Página 1 dos resultados de 258 itens digitais encontrados em 0.006 segundos

- AMER SOC AGRICULTURAL & BIOLOGICAL ENGINEERS
- ELSEVIER SCI LTD
- Academy of Dental Materials; Washington
- Biblioteca Digitais de Teses e Dissertações da USP
- Fundação Odontológica de Ribeirão Preto
- American Physiological Society
- Massachusetts Institute of Technology
- Université de Montréal
- Amer Physiological Soc
- Univ Zagreb, Fac Kinesiology
- Journal of Manual & Manipulative Therapy, Inc.
- Brazilian Society of Chemical Engineering
- Elsevier
- Universidade Cornell
- Mais Publicadores...

## LOCAL HEAD LOSS OF NON-COAXIAL EMITTERS INSERTED IN POLYETHYLENE PIPE

Fonte: AMER SOC AGRICULTURAL & BIOLOGICAL ENGINEERS
Publicador: AMER SOC AGRICULTURAL & BIOLOGICAL ENGINEERS

Tipo: Artigo de Revista Científica

ENG

Relevância na Pesquisa

46.11%

#Contraction coefficient#Emitter#Head loss#Hydraulic radius#DRIP IRRIGATION LATERALS#DIAMETER PLASTIC PIPES#FRICTION FACTORS#PRESSURE LOSSES#FLOW#EQUATION#DESIGN

The design of a lateral line for drip irrigation requires accurate evaluation of head losses in not only the pipe but in the emitters as well. A procedure was developed to determine localized head losses within the emitters by the formulation of a mathematical model that accounts for the obstruction caused by the insertion point. These localized losses can be significant when compared with tire total head losses within the system due to the large number of emitters typically installed along the lateral line. Air experiment was carried out by altering flow characteristics to create Reynolds numbers (R) from 7,480 to 32,597 to provide turbulent flow and a maximum velocity of 2.0 m s(-1). The geometry of the emitter was determined by an optical projector and sensor An equation was formulated to facilitate the localized head loss calculation using the geometric characteristics of the emitter (emitter length, obstruction ratio, and contraction coefficient). The mathematical model was tested using laboratory measurements on four emitters. The local head loss was accurately estimated for the Uniram (difference of +13.6%) and Drip Net (difference of +7.7%) emitters, while appreciable deviations were found for the Twin Plus (-21.8%) and Tiran (+50%) emitters. The head loss estimated by the model was sensitive to the variations in the obstruction area of the emitter However...

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## Influence of curing protocol on selected properties of light-curing polymers: Degree of conversion, volume contraction, elastic modulus, and glass transition temperature

Fonte: ELSEVIER SCI LTD
Publicador: ELSEVIER SCI LTD

Tipo: Artigo de Revista Científica

ENG

Relevância na Pesquisa

36.05%

#Light-curing#Soft-start#Degree of conversion#Volume contraction#Elastic modulus#Glass transition temperature#Dental materials#Dental polymers#Resin composite#CROSS-LINK DENSITY#RESIN-COMPOSITE

Objectives. The purpose of this study was to investigate the effect of light-curing protocol on degree of conversion (DC), volume contraction (C), elastic modulus (E), and glass transition temperature (T(g)) as measured on a model polymer. It was a further aim to correlate the measured values with each other. Methods. Different light-curing protocols were used in order to investigate the influence of energy density (ED), power density (PD), and mode of cure on the properties. The modes of cure were continuous, pulse-delay, and stepped irradiation. DC was measured by Raman micro-spectroscopy. C was determined by pycnometry and a density column. E was measured by a dynamic mechanical analyzer (DMA), and T(g) was measured by differential scanning calorimetry (DSC). Data were submitted to two-and three-way ANOVA, and linear regression analyses. Results. ED, PD, and mode of cure influenced DC, C, E, and T(g) of the polymer. A significant positive correlation was found between ED and DC (r = 0.58), ED and E (r = 0.51), and ED and T(g) (r = 0.44). Taken together, ED and PD were significantly related to DC and E. The regression coefficient was positive for ED and negative for PD. Significant positive correlations were detected between DC and C (r = 0.54)...

