88 research outputs found
Oscillatory instability of fully 3D flow in a cubic diagonally lid-driven cavity
A transition to unsteadiness of a flow inside a cubic diagonally lid-driven
cavity with no-slip boundaries is numerically investigated by a series of
direct numerical simulations (DNS) performed on 100^3 and 200^3 stretched
grids. It is found that the observed oscillatory instability is setting in via
a subcritical symmetry-breaking Hopf bifurcation. The instability evolves on
two vortices in a coupled manner. Critical values of Reynolds number Recr=2320
and non-dimensional angular oscillating frequency omegacr=0.249 for transition
from steady to oscillatory flow are accurately estimated. Characteristic
patterns of the 3D oscillatory flow are presented
Fluttering-induced flow in a closed chamber
We study the emergence of fluid flow in a closed chamber that is driven by
dynamical deformations of an elastic sheet. The sheet is compressed between the
sidewalls of the chamber and partitions it into two separate parts, each of
which is initially filled with an inviscid fluid. When fluid exchange is
allowed between the two compartments of the chamber, the sheet becomes
unstable, and its motion displaces the fluid from rest. We derive an analytical
model that accounts for the coupled, two-way, fluid-sheet interaction. We show
that the system depends on four dimensionless parameters: the normalized excess
length of the sheet compared to the lateral dimension of the chamber, ;
the normalized vertical dimension of the chamber; the normalized initial volume
difference between the two parts of the chamber, ; and the
structure-to-fluid mass ratio, . We investigate the dynamics at the
early times of the system's evolution and then at moderate times. We obtain the
growth rates and the frequency of vibrations around the second and the first
buckling modes, respectively. Analytical solutions are derived for these linear
stability characteristics within the limit of the small-amplitude
approximation. At moderate times, we investigate how the sheet escapes from the
second mode. Given the chamber's dimensions, we show that the initial energy of
the sheet is mostly converted into hydrodynamic energy of the fluid if
, and into kinetic energy of the sheet if . In both
cases most of the initial energy is released at time , where is the growth rate and
is a constant.Comment: 25 pages, 12 figure
Time domain Dielectric Spectroscopy Study of Human Cells. II. Normal and Malignant White Blood Cells
open access articleThe dielectric properties of human lymphocyte suspensions were studied by time domain dielectric spectroscopy (TDDS). Nine populations of malignant and normal lymphocytes were investigated. Analysis of the dielectric parameters of cell structural parts were performed in the framework of Maxwell^Wagner mixture formula and the double-shell model of cell. The specific capacitance of the cell membranes was estimated by the Hanai^Asami^Koisumi formula. It was shown that the dielectric permittivity, capacitance and conductivity values of cell membranes are higher for normal lymphocytes than for the malignant ones. The difference of the same parameters for normal B- and T-cells is also discussed
Inverse melting of the vortex lattice
Inverse melting, in which a crystal reversibly transforms into a liquid or
amorphous phase upon decreasing the temperature, is considered to be very rare
in nature. The search for such an unusual equilibrium phenomenon is often
hampered by the formation of nonequilibrium states which conceal the
thermodynamic phase transition, or by intermediate phases, as was recently
shown in a polymeric system. Here we report a first-order inverse melting of
the magnetic flux line lattice in Bi2Sr2CaCu2O8 superconductor. At low
temperatures, the material disorder causes significant pinning of the vortices,
which prevents observation of their equilibrium properties. Using a newly
introduced 'vortex dithering' technique we were able to equilibrate the vortex
lattice. As a result, direct thermodynamic evidence of inverse melting
transition is found, at which a disordered vortex phase transforms into an
ordered lattice with increasing temperature. Paradoxically, the structurally
ordered lattice has larger entropy than the disordered phase. This finding
shows that the destruction of the ordered vortex lattice occurs along a unified
first-order transition line that gradually changes its character from
thermally-induced melting at high temperatures to a disorder-induced transition
at low temperatures.Comment: 13 pages, 4 figures, Nature, In pres
Validation and Calibration of Models for Reaction-Diffusion Systems
