426 research outputs found
Traveling Wave Fronts and Localized Traveling Wave Convection in Binary Fluid Mixtures
Nonlinear fronts between spatially extended traveling wave convection (TW)
and quiescent fluid and spatially localized traveling waves (LTWs) are
investigated in quantitative detail in the bistable regime of binary fluid
mixtures heated from below. A finite-difference method is used to solve the
full hydrodynamic field equations in a vertical cross section of the layer
perpendicular to the convection roll axes. Results are presented for
ethanol-water parameters with several strongly negative separation ratios where
TW solutions bifurcate subcritically. Fronts and LTWs are compared with each
other and similarities and differences are elucidated. Phase propagation out of
the quiescent fluid into the convective structure entails a unique selection of
the latter while fronts and interfaces where the phase moves into the quiescent
state behave differently. Interpretations of various experimental observations
are suggested.Comment: 46 pages, 11 figures. Accepted for publication in Phys. Rev.
Java Card:An analysis of the most successful smart card operating system
To explain why the Java Card operating system has become the most successful smart card operating system to date, we analyze the realized features of the current Java Card version, we argue it could be enhanced by adding a number of intended features and we discuss a set of complementary features that have been suggested. No technology can be successful without the right people and the right circumstances, so we provide some insights in the personal and historical historic aspects of the success of Java Card
Resolving singular forces in cavity flow: Multiscale modeling from atoms to millimeters
A multiscale approach for fluid flow is developed that retains an atomistic
description in key regions. The method is applied to a classic problem where
all scales contribute: The force on a moving wall bounding a fluid-filled
cavity. Continuum equations predict an infinite force due to stress
singularities. Following the stress over more than six decades in length in
systems with characteristic scales of millimeters and milliseconds allows us to
resolve the singularities and determine the force for the first time. The
speedup over pure atomistic calculations is more than fourteen orders of
magnitude. We find a universal dependence on the macroscopic Reynolds number,
and large atomistic effects that depend on wall velocity and interactions.Comment: 4 pages,3 figure
Friction Drag on a Particle Moving in a Nematic Liquid Crystal
The flow of a liquid crystal around a particle does not only depend on its
shape and the viscosity coefficients but also on the direction of the
molecules. We studied the resulting drag force on a sphere moving in a nematic
liquid crystal (MBBA) in a low Reynold's number approach for a fixed director
field (low Ericksen number regime) using the computational artificial
compressibility method. Taking the necessary disclination loop around the
sphere into account, the value of the drag force anisotropy
(F_\perp/F_\parallel=1.50) for an exactly computed field is in good agreement
with experiments (~1.5) done by conductivity diffusion measurements. We also
present data for weak anchoring of the molecules on the particle surface and of
trial fields, which show to be sufficiently good for most applications.
Furthermore, the behaviour of the friction close to the transition point
nematic isotropic and for a rod-like and a disc-like liquid crystal will be
given.Comment: 23 pages RevTeX, including 3 PS figures, 1 PS table and 1 PS-LaTeX
figure; Accepted for publication in Phys. Rev.
A Method for Calculating the Structure of (Singular) Spacetimes in the Large
A formalism and its numerical implementation is presented which allows to
calculate quantities determining the spacetime structure in the large directly.
This is achieved by conformal techniques by which future null infinity
(\Scri{}^+) and future timelike infinity () are mapped to grid points on
the numerical grid. The determination of the causal structure of singularities,
the localization of event horizons, the extraction of radiation, and the
avoidance of unphysical reflections at the outer boundary of the grid, are
demonstrated with calculations of spherically symmetric models with a scalar
field as matter and radiation model.Comment: 29 pages, AGG2
Vortex disruption by magnetohydrodynamic feedback
In an electrically conducting fluid, vortices stretch out a weak, large-scale magnetic field to form strong current sheets on their edges. Associated with these current sheets are magnetic stresses, which are subsequently released through reconnection, leading to vortex disruption, and possibly even destruction. This disruption phenomenon is investigated here in the context of two-dimensional, homogeneous, incompressible magnetohydrodynamics. We derive a simple order of magnitude estimate for the magnetic stresses—and thus the degree of disruption—that depends on the strength of the background magnetic field (measured by the parameter M, a ratio between the Alfvén speed and a typical flow speed) and on the magnetic diffusivity (measured by the magnetic Reynolds number Rm). The resulting estimate suggests that significant disruption occurs when M²Rm=O(1). To test our prediction, we analyze direct numerical simulations of vortices generated by the breakup of unstable shear flows with an initially weak background magnetic field. Using the Okubo-Weiss vortex coherence criterion, we introduce a vortex disruption measure, and show that it is consistent with our predicted scaling, for vortices generated by instabilities of both a shear layer and a jet
Localized states in sheared electroconvection
Electroconvection in a thin, sheared fluid film displays a rich sequence of
bifurcations between different flow states as the driving voltage is increased.