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## Residual stresses in Y-TZP crowns due to changes in the thermal contraction coefficient of veneers

Fonte: Academy of Dental Materials; Washington
Publicador: Academy of Dental Materials; Washington

Tipo: Artigo de Revista Científica

ENG

Relevância na Pesquisa

46.27%

#Finite element analysis#Veneered zirconia crowns#Coefficient of thermal contraction#Veneer chipping#Glass transition temperature#Materiais Dentarios

Objective. To test the hypothesis that the difference in the coefficient of thermal contraction
of the veneering porcelain above (˛liquid) and below (˛solid) its Tg plays an important role in
stress development during a fast cooling protocol of Y-TZP crowns.
Methods. Three-dimensional finite element models of veneered Y-TZP crowns were developed.
Heat transfer analyses were conducted with two cooling protocols: slow (group A) and
fast (groups B–F). Calculated temperatures as a function of time were used to determine
the thermal stresses. Porcelain ˛solid was kept constant while its ˛liquid was varied, creating
different ˛/˛solid conditions: 0, 1, 1.5, 2 and 3 (groups B–F, respectively). Maximum ( 1) and
minimum ( 3) residual principal stress distributions in the porcelain layer were compared.
Results. For the slowly cooled crown, positive 1 were observed in the porcelain, orientated
perpendicular to the core–veneer interface (“radial” orientation). Simultaneously, negative
3 were observed within the porcelain, mostly in a hoop orientation (“hoop–arch”). For
rapidly cooled crowns, stress patterns varied depending on ˛/˛solid ratios. For groups B and
C, the patterns were similar to those found in group A for 1 (“radial”) and 3 (“hoop–arch”).
For groups D–F...

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## Metodologia para determinação da perda de carga localizada em emissores não coaxiais integrados a tubos de polietileno; Methodology for Localized head loss determination of non coaxial emitters inserted in polyethylene pipe

Fonte: Biblioteca Digitais de Teses e Dissertações da USP
Publicador: Biblioteca Digitais de Teses e Dissertações da USP

Tipo: Dissertação de Mestrado
Formato: application/pdf

Publicado em 25/02/2008
PT

Relevância na Pesquisa

46.11%

#Contraction coefficient#Emitter#Head loss#Hydraulic radius.#Irrigação localizada#Modelos matemáticos#Perda de carga.

O procedimento de dimensionamento de uma linha lateral de irrigação localizada necessita ser avaliado com precisão, devido às perdas de carga tanto distribuídas na tubulação como nas inserções dos emissores com os tubos. Devido a isso, desenvolveu-se uma metodologia para a determinação da perda de carga localizada mediante a formulação de um modelo matemático a partir do índice de obstrução. Estas perdas localizadas podem ser significativas quando comparadas com as perdas de carga totais, devido ao grande número de emissores instalados ao longo da linha lateral. O experimento foi junto ao Departamento de Engenharia Rural (ESALQ-USP) e ajustado para valores número de Reynolds (R), no intervalo de 7.480 a 32.597 para proporcionar fluxo turbulento e velocidade máxima de 2,0 m s-1. A pesquisa foi conduzida em duas etapas: 1) ajuste do fator f para um seguimento de 0,5 m de tubo,sendo realizada 10 repetições, com o intuito de validação da metodologia empregada comparando-se o ajuste dos dados encontrados com os trabalhos existentes na literatura, 2) determinação da perda de carga (hf) em um seguimento de tubo de 1 m de comprimento com emissor devidamente vedado, portanto, pela diferença entre a hf do tubo mais o emissor e a hf do tubo obtido na primeira etapa...

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## Evaluation of the coefficient of thermal expansion of human and bovine dentin by thermomechanical analysis

Fonte: Fundação Odontológica de Ribeirão Preto
Publicador: Fundação Odontológica de Ribeirão Preto

Tipo: Artigo de Revista Científica
Formato: text/html

Publicado em 01/01/2012
EN

Relevância na Pesquisa

36.18%

#dentin#thermal diffusion#physical properties#coefficient of thermal expansion#bovine teeth#human teeth

