130,420 research outputs found
A Note on Positive Solutions for Conservation Laws with Singular Source
No full textWe consider the Cauchy problem for the scalar conservation law
∂t u + ∂x f (u) = 1/g(u) , t > 0, x ∈ R,
with g ∈ C^1 (R), g(0) = 0, g(u) > 0 for u > 0, and assume that the initial datum u0 is nonnegative.
We show the existence of entropy solutions that are positive a.e., by means of an approximation of the equation that preserves positive solutions, and by passing to the limit using a monotonicity argument. The difficulty lies in handling the singularity of the right hand side (the source term) as
u possibly vanishes at the initial time. The source term is shown to be locally integrable. Moreover, we prove an uniqueness and stability result for the above equation
A Lattice Boltzmann dynamic-Immersed Boundary scheme for the transport of deformable inertial capsules in low-Re flows
No full textIn this work, a dynamic-Immersed–Boundary method combined with a BGK-Lattice–Boltzmann technique is developed and critically discussed. The fluid evolution is obtained on a three-dimensional lattice with 19 reticular velocities (D3Q19 computational molecule) while the immersed body surface is modeled as a collection of Lagrangian points responding to an elastic potential and a bending resistance. A moving least squares reconstruction is used to accurately interpolate flow quantities and the forcing field needed to enforce the boundary condition on immersed bodies. The proposed model is widely validated against well known benchmark data for rigid and deformable objects. Rigid transport is validated by computing the settling of a sphere under gravity for five different conditions. Then, the tumbling of inertial particles with different shape is considered, recovering the Jefferey orbit for a prolate spheroid. Moreover, the revolution period for an oblate spheroid and for a disk-like particle is obtained as a function of the Reynolds number. The existence of a critical Reynolds number is demonstrated for both cases above which revolution is inhibited. The transport of deformable objects is also considered. The steady deformation of a membrane under shear for three different values of the mechanical stiffness is assessed. Then, the tumbling of a weakly-deformable spheroid under shear is systematically analyzed as a function strain stiffness, bending resistance and membrane mass
Vapor phase infiltration of zinc oxide into thin films of: Cis -polyisoprene rubber
No full textElastomers are an important class of polymers for many applications. Often, additives are added to the polymer matrix of elastomers to promote vulcanization or enhance physical or chemical properties. In this study, vapor phase infiltration (VPI) is investigated for transforming unvulcanized cis-polyisoprene (from natural rubber) into an organic/inorganic hybrid material. Specifically, we examine single-cycle infiltration with diethylzinc (DEZ) and water to form infiltrated zinc oxide species. Interestingly, low-temperature pre-heating of the cis-polyisoprene acutely affects the processes of infiltration, including diffusivity, maximum solubility, and chemical reactivity. We attribute these effects to a combination of film relaxation and oxidation. Independent of thermal pre-treatments, all infiltration processes exhibited consistent zinc oxide loading irrespective of purge time between the DEZ and water doses, indicating the presence of a strongly bound intermediate state between the DEZ precursor and the cis-polyisoprene polymer. Increasing infiltration process temperature accelerates diffusion and lowers the maximum solubility, in accordance with Fick's law and gas phase sorption equilibrium. Resulting organic-inorganic hybrid films show enhanced resistance to dissolution in toluene, a good solvent for the pure polymer
LES of the Sandia Flame D Using an FPV Combustion Model
No full textAbstractThe simulation of turbulent combustion phenomena is still an open problem in modern fluid dynamics. Considering the economical importance of hydrocarbon combustion in energy production processes, it is evident the need of an accurate tool with a relatively low computational cost for the prediction of this kind of reacting flows. In the present work, a comparative study is carried out among large eddy simulations, performed with various grid resolutions, a Reynolds averaged Navier-Stokes simulation, and experimental data concerning the well-known Sandia D flame test case. In all the simulations, a flamelet progress variable model has been employed using various hypotheses for the joint probability density function closure. The filtered approach proved to be more accurate than the averaged one, even for the coarser grid used in this work. In fact both approaches have shown poorly accurate predictions in the first part of the combustion chamber, but only by the large eddy simulation one is capable to recover the inlet discrepancies with respect to the experimental data going along the streamwise direction
Conservation Laws with Time Dependent Discontinuous Coefficients
No full textWe consider scalar conservation laws where the flux function depends discontinuously
on both the spatial and temporal locations. Our main results are the existence and well-posedness of
an entropy solution to the Cauchy problem. The existence is established by showing that a sequence
of front tracking approximations is compact in L1 , and that the limits are entropy solutions. Then,
using the definition of an entropy solution taken from [K. H. Karlsen, N. H. Risebro, and J. D.
