940 research outputs found
Three-dimensional structure of the flow inside the left ventricle of the human heart
The laboratory models of the human heart left ventricle developed in the last
decades gave a valuable contribution to the comprehension of the role of the
fluid dynamics in the cardiac function and to support the interpretation of the
data obtained in vivo. Nevertheless, some questions are still open and new ones
stem from the continuous improvements in the diagnostic imaging techniques.
Many of these unresolved issues are related to the three-dimensional structure
of the left-ventricular flow during the cardiac cycle. In this paper we
investigated in detail this aspect using a laboratory model. The ventricle was
simulated by a flexible sack varying its volume in time according to a
physiologically shaped law. Velocities measured during several cycles on series
of parallel planes, taken from two orthogonal points of view, were combined
together in order to reconstruct the phase averaged, three-dimensional velocity
field. During the diastole, three main steps are recognized in the evolution of
the vortical structures: i) straight propagation in the direction of the long
axis of a vortex-ring originated from the mitral orifice; ii) asymmetric
development of the vortex-ring on an inclined plane; iii) single vortex
formation. The analysis of three-dimensional data gives the experimental
evidence of the reorganization of the flow in a single vortex persisting until
the end of the diastole. This flow pattern seems to optimize the cardiac
function since it directs velocity towards the aortic valve just before the
systole and minimizes the fraction of blood residing within the ventricle for
more cycles
Mixing-induced fluid destratification and ullage condensation
In many applications, on-orbit storage and transfer of cryogens will require forced mixing to control tank pressure without direct venting to space. During a no-vent transfer or during operation of a thermodynamic vent system in a cryogen storage tank, pressure control is achieved by circulating cool liquid to the liquid-vapor interface to condense some of the ullage vapor. To measure the pressure and temperature response rates in mixing-induced condensation, an experiment has been developed using Freon 11 to simulate the two-phase behavior of a cryogen. A thin layer at the liquid surface is heated to raise the tank pressure, and then a jet mixer is turned on to circulate the liquid, cool the surface, and reduce the pressure. Many nozzle configurations and flow rates are used. Tank pressure and the temperature profiles in the ullage and the liquid are measured. Initial data from this ground test are shown correlated with normal-gravity and drop-tower dye-mixing data. Pressure collapse times are comparable to the dye-mixing times, whereas the times needed for complete thermal mixing are much longer than the dye-mixing times
Crystallization and gelation in colloidal systems with short-ranged attractive interactions
We systematically study the relationship between equilibrium and
non-equilibrium phase diagrams of a system of short-ranged attractive colloids.
Using Monte Carlo and Brownian dynamics simulations we find a window of
enhanced crystallization that is limited at high interaction strength by a
slowing down of the dynamics and at low interaction strength by the high
nucleation barrier. We find that the crystallization is enhanced by the
metastable gas-liquid binodal by means of a two-stage crystallization process.
First, the formation of a dense liquid is observed and second the crystal
nucleates within the dense fluid. In addition, we find at low colloid packing
fractions a fluid of clusters, and at higher colloid packing fractions a
percolating network due to an arrested gas-liquid phase separation that we
identify with gelation. We find that this arrest is due to crystallization at
low interaction energy and it is caused by a slowing down of the dynamics at
high interaction strength. Likewise, we observe that the clusters which are
formed at low colloid packing fractions are crystalline at low interaction
energy, but glassy at high interaction energy. The clusters coalesce upon
encounter.Comment: 8 pages, 8 figure
Turbulence investigation in a laboratory model of the ascending aorta
This study aims to investigate turbulence inside a model of the
human ascending aorta as a function of the main flow control
parameters. For this purpose, we performed a two-dimensional
in vitro investigation of the pulsatile flow inside a laboratory
model of a healthy aorta by varying both the Reynolds and
Womersley numbers. Our findings indicate that the velocity
fluctuations become significant particularly during the
deceleration phase of the flow, reach the maximum near the
systolic peak and then decay during the rest of the diastole
phase. Higher levels of turbulence were recovered for
increasing Stroke Volumes, in particular maxima of Turbulent
Kinetic Energy occurred in the bulk region while higher
values of Reynolds shear stresses were found in
correspondence of the sinus of Valsalva
Phase behavior of hard spheres confined between parallel hard plates: Manipulation of colloidal crystal structures by confinement
We study the phase behavior of hard spheres confined between two parallel
hard plates using extensive computer simulations. We determine the full
equilibrium phase diagram for arbitrary densities and plate separations from
one to five hard-sphere diameters using free energy calculations. We find a
first-order fluid-solid transition, which corresponds to either capillary
freezing or melting depending on the plate separation. The coexisting solid
phase consists of crystalline layers with either triangular or square symmetry.
