratio of the channels are also reviewed. The role of instability and sensitivity analyses in highlighting the mechanisms of the onsets of the different regimes is then described. These analyses also suggest possible geometrical modifications to promote mixing. We focus on that consisting of the downward tilting of the inlet channels (arrow mixers). Arrow mixers are interesting because the onset of the engulfment regime is anticipated at lower Reynolds numbers. Hence, the mixing performances of arrow mixers with varying Reynolds number are described
A study on the effect of flow unsteadiness on the yield of a chemical reaction in a t micro-reactor
Despite the very simple geometry and the laminar flow, T-shaped microreactors have been found to be characterized by different and complex steady and unsteady flow regimes, depending on the Reynolds number. In particular, flow unsteadiness modifies strongly the mixing process; however, little is known on how this change may affect the yield of a chemical reaction. In the present work, experiments and 3-dimensional numerical simulations are carried out jointly to analyze mixing and reaction in a T-shaped microreactor with the ultimate goal to investigate how flow unsteadiness affects the reaction yield. The onset of the unsteady asymmetric regime enhances the reaction yield by more than 30%; however, a strong decrease of the yield back to values typical of the vortex regime is observed when the flow undergoes a transition to the unsteady symmetric regime
Influence of the inlet conditions on the degree of mixing of a t-shaped micro-mixer
Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.The degree of mixing of a T-shaped micro-mixer depends strongly on the inlet flow conditions. Specifically, through a series of numerical simulations, we compared the case where the flow at the micro-mixer confluence is fully developed with that when it is not, and found that in the former case engulfment occurs at smaller Reynolds number, with a different flow pattern and with much larger mixing efficiencies than in the latter case. In particular for fully developed flow conditions the engulfment shows S-shaped morphology, whereas for non- fully developed inlets a symmetry breaking occurs, leading to a C-shaped engulfment pattern. This is characterized by much lower mixing efficiencies.dc201
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