The mismatch of thermal expansion and contraction between restorative materials and tooth may cause stresses at their interface, which may lead to microleakage. The present work compared the coefficient of thermal expansion (CTE) with the thermomechanical behavior of human and bovine teeth and determined if the CTE is a suitable parameter to describe tooth behavior. Fifteen human third molar and 15 bovine incisor tooth slices (6×5×2 mm) were allocated to 3 groups according to the test environment: G1 - room condition, G2 - 100% humidity, G3 - desiccated and tested in dry condition. Each specimen was weighed, heated from 20 to 70ºC at 10ºC min−1 and reweighed. The CTE was measured between 20 and 50ºC. Fresh dentin (human -0.49% ± 0.27, bovine -0.22% ± 0.16) contracted on heating under dry condition. Under wet conditions, only human teeth (-0.05% ± 0.04) showed contraction (bovine 0.00% ± 0.03) accompanied by a significantly lower (p<0.05) weight loss than in dry specimens (human 0.35% ± 0.15, bovine 0.45% ± 0.20). The desiccated dentin expanded on heating without obvious weight changes (0.00% ± 0.00). The CTE found was, respectively, in dry, wet and dissected conditions in ºC-1: human (-66.03×10-6, -6.82×10-6, 5.52×10-6) and bovine (-33.71×10-6...

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## Endothelin-1 stimulates contraction of rat glomerular mesangial cells and potentiates beta-adrenergic-mediated cyclic adenosine monophosphate accumulation.

Fonte: PubMed
Publicador: PubMed

Tipo: Artigo de Revista Científica

Publicado em /03/1990
EN

Relevância na Pesquisa

26.17%

The newly isolated peptide, endothelin-1 (ET-1), is a potent pressor agent that reduces GFR and the glomerular ultrafiltration coefficient. Recent evidence demonstrates that ET-1 mobilizes intracellular Ca2+ [( Ca2+]i) in glomerular mesangial cells by activating the phosphoinositide cascade. The present experiments were designed to examine whether ET-1 stimulates mesangial cell contraction and regulates the synthesis of PGE2 and cAMP, which dampen vasoconstrictor-induced mesangial contraction. ET-1 (greater than or equal to 1 nM) reduced the cross-sectional area of rat mesangial cells cultured on three-dimensional gels of collagen type I. ET-1 also caused complex rearrangements of F-actin microfilaments consistent with a motile response. Contraction in response to ET-1 occurred only at concentrations that activate phospholipase C, and contraction was unaffected by blockade of dihydropyridine-sensitive Ca2+ channels. Elevation of [Ca2+]i with ionomycin, to equivalent concentrations of [Ca2+]i achieved with ET-1, also reduced mesangial cell cross-sectional area. ET-1 (0.1 microM) also evoked [3H]arachidonate release and a fivefold increase in PGE2 synthesis as well as increased synthesis of PGF2 alpha and small changes of TXB2. ET-1 caused a minor increase in intracellular cAMP accumulation only in the presence of 3-isobutyl-1-methylxanthine. ET-1 also amplified cAMP production in response to isoproterenol. TPA and ionomycin...

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## Histamine modulates contraction and cyclic nucleotides in cultured rat mesangial cells. Differential effects mediated by histamine H1 and H2 receptors.

Fonte: PubMed
Publicador: PubMed

Tipo: Artigo de Revista Científica

Publicado em /05/1985
EN

Relevância na Pesquisa

26.33%

Histamine influences the glomerular microcirculation and modulates immune-inflammatory responses. In the rat kidney, histamine is synthesized by glomeruli and stimulates cyclic nucleotide production specifically in glomeruli. We investigated the in vitro effect of histamine on cyclic nucleotide accumulation in rat cultured glomerular mesangial and epithelial cells. Histamine stimulated cyclic AMP (cAMP) accumulation in cultured mesangial cells (64.0 +/- 22.1 to 511.4 +/- 86.6 pmol/mg protein, n = 9) but had no effect on cAMP accumulation in epithelial cells. This effect was dose-dependent and time-dependent. Stimulation of cAMP accumulation occurred in the range of 5 X 10(-6) M-10(-4) M histamine with a half maximal stimulatory effect of 2 X 10(-5) M. Initial stimulation was noted by 30 s, and maximum stimulation was observed at 5 min. The H2 antagonist cimetidine (10(-4) M) abolished the stimulatory effect of histamine (10(-4) M), while equimolar concentrations of the H1 antagonist diphenhydramine had no significant effect on cAMP accumulation. Moreover, the specific H2 agonist dimaprit, but not the H1 agonist 2-pyridylethylamine, stimulated cAMP accumulation. Histamine had no effect on cAMP accumulation in epithelial cells or on cyclic guanosine monophosphate accumulation in epithelial or mesangial cells. Since the in vivo infusion of histamine reduces ultrafiltration coefficient and since mesangial cell contraction is thought to be responsible for the reduction in the ultrafiltration coefficient...

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## The effects of ADP and phosphate on the contraction of muscle fibers.

