629,698 research outputs found
Study of the development of three-dimensional sets of fluid particles and iso-concentration fields using kinematic simulation
We use kinematic simulation (KS) to study the development of a material line immersed in a three-dimensional turbulent flow. We generalize this study to a material surface, cube and sphere. We find that the fractal dimension of the surface can be explained by the same mechanism as that proposed by Villermaux & Gagne (Phys. Rev. Lett. vol. 73, 1994, p. 252) for the line. The fractal dimension of the line or the surface is a linear function of time up to times of the order of the smallest characteristic time of turbulence (or Kolmogorov timescale).
For volume objects we describe the respective role of the Reynolds number and of the object's characteristic size. Using the method of characteristics with KS we compute the evolution with time of a concentration field and measure the fractal dimension of the intersection of this scalar field with a given plane. For these objects, we retrieve the result of Villermaux & Innocenti (J. Fluid Mech. vol. 393, 1999, p. 123) that the Reynolds number does not affect the development of the fractal dimension of the iso-scalar surface and extend this result to volume geometries. We also find that for volume objects the characteristic time of development of the fractal dimension is the large scales' characteristic time and not the Kolmogorov timescale
A finite membrane element formulation for surfactants
Surfactants play an important role in various physiological and biomechanical
applications. An example is the respiratory system, where pulmonary surfactants
facilitate the breathing and reduce the possibility of airway blocking by
lowering the surface tension when the lung volume decreases during exhalation.
This function is due to the dynamic surface tension of pulmonary surfactants,
which depends on the concentration of surfactants spread on the liquid layer
lining the interior surface of the airways and alveoli. Here, a finite membrane
element formulation for liquids is introduced that allows for the dynamics of
concentration-dependent surface tension, as is the particular case for
pulmonary surfactants. A straightforward approach is suggested to model the
contact line between liquid drops/menisci and planar solid substrates, which
allows the presented framework to be easily used for drop shape analysis. It is
further shown how line tension can be taken into account. Following an
isogeometric approach, NURBS-based finite elements are used for the
discretization of the membrane surface. The capabilities of the presented
computational model is demonstrated by different numerical examples - such as
the simulation of liquid films, constrained and unconstrained sessile drops,
pendant drops and liquid bridges - and the results are compared with
experimental data.Comment: Some typos are removed. Eqs. 13 and 105 are modified. Eqs. 64 and 73
are added; thus, the rest of equations is renumbered. All the numerical
experiments are repeated. The example of Sec. 6.3 is slightly modifie
Seizure-onset mapping based on time-variant multivariate functional connectivity analysis of high-dimensional intracranial EEG : a Kalman filter approach
The visual interpretation of intracranial EEG (iEEG) is the standard method used in complex epilepsy surgery cases to map the regions of seizure onset targeted for resection. Still, visual iEEG analysis is labor-intensive and biased due to interpreter dependency. Multivariate parametric functional connectivity measures using adaptive autoregressive (AR) modeling of the iEEG signals based on the Kalman filter algorithm have been used successfully to localize the electrographic seizure onsets. Due to their high computational cost, these methods have been applied to a limited number of iEEG time-series (< 60). The aim of this study was to test two Kalman filter implementations, a well-known multivariate adaptive AR model (Arnold et al. 1998) and a simplified, computationally efficient derivation of it, for their potential application to connectivity analysis of high-dimensional (up to 192 channels) iEEG data. When used on simulated seizures together with a multivariate connectivity estimator, the partial directed coherence, the two AR models were compared for their ability to reconstitute the designed seizure signal connections from noisy data. Next, focal seizures from iEEG recordings (73-113 channels) in three patients rendered seizure-free after surgery were mapped with the outdegree, a graph-theory index of outward directed connectivity. Simulation results indicated high levels of mapping accuracy for the two models in the presence of low-to-moderate noise cross-correlation. Accordingly, both AR models correctly mapped the real seizure onset to the resection volume. This study supports the possibility of conducting fully data-driven multivariate connectivity estimations on high-dimensional iEEG datasets using the Kalman filter approach
Comparison of Two Methods of Pediatric Triage Education
Introduction. The majority of pediatric emergency patients are seen in mixed-age emergency departments and triaged by general emergency nurses. Educational methods for teaching pediatric triage education to general emergency nurses have not been well studied, and previous studies of the use of the Emergency Severity Index in children have been performed primarily in centers that are high volume for pediatrics. Methods. A repeated measures, randomized crossover study comparing two different methods of pediatric triage education was conducted. Participants were general emergency triage nurses recruited from a general emergency department that is classified as low volume for pediatrics. Each participant was exposed to both educational methods: paper case studies and high fidelity simulation. Results. All participants had substantial improvement in pediatric triage accuracy as measured by a standardized set of pediatric cases. The previously reported trend towards undertriage of the pediatric patient was observed despite a mean triage agreement rate of 73% at the end of the study period. No differences were observed between groups; the order of the educational intervention did not result in statistically significant differences in triage accuracy. Discussion. A combined approach of paper case studies and high fidelity simulation was effective at improving pediatric triage accuracy among a group of general emergency department nurses with limited exposure to pediatric patients. Persistent trends in undertriage should be studied further
A Moving Boundary Flux Stabilization Method for Cartesian Cut-Cell Grids using Directional Operator Splitting
An explicit moving boundary method for the numerical solution of
time-dependent hyperbolic conservation laws on grids produced by the
intersection of complex geometries with a regular Cartesian grid is presented.