Increasing the plate separation, we find a sequence of crystal structures from
n triangular to (n+1) square to (n+1) triangular, where n is the number of
crystal layers, in agreement with experiments on colloids. At high densities,
the transition between square to triangular phases are intervened by
intermediate structures, e.g., prism, buckled, and rhombic phases.Comment: 9 pages, 4 figures. Accepted for publication in J. Phys.: Condens.
Matte
Lagrangian planetary equations in Schwarzschild space--time
We have developed a method to study the effects of a perturbation to the
motion of a test point--like object in a Schwarzschild spacetime. Such a method
is the extension of the Lagrangian planetary equations of classical celestial
mechanics into the framework of the full theory of general relativity. The
method provides a natural approach to account for relativistic effects in the
unperturbed problem in an exact way.Comment: 7 pages; revtex; accepted for publication in Class. Quantum Gra
Phase behavior and structure of model colloid-polymer mixtures confined between two parallel planar walls
Using Gibbs ensemble Monte Carlo simulations and density functional theory we
investigate the fluid-fluid demixing transition in inhomogeneous
colloid-polymer mixtures confined between two parallel plates with separation
distances between one and ten colloid diameters covering the complete range
from quasi two-dimensional to bulk-like behavior. We use the
Asakura-Oosawa-Vrij model in which colloid-colloid and colloid-polymer
interactions are hard-sphere like, whilst the pair potential between polymers
vanishes. Two different types of confinement induced by a pair of parallel
walls are considered, namely either through two hard walls or through two
semi-permeable walls that repel colloids but allow polymers to freely
penetrate. For hard (semi-permeable) walls we find that the capillary binodal
is shifted towards higher (lower) polymer fugacities and lower (higher) colloid
fugacities as compared to the bulk binodal; this implies capillary condensation
(evaporation) of the colloidal liquid phase in the slit. A macroscopic
treatment is provided by a novel symmetric Kelvin equation for general binary
mixtures, based on the proximity in chemical potentials of statepoints at
capillary coexistence and the reference bulk coexistence. Results for capillary
binodals compare well with those obtained from the classic version of the
Kelvin equation due to Evans and Marini Bettolo Marconi [J. Chem. Phys. 86,
7138 (1987)], and are quantitatively accurate away from the fluid-fluid
critical point, even at small wall separations. For hard walls the density
profiles of polymers and colloids inside the slit display oscillations due to
packing effects for all statepoints. For semi-permeable walls either similar
structuring or flat profiles are found, depending on the statepoint considered.Comment: 15 pages, 13 figure
Wall-Fluid and Liquid-Gas Interfaces of Model Colloid-Polymer Mixtures by Simulation and Theory
We perform a study of the interfacial properties of a model suspension of
hard sphere colloids with diameter and non-adsorbing ideal polymer
coils with diameter . For the mixture in contact with a planar hard
wall, we obtain from simulations the wall-fluid interfacial free energy,
, for size ratios and 1, using
thermodynamic integration, and study the (excess) adsorption of colloids,
, and of polymers, , at the hard wall. The interfacial
tension of the free liquid-gas interface, , is obtained following
three different routes in simulations: i) from studying the system size
dependence of the interfacial width according to the predictions of capillary
wave theory, ii) from the probability distribution of the colloid density at
coexistence in the grand canonical ensemble, and iii) for statepoints where the
colloidal liquid wets the wall completely, from Young's equation relating
to the difference of wall-liquid and wall-gas interfacial
tensions, . In addition, we calculate , and using density functional theory and a scaled particle
theory based on free volume theory. Good agreement is found between the
simulation results and those from density functional theory, while the results
from scaled particle theory quantitatively deviate but reproduce some essential
features. Simulation results for obtained from the three
different routes are all in good agreement. Density functional theory predicts
with good accuracy for high polymer reservoir packing fractions,
but yields deviations from the simulation results close to the critical point.Comment: 23 pages, 10 figures, REVTEX. Fig 5a changed. Final versio
Developing a brief motivational intervention for young adults admitted with alcohol intoxication in the emergency department - Results from an iterative qualitative design.