Fonte: PubMed
Publicador: PubMed

Tipo: Artigo de Revista Científica

Publicado em /11/1985
EN

Relevância na Pesquisa

26.14%

The products of MgATP hydrolysis bind to the nucleotide site of myosin and thus may be expected to inhibit the contraction of muscle fibers. We measured the effects of phosphate and MgADP on the isometric tensions and isotonic contraction velocities of glycerinated rabbit psoas muscle at 10 degrees C. Addition of phosphate decreased isometric force but did not affect the maximum velocity of shortening. To characterize the effects of ADP on fiber contractions, force-velocity curves were measured for fibers bathed in media containing various concentrations of MgATP (1.5-4 mM) and various concentrations of MgADP (1-4 mM). As the [MgADP]/[MgATP] ratio in the fiber increases, the maximum velocity achieved by the fiber decreases while the isometric tension increases. The inhibition of fiber velocities and the potentiation of fiber tension by MgADP is not altered by the presence of 12 mM phosphate. The concentration of both MgADP and MgATP within the fiber was calculated from the diffusion coefficient for nucleotides within the fiber, and the rate of MgADP production within the fiber. Using the calculated values for the nucleotide concentration inside the fiber, observed values of the maximum contraction velocity could be described, within experimental accuracy...

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## Kinetics of cardiac muscle contraction and relaxation are linked and determined by properties of the cardiac sarcomere

Fonte: American Physiological Society
Publicador: American Physiological Society

Tipo: Artigo de Revista Científica

EN

Relevância na Pesquisa

26.23%

The regulation of myocardial contraction and relaxation kinetics is currently incompletely understood. When the amplitude of contraction is increased via the Frank-Starling mechanism, the kinetics of the contraction slow down, but when the amplitude of contraction is increased with either an increase in heart rate or via β-adrenergic stimulation, the kinetics speed up. It is also unknown how physiological mechanisms affect the kinetics of contraction versus those of relaxation. We investigated contraction-relaxation coupling in isolated trabeculae from the mouse and rat and stimulated them to contract at various temperatures, frequencies, preloads, and in the absence and presence of β-adrenergic stimulation. In each muscle at least 16 different conditions were assessed, and the correlation coefficient of the speed of contraction and relaxation was very close (generally >0.98). Moreover, in all but one of the analyzed murine strains, the ratio of the minimum rate of the derivative of force development (dF/dt) over maximum dF/dt was not significantly different. Only in trabeculae isolated from myosin-binding protein-C mutant mice was this ratio significantly lower (0.61 ± 0.07 vs. 0.84 ± 0.02 in 11 other strains of mice). Within each strain...

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## Structural, vibrational and thermodynamic properties of carbon allotropes from first-principles : diamond, graphite, and nanotubes

Fonte: Massachusetts Institute of Technology
Publicador: Massachusetts Institute of Technology

Tipo: Tese de Doutorado
Formato: 104 p.; 4519769 bytes; 4524067 bytes; application/pdf; application/pdf

ENG

Relevância na Pesquisa

26.25%

The structural, dynamical, and thermodynamic properties of different carbon allotropes are computed using a combination of ab-initio methods: density-functional theory for total-energy calculations and density-functional perturbation theory for lattice dynamics. For diamond, graphite, graphene, and armchair or zigzag single-walled nanotubes we first calculate the ground-state properties: lattice parameters, elastic constants and phonon dispersions and density of states. Very good agreement with available experimental data is found for all these, with the exception of the c/a ratio in graphite and the associated elastic constants and phonon dispersions. Agreement with experiments is recovered once the experimental c/a is chosen for the calculations. Results for carbon nanotubes confirm and expand available, but scarce, experimental data. The vibrational free energy and the thermal expansion, the temperature dependence of the elastic moduli and the specific heat are calculated using the quasi-harmonic approximation. Graphite shows a distinctive in-plane negative thermal-expansion coefficient that reaches its lowest value around room temperature, in very good agreement with experiments. The predicted value for the thermal-contraction coefficient of narrow single-walled nanotubes is half that of graphite...