We present a numerical study of an annular film in which a radial potential
difference acts on induced surface charges to drive convection. The film is
also sheared by independently rotating the inner edge of the annulus. This
simulation models laboratory experiments on electroconvection in sheared
smectic liquid crystal films. The applied shear competes with the electrical
forces, resulting in oscillatory and strongly subcritical bifurcations between
localized vortex states close to onset. At higher forcing, the flow becomes
chaotic via a Ruelle-Takens-Newhouse scenario. The simulation allows flow
visualization not available in the physical experiments, and sheds light on
previously observed transitions in the current-voltage characteristics of
electroconvecting smectic films.Comment: To be published in EuroPhysics Letters, 6 pages, 6 figures: final
versio
The UV-A and visible solar irradiance spectrum: inter-comparison of absolutely calibrated, spectrally medium resolution solar irradiance spectra from balloon- and satellite-borne measurements
International audienceWithin the framework of the ENVISAT/-SCIAMACHY satellite validation, solar irradiance spectra are absolutely measured at moderate resolution in the UV/visible spectral range (in the UV from 316.7–418 nm and the visible from 400–652 nm at a full width half maximum resolution of 0.55 nm and 1.48 nm, respectively) from aboard the azimuth-controlled LPMA/DOAS balloon gondola at around 32 km balloon float altitude. After accounting for the atmospheric extinction due to Rayleigh scattering and gaseous absorption (O3, and NO2), the measured solar spectra are compared with previous observations. Our solar irradiance is +1.6% larger than the re-calibrated Kurucz et al. (1984) solar spectrum (Fontenla et al., 1999, called MODTRAN 3.5) in the visible spectral range (435–650 nm), +1.5% larger in the (370–415 nm) wavelength interval, but -4% smaller in the UV spectral range (316.7–370 nm), when the Kurucz spectrum is convolved to the spectral resolution of our instrument. The same comparison with the SOLSPEC solar spectrum (Thuillier et al., 1997, 1998a, b) confirms the somewhat larger solar irradiance (+1.7%) measured by the balloon instrument from 435–500 nm, but not from 500–650 nm, where the SOLSPEC is -1.3% lower than MODTRAN 3.5. Comparison of the SCIAMACHY solar spectrum from channels 1 to 4 (– re-calibrated by the University of Bremen –) with MODTRAN 3.5 indicates an agreement of +0.2% in the visible spectral range (435–585 nm). With this calibration, the SCIAMACHY solar spectrum is congruent with the balloon observations (-1%) in the 316.7–370 nm wavelength range, but both are up to -5%/-3% smaller than MODTRAN 3.5 and SOLSPEC, respectively. In agreement with findings of Skupin et al. (2002) our study emphasizes that the present ESA SCIAMACHY level 1 calibration is systematically +15% larger in the considered wavelength intervals when compared to all available other solar irradiance measurements
Recombinant expression of tandem-HBc virus-like particles (VLPs)
The hepatitis B virus (HBV) core protein (HBc) has formed the building block for virus-like particle (VLP) production for more than 30 years. The ease of production of the protein, the robust ability of the core monomers to dimerize and assemble into intact core particles, and the strong immune responses they elicit when presenting antigenic epitopes all demonstrate its promise for vaccine development (reviewed in Pumpens and Grens (Intervirology 44: 98–114, 2001)). HBc has been modified in a number of ways in attempts to expand its potential as a novel vaccine platform. The HBc protein is predominantly α-helical in structure and folds to form an L-shaped molecule. The structural subunit of the HBc particle is a dimer of monomeric HBc proteins which together form an inverted T-shaped structure. In the assembled HBc particle the four-helix bundle formed at each dimer interface appears at the surface as a prominent “spike.” The tips of the “spikes” are the preferred sites for the insertion of foreign sequences for vaccine purposes as they are the most highly exposed regions of the assembled particles. In the tandem-core modification two copies of the HBc protein are covalently linked by a flexible amino acid sequence which allows the fused dimer to fold correctly and assemble into HBc particles. The advantage of the modified structure is that the assembly of the dimeric subunits is defined and not formed by random association. This facilitates the introduction of single, larger sequences at the tip of each surface “spike,” thus overcoming the conformational clashes contingent on insertion of large structures into monomeric HBc proteins. Differences in inserted sequences influence the assembly characteristics of the modified proteins, and it is important to optimize the design of each novel construct to maximize efficiency of assembly into regular VLPs. In addition to optimization of the construct, the expression system used can also influence the ability of recombinant structures to assemble into regular isometric particles. Here, we describe the production of recombinant tandem-core particles in bacterial, yeast and plant expression systems
Nonequilibrium effects in DNA microarrays: a multiplatform study
It has recently been shown that in some DNA microarrays the time needed to
reach thermal equilibrium may largely exceed the typical experimental time,
which is about 15h in standard protocols (Hooyberghs et al. Phys. Rev. E 81,
012901 (2010)). In this paper we discuss how this breakdown of thermodynamic
equilibrium could be detected in microarray experiments without resorting to
real time hybridization data, which are difficult to implement in standard
experimental conditions. The method is based on the analysis of the
distribution of fluorescence intensities I from different spots for probes
carrying base mismatches. In thermal equilibrium and at sufficiently low
concentrations, log I is expected to be linearly related to the hybridization
free energy with a slope equal to , where is
the experimental temperature and R is the gas constant. The breakdown of
equilibrium results in the deviation from this law. A model for hybridization
kinetics explaining the observed experimental behavior is discussed, the
so-called 3-state model. It predicts that deviations from equilibrium yield a
proportionality of to . Here, is an
effective temperature, higher than the experimental one. This behavior is
indeed observed in some experiments on Agilent arrays. We analyze experimental
data from two other microarray platforms and discuss, on the basis of the
results, the attainment of equilibrium in these cases. Interestingly, the same
3-state model predicts a (dynamical) saturation of the signal at values below
the expected one at equilibrium.Comment: 27 pages, 9 figures, 1 tabl
- …