Towers, Skr. K. Nor. Vidensk. Selsk., 3 (2003), pp. 1–49], we show that the solution operator is L1
contractive. These results generalize the corresponding results from [S. N. Kruˇzkov, Math. USSR-Sb.,
10 (1970), pp. 217–243] and also partially those from Karlsen, Risebro, and Towers
MeSH term explosion and author rank improve expert recommendations
No full textInformation overload is an often-cited phenomenon that reduces the productivity, efficiency and efficacy of scientists. One challenge for scientists is to find appropriate collaborators in their research. The literature describes various solutions to the problem of expertise location, but most current approaches do not appear to be very suitable for expert recommendations in biomedical research. In this study, we present the development and initial evaluation of a vector space model-based algorithm to calculate researcher similarity using four inputs: 1) MeSH terms of publications; 2) MeSH terms and author rank; 3) exploded MeSH terms; and 4) exploded MeSH terms and author rank. We developed and evaluated the algorithm using a data set of 17,525 authors and their 22,542 papers. On average, our algorithms correctly predicted 2.5 of the top 5/10 coauthors of individual scientists. Exploded MeSH and author rank outperformed all other algorithms in accuracy, followed closely by MeSH and author rank. Our results show that the accuracy of MeSH term-based matching can be enhanced with other metadata such as author rank
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Chemical vapor deposition of carbohydrate-based polymers: a proof of concept study
No full textThe aim of this work is to investigate if vinyl-modified carbohydrate compounds are suitable monomers for thin film polymerization via chemical vapor deposition in a proof-of-concept study. Synthetic carbohydrate-based polymers are explored as biodegradable, biocompatible, and biorenewable materials. A thin film of synthetic polymers bearing sugar residues can also offer a good surface for cell attachment, and thus might be applied in biomaterials and tissue engineering. The possibility of having such thin film deposited from the vapor phase would ease the implementation in complex device architectures. For a proof-of-concept study, sugar vinyl compound monomers are synthesized starting from methyl α-d-glucopyranoside and polymerized by initiated chemical vapor deposition (iCVD) leading to a thin polymer layer on a Si-substrate. Thus, a successful vapor polymerization of the sugar compounds could be demonstrated. Infrared spectroscopy shows that no unwanted crosslinking reactions take place during the vapor deposition. The solubility of the polymers in water was observed in situ by spectroscopic ellipsometry. Graphical abstract: [Figure not available: see fulltext.]
A fluid-structure interaction method for soft particle transport in curved microchannels
No full textA numerical framework is presented to predict the transport of soft elastic capsules immersed in an incompressible fluid with the aim of simulating inertial microfluidics applications. The flow evolution is modeled by a fully incompressible lattice Boltzmann method whereas a finite element model is considered for describing the dynamics of deformable structures. An immersed boundary technique is adopted to reconstruct the solution in the vicinity of the immersed surface and the time integration of the fluid-structure interaction problem is obtained by following an explicit procedure. The effectiveness of the framework is validated by means of several test cases involving: flows between two parallel plates for Reynolds numbers in the range 1÷100; capsules immersed in a fluid at Stokes regime exhibiting small and large deformations under shear; capsules migrating in a long straight microchannel with square cross-section at low-to-moderate Reynolds number. A very good agreement between the present results and literature data obtained using different numerical methods is found for all test cases. Finally, the method allowed to accurately simulate incompressible flows subjected to large pressure difference between the inlet and the outlet, characteristic of long curved microchannels, obtaining very smooth particle dynamics
Triage protocol for allocation of critical health resources during the COVID-19 health emergency. A review
No full textBackground and aim of the work:Triage during the Covid-19 pandemic can impose difficult allocation deci-sions when demand for mechanical ventilation or intensive care beds greatly exceeds available resources. Tri-age criteria should be objective, ethical, transparent, applied equitably and publically disclosed. The aim of this review is to describe the triage tools and process for critical care resources in a pandemic health emergency. Methods:A narrative review was conducted of the literature on five electronic databases, namely PubMed, CINHAL, Web of Science, Cochrane and Embase, searching for studies published from 2006 to June 2020. Results:The results describe different triage tools. A gold standard of triage does not exist for the adult or paediatric population. Using probability of short-term survival as the sole allocation principle is problematic. In general, each triage protocol should be applied with a specific ethical justification, including transparency, duty to care, duty to steward resources, duty to plan, and distributive justice. Conclusions:Clinical triage deci-sions based on clinical judgment alone are prone to inconsistent application by triage officers in a pandemic. An ethical framework can inform decision-making and improve accountability. It remains difficult to connect clinical criteria and ethical criteria, because of the models on offer for health services
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