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## Perception de la force musculaire et de la distribution du poids lors d’activités motrices chez le sujet sain

Fonte: Université de Montréal
Publicador: Université de Montréal

Tipo: Thèse ou Mémoire numérique / Electronic Thesis or Dissertation

FR

Relevância na Pesquisa

26.2%

#perception#force#distribution du poids#contraction musculaire statique#passage assis-debout#réadaptation#weight-bearing distribution#static contraction#sit-to-stand#rehabilitation#Health Sciences - Rehabilitation and Therapy / Sciences de la santé - Réhabilitation et thérapie (UMI : 0382)

Les résultats de récentes études supportent l’idée que la perception de la force aurait un rôle important dans la réalisation des tâches fonctionnelles telles que le passage assis à debout (PAD). Cependant, très peu d’études se sont attardées à quantifier la précision avec laquelle les individus en santé sont capables de percevoir la force musculaire. De plus, aucune étude n’a évalué la perception de la distribution du poids lors du PAD chez la clientèle saine. L’objectif principal était de recueillir des données sur la capacité des individus en santé, jeunes et âgés, à percevoir la force musculaire lors d’une contraction statique des extenseurs du genou et à percevoir la distribution du poids (DP) lorsqu’ils réalisent le passage assis à debout. Cette étude a été effectuée auprès de 31 individus, divisés en deux groupes d’âge (< 50 ans et > 60 ans). Les résultats du premier objectif visant à quantifier les erreurs de perception de la force lors d’une contraction statique des extenseurs du genou ont démontré que les erreurs absolues et brutes sont plus élevées pour les niveaux de force supérieurs à 50 % CVM et que les erreurs brutes sont plus importantes chez les jeunes pour les hauts niveaux de force. Les résultats du second objectif visant à évaluer l’effet de différentes références sensorimotrices sur les erreurs de perception de la force musculaire ont montré qu’une référence à 50 % CVM préalablement montrée au participant et qu’une contraction simultanée des muscles de la préhension de la main diminuent les erreurs brutes et absolues de perception pour le niveau de force 70%. Les résultats du troisième objectif visant à quantifier les erreurs de perception de la DP lors du PAD ont démontré que les sujets jeunes et âgés ont une bonne capacité à percevoir leur DP avec des erreurs absolues et brutes variant respectivement entre 2...

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## Motor-unit activity differs with load type during a fatiguing contraction

Fonte: Amer Physiological Soc
Publicador: Amer Physiological Soc

Tipo: Artigo de Revista Científica

Publicado em //2005
EN

Relevância na Pesquisa

36.05%

#Humans#Muscle, Skeletal#Elbow Joint#Electromyography#Task Performance and Analysis#Psychomotor Performance#Physical Endurance#Muscle Contraction#Evoked Potentials, Motor#Action Potentials#Adult

Despite a similar rate of change in average electromyographic (EMG) activity, previous studies have observed different rates of change in mean arterial pressure, heart rate, perceived exertion, and fluctuations in motor output during the performance of fatiguing contractions that involved different types of loads. To obtain a more direct measure of the motor output from the spinal cord, the purpose of this study was to compare the discharge characteristics of the same motor unit in biceps brachii during the performance of two types of fatiguing contractions. In separate tests with the upper arm vertical and the elbow flexed to 1.57 rad, the seated subjects maintained either a constant upward force at the wrist (force task) or a constant elbow angle (position task) for a prescribed duration. The force and position tasks were performed in random order at a target force equal to 3.5 ± 2.1% (mean ± SD) of the maximal voluntary contraction (MVC) force above the recruitment threshold of the isolated motor unit. Each subject maintained the two tasks for an identical duration (161 ± 96 s) at a mean target force of 22.2 ± 13.4% MVC (range: 3–49% MVC). The dependent variables included the discharge characteristics of the same motor unit in biceps brachii...

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## Influence of neuromuscular fatigue on co-contraction between vastus medialis and vastus lateralis during isometric contractions

Fonte: Univ Zagreb, Fac Kinesiology
Publicador: Univ Zagreb, Fac Kinesiology

Tipo: Artigo de Revista Científica
Formato: 179-185

ENG

Relevância na Pesquisa

26.22%

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP); Processo FAPESP: 2008/11639-4; The aim of this study was to evaluate the effect of neuromuscular fatigue on vastus medialis and vastus lateralis co-contraction. Fifteen healthy young subjects performed an isometric leg extension test in two different condition phases: i) maximum test for determining the maximum voluntary isometric contraction, and ii) fatigue test for exercises executed at 50% of the maximum voluntary contraction in three distinct periods with seven-day intervals between the tests. To quantify the co-contraction between the vastus medialis and vastus lateralis muscles, the signals overlapped and the intersection area was calculated. The data reliability was verified with the intraclass correlation coefficient (ICC) and standard error measurement (SEM). Values of median frequency and root mean square for both muscles showed a significant difference between the beginning and the end of the test, which indicates occurrence of the neuromuscular fatigue. The median frequency and root mean square ICC values ranged from moderate (ICC .66) to high (ICC .74) reliability with low SEM both for vastus medialis and vastus lateralis. The co-contraction values in the beginning of the test varied from 0.76 to 0.77...