Space and time scales are not independent in diffusion. In fact, numerical
simulations show that different patterns are obtained when space and time steps
( and ) are varied independently. On the other hand,
anisotropy effects due to the symmetries of the discretization lattice prevent
the quantitative calibration of models. We introduce a new class of explicit
difference methods for numerical integration of diffusion and
reaction-diffusion equations, where the dependence on space and time scales
occurs naturally. Numerical solutions approach the exact solution of the
continuous diffusion equation for finite and , if the
parameter assumes a fixed constant value,
where is an odd positive integer parametrizing the alghorithm. The error
between the solutions of the discrete and the continuous equations goes to zero
as and the values of are dimension
independent. With these new integration methods, anisotropy effects resulting
from the finite differences are minimized, defining a standard for validation
and calibration of numerical solutions of diffusion and reaction-diffusion
equations. Comparison between numerical and analytical solutions of
reaction-diffusion equations give global discretization errors of the order of
in the sup norm. Circular patterns of travelling waves have a maximum
relative random deviation from the spherical symmetry of the order of 0.2%, and
the standard deviation of the fluctuations around the mean circular wave front
is of the order of .Comment: 33 pages, 8 figures, to appear in Int. J. Bifurcation and Chao
The Impact of Ca2+ on Intracellular Distribution of Hemoglobin in Human Erythrocytes
The membrane-bound hemoglobin (Hb) fraction impacts red blood cell (RBC) rheology and metabolism. Therefore, Hb–RBC membrane interactions are precisely controlled. For instance, the signaling function of membrane-bound deoxy-Hb and the structure of the docking sites in the cytosolic domain of the anion exchanger 1 (AE-1) protein are well documented; however, much less is known about the interaction of Hb variants with the erythrocyte’s membrane. Here, we identified factors other than O2 availability that control Hb abundance in the membrane-bound fraction and the possible variant-specific binding selectivity of Hb to the membrane. We show that depletion of extracellular Ca2+ by chelators, or its omission from the extracellular medium, leads to membrane-bound Hb release into the cytosol. The removal of extracellular Ca2+ further triggers the redistribution of HbA0 and HbA2 variants between the membrane and the cytosol in favor of membrane-bound HbA2. Both effects are reversible and are no longer observed upon reintroduction of Ca2+ into the extracellular medium. Fluctuations of cytosolic Ca2+ also impact the pre-membrane Hb pool, resulting in the massive transfer of Hb to the cellular cytosol. We hypothesize that AE-1 is the specific membrane target and discuss the physiological outcomes and possible clinical implications of the Ca2+ regulation of the intracellular Hb distribution
Quantum effects, soft singularities and the fate of the universe in a braneworld cosmology
We examine a class of braneworld models in which the expanding universe
encounters a "quiescent" future singularity. At a quiescent singularity, the
energy density and pressure of the cosmic fluid as well as the Hubble parameter
remain finite while all derivatives of the Hubble parameter diverge (i.e.,
, , etc. ). Since the Kretschmann invariant
diverges () at the singularity, one expects
quantum effects to play an important role as the quiescent singularity is
approached. We explore the effects of vacuum polarization due to massless
conformally coupled fields near the singularity and show that these can either
cause the universe to recollapse or, else, lead to a softer singularity at
which , , and remain finite while {\dddot H} and
higher derivatives of the Hubble parameter diverge. An important aspect of the
quiescent singularity is that it is encountered in regions of low density,
which has obvious implications for a universe consisting of a cosmic web of
high and low density regions -- superclusters and voids. In addition to vacuum
polarization, the effects of quantum particle production of non-conformal
fields are also likely to be important. A preliminary examination shows that
intense particle production can lead to an accelerating universe whose Hubble
parameter shows oscillations about a constant value.Comment: 19 pages, 3 figures, text slightly improved and references added.
Accepted for publication in Classical and Quantum Gravit
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