Unhealthy alcohol use among young adults is a major public health concern. Brief motivational interventions for young adults in the Emergency Department (ED) have shown promising but inconsistent results.
Based on the literature on brief intervention and motivational interviewing efficacy and active ingredients, we developed a new motivational intervention model for young adults admitted in the ED with alcohol intoxication. Using an iterative qualitative design, we first pre-tested this model by conducting 4 experimental sessions and 8 related semi-structured interviews to evaluate clinicians' and patients' perceptions of the intervention's acceptability and feasibility. We then conducted a consultation meeting with 9 international experts using a nominal group technique. The intervention model was adjusted and finally re-tested by conducting 6 new experimental sessions and 12 related semi-structured interviews. At each round, data collected were analyzed and discussed, and the intervention model updated accordingly.
Based on the literature, we found 6 axes for developing a new model: High level of relational factors (e.g. empathy, alliance, avoidance of confrontation); Personalized feedback; Enhance discrepancy; Evoke change talk while softening sustain talk, strengthen ability and commitment to change; Completion of a change plan; Devote more time: longer sessions and follow-up options (face-to-face, telephone, or electronic boosters; referral to treatment). A qualitative analysis of the semi-structured interviews gave important insights regarding acceptability and feasibility of the model. Adjustments were made around which information to provide and how, as well as on how to deepen discussion about change with patients having low levels of self-exploration. The experts' consultation addressed numerous points, such as information and advice giving, and booster interventions.
This iterative, multi-component design resulted in the development of an intervention model embedded in recent research findings and theory advances, as well as feasible in a complex environment. The next step is a randomized controlled trial testing the efficacy of this model
Metallurgical and statistical approaches to the study of cast iron street furniture
The evolution of microstructure in relation to dating and nationality of origin was investigated in twenty-four cast iron objects of street furniture produced between XIX and XX centuries in United Kingdom, France, and Italy. Chemical composition of the metalworks was evaluated by glow-discharge optical emission spectrometry. Fragments from the cast irons were analyzed by optical microscopy and scanning electron microscopy with energy-dispersive spectroscopy. Form, distribution, and size of graphite were evaluated in the microstructure according to standard EN ISO 945-1:2008. An image analysis software was employed to quantify the area fraction of graphite in the matrix, major axis, and shape factor of graphite lamellae, area fraction of manganese sulfides (ψS), area fraction of steadite, and number of eutectic cells per area unit. All data were grouped and linear discrimination analysis (LDA) was applied to assess the group assignment and the probability of correct classification for each metalwork. The results showed that the microstructural features were compatible with those of cast irons produced in the XIX and XX centuries. Values of ψS also suggested re-melting of cast irons, associated with recycling of cast iron and/or steel scraps. The high values of steadite found in the metalworks are probably due to the excellent castability required for complex shape castings in these centuries. The LDA multivariate analysis allowed to discriminate cast irons based on the year of manufacturing and the nationality of origin
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