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## Correlation of Digital Palpation and Transabdominal Ultrasound for Assessment of Pelvic Floor Muscle Contraction

Fonte: Journal of Manual & Manipulative Therapy, Inc.
Publicador: Journal of Manual & Manipulative Therapy, Inc.

Tipo: Artigo de Revista Científica

Publicado em //2009
EN

Relevância na Pesquisa

26.24%

Pelvic floor muscle (PFM) dysfunction has been commonly associated with urinary disorders and lumbo-pelvic pain. Transabdominal (TA) ultrasound is currently used by physical therapists to assess PFM function. Controversy exists regarding the correlation between TA ultrasound measurement and vaginal palpation for assessment of PFM contraction, and this relationship has not yet been examined concurrently during the same contraction. The purpose of this study was to determine the correlation of digital palpation and TA ultrasound to assess PFM contraction when recorded 1) simultaneous to digital palpation during one contraction and 2) following digital palpation testing in another contraction. A descriptive correlational design was used to describe the relationship between variables. A total of 19 women (both asymptomatic women and those with incontinence or lumbo-pelvic pain) participated in the study. The modified Oxford scale was used to grade PFM contraction in digital palpation testing. The amount of bladder base movement on ultrasound was measured and considered as an indicator of PFM activity. Two trials were performed for TA ultrasound measurement: 1) simultaneous to digital palpation during one contraction, and 2) following digital palpation testing in another contraction. Spearman's correlation coefficient was used for analysis. There was a significant correlation between digital palpation and TA ultrasound for PFM assessment when measured simultaneously in one contraction (rho=0.62...

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## Drying and shrinkage of polymer gels

Fonte: Brazilian Society of Chemical Engineering
Publicador: Brazilian Society of Chemical Engineering

Tipo: Artigo de Revista Científica
Formato: text/html

Publicado em 01/06/2005
EN

Relevância na Pesquisa

46.18%

#Polymer gel#Drying characteristics#Diffusion coefficient#Modified gel thickness#Contraction coefficient

The polymer hydrogel was synthesized by photo-polymerization process (UV light, 60 ºC) in presence of Photo-initiator (IrgacureR) and Cross-linker (NN'-methylene bisacrylamide; MBAM). In the present work, the drying of polymer hydrogel was carried out to study the effect of temperature, gel-sheet thickness, monomer ratio of acryl acid to acrylamide (AA/AM), concentration of MBAM and quantity of monomers. A correlation has been developed for modified sheet thickness as a function of contraction coefficient and degree of drying. Effective diffusivity was estimated from Fickian-diffusive model considering modified sheet thickness and was found to be in the range of 1.1 ´ 10-10-5.93 ´ 10-10 m²/s. The activation energy obtained using Arrhenius type equation was found to be in the range of 2979-10737 kJ/kmol H2O. The drying behavior shows an initial shoot-up in drying rate followed by constant rate and two falling rate periods.

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## Slip flows of Newtonian and viscoelastic fluids in a 4:1 contraction

Fonte: Elsevier
Publicador: Elsevier

Tipo: Artigo de Revista Científica

Publicado em //2014
ENG

Relevância na Pesquisa

36.34%

This work presents a numerical study of the 4:1 planar contraction flow of a viscoelastic fluid described
by the simplified Phan-Thien–Tanner model under the influence of slip boundary conditions at the channel
walls. The linear Navier slip law was considered with the dimensionless slip coefficient varying in the
range ½0; 4500. The simulations were carried out for a small constant Reynolds number of 0.04 and Deborah
numbers (De) varying between 0 and 5. Convergence could not be achieved for higher values of the
Deborah number, especially for large values of the slip coefficient, due to the large stress gradients near
the singularity of the reentrant corner.
Increasing the slip coefficient leads to the formation of two vortices, a corner and a lip vortex. The lip
vortex grows with increasing slip until it absorbs the corner vortex, creating a single large vortex that
continues to increase in size and intensity. In the range De = 3–5 no lip vortex was formed. The flow is
characterized in detail for De ¼ 1 as function of the slip coefficient, while for the remaining De only
the main features are shown for specific values of the slip coefficient.