As it employs directional operator splitting, implementation of the scheme is
rather straightforward. Extending the method for static walls from Klein et
al., Phil. Trans. Roy. Soc., A367, no. 1907, 4559-4575 (2009), the scheme
calculates fluxes needed for a conservative update of the near-wall cut-cells
as linear combinations of standard fluxes from a one-dimensional extended
stencil. Here the standard fluxes are those obtained without regard to the
small sub-cell problem, and the linear combination weights involve detailed
information regarding the cut-cell geometry. This linear combination of
standard fluxes stabilizes the updates such that the time-step yielding
marginal stability for arbitrarily small cut-cells is of the same order as that
for regular cells. Moreover, it renders the approach compatible with a wide
range of existing numerical flux-approximation methods. The scheme is extended
here to time dependent rigid boundaries by reformulating the linear combination
weights of the stabilizing flux stencil to account for the time dependence of
cut-cell volume and interface area fractions. The two-dimensional tests
discussed include advection in a channel oriented at an oblique angle to the
Cartesian computational mesh, cylinders with circular and triangular
cross-section passing through a stationary shock wave, a piston moving through
an open-ended shock tube, and the flow around an oscillating NACA 0012 aerofoil
profile.Comment: 30 pages, 27 figures, 3 table
Modelling of the diffusion of carbon dioxide in polyimide matrices by computer simulation
Computer aided molecular modelling is used to visualize the motion of CO2 gas molecules inside a polyimide polymer matrix. The polymers simulated are two 6FDA-bases polyimides, 6FDA-4PDA and 6FDA-44ODA. These polymers have also been synthesized in our laboratory, and thus the simulated properties could directly be compared with “real-world” data. The simulation experiments have been performed using the GROMOS1 package. The polymer boxes were created using the soft-core method, with short (11 segments) chains. This results in highly relaxed and totally amorphous polyimide matrices. The motion of randomly placed CO2 molecules in the boxes during molecular dynamics runs was followed, revealing three types of motion: jumping, continuous- and trapped motion. The calculated diffusivities are unrealistic, but possible shortcomings in our model are given
Crowding of Polymer Coils and Demixing in Nanoparticle-Polymer Mixtures
The Asakura-Oosawa-Vrij (AOV) model of colloid-polymer mixtures idealizes
nonadsorbing polymers as effective spheres that are fixed in size and
impenetrable to hard particles. Real polymer coils, however, are intrinsically
polydisperse in size (radius of gyration) and may be penetrated by smaller
particles. Crowding by nanoparticles can affect the size distribution of
polymer coils, thereby modifying effective depletion interactions and
thermodynamic stability. To analyse the influence of crowding on polymer
conformations and demixing phase behaviour, we adapt the AOV model to mixtures
of nanoparticles and ideal, penetrable polymer coils that can vary in size. We
perform Gibbs ensemble Monte Carlo simulations, including trial
nanoparticle-polymer overlaps and variations in radius of gyration. Results are
compared with predictions of free-volume theory. Simulation and theory
consistently predict that ideal polymers are compressed by nanoparticles and
that compressibility and penetrability stabilise nanoparticle-polymer mixtures.Comment: 18 pages, 4 figure
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