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## Dobrushin's ergodicity coefficient for Markov operators on cones

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

26.34%

We give a characterization of the contraction ratio of bounded linear maps in
Banach space with respect to Hopf's oscillation seminorm, which is the
infinitesimal distance associated to Hilbert's projective metric, in terms of
the extreme points of a certain abstract "simplex". The formula is then applied
to abstract Markov operators defined on arbitrary cones, which extend the row
stochastic matrices acting on the standard positive cone and the completely
positive unital maps acting on the cone of positive semidefinite matrices. When
applying our characterization to a stochastic matrix, we recover the formula of
Dobrushin's ergodicity coefficient. When applying our result to a completely
positive unital map, we therefore obtain a noncommutative version of
Dobrushin's ergodicity coefficient, which gives the contraction ratio of the
map (representing a quantum channel or a "noncommutative Markov chain") with
respect to the diameter of the spectrum. The contraction ratio of the dual
operator (Kraus map) with respect to the total variation distance will be shown
to be given by the same coefficient. We derive from the noncommutative
Dobrushin's ergodicity coefficient an algebraic characterization of the
convergence of a noncommutative consensus system or equivalently the ergodicity
of a noncommutative Markov chain.; Comment: An announcement of some of the present results has appeared in the
Proceedings of the ECC 2013 conference (Zurich). Further results can be found
in the companion arXiv:1302.5226

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## Dobrushin ergodicity coefficient for Markov operators on cones, and beyond

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

26.19%

#Mathematics - Operator Algebras#Computer Science - Multiagent Systems#Mathematics - Differential Geometry

The analysis of classical consensus algorithms relies on contraction
properties of adjoints of Markov operators, with respect to Hilbert's
projective metric or to a related family of seminorms (Hopf's oscillation or
Hilbert's seminorm). We generalize these properties to abstract consensus
operators over normal cones, which include the unital completely positive maps
(Kraus operators) arising in quantum information theory. In particular, we show
that the contraction rate of such operators, with respect to the Hopf
oscillation seminorm, is given by an analogue of Dobrushin's ergodicity
coefficient. We derive from this result a characterization of the contraction
rate of a non-linear flow, with respect to Hopf's oscillation seminorm and to
Hilbert's projective metric.

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## Dissipation of information in channels with input constraints

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

26.32%

One of the basic tenets in information theory, the data processing inequality
states that output divergence does not exceed the input divergence for any
channel. For channels without input constraints, various estimates on the
amount of such contraction are known, Dobrushin's coefficient for the total
variation being perhaps the most well-known. This work investigates channels
with average input cost constraint. It is found that while the contraction
coefficient typically equals one (no contraction), the information nevertheless
dissipates. A certain non-linear function, the \emph{Dobrushin curve} of the
channel, is proposed to quantify the amount of dissipation. Tools for
evaluating the Dobrushin curve of additive-noise channels are developed based
on coupling arguments. Some basic applications in stochastic control,
uniqueness of Gibbs measures and fundamental limits of noisy circuits are
discussed.
As an application, it shown that in the chain of $n$ power-constrained relays
and Gaussian channels the end-to-end mutual information and maximal squared
correlation decay as $\Theta(\frac{\log\log n}{\log n})$, which is in stark
contrast with the exponential decay in chains of discrete channels. Similarly,
the behavior of noisy circuits (composed of gates with bounded fan-in) and
broadcasting of information on trees (of bounded degree) does not experience
threshold behavior in the signal-to-noise ratio (SNR). Namely...

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## Bounds between Contraction Coefficients

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 07/10/2015

Relevância na Pesquisa

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In this paper, we delineate how the contraction coefficient of the strong
data processing inequality for KL divergence can be used to learn likelihood
models. We then present an alternative formulation to learn likelihood models
that forces the input KL divergence of the data processing inequality to
vanish, and achieves a contraction coefficient equivalent to the squared
maximal correlation. This formulation turns out to admit a linear algebraic
solution. To analyze the performance loss in using this simple but suboptimal
procedure, we bound these contraction coefficients in the discrete and finite
regime, and prove their equivalence in the Gaussian regime.; Comment: Part of this work has been published in the 53rd Annual Allerton
Conference on Communication, Control, and Computing, 2015. Minor changes made
from first version

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