5,766 research outputs found
Dynamical Models of Extreme Rolling of Vessels in Head Waves
Rolling of a ship is a swinging motion around its length axis. In particular vessels transporting containers may show large amplitude roll when sailing in seas with large head waves. The dynamics of the ship is such that rolling interacts with heave being the motion of the mass point of the ship in vertical direction. Due to the shape of the hull of the vessel its heave is influenced considerably by the phase of the wave as it passes the ship. The interaction of heave and roll can be modeled by a mass-spring-pendulum system. The effect of waves is then included in the system by a periodic forcing term. In first instance the damping of the spring can be taken infinitely large making the system a pendulum with an in vertical direction periodically moving suspension. For a small angular deflection the roll motion is then described by the Mathieu equation containing a periodic forcing. If the period of the solution of the equation without forcing is about twice the period of the forcing then the oscillation gets unstable and the amplitude starts to grow. After describing this model we turn to situation that the ship is not anymore statically fixed at the fluctuating water level. It may move up and down showing a motion
modeled by a damped spring. One step further we also allow for pitch, a swinging motion around a horizontal axis perpendicular to the ship. It is recommended to investigate the way waves may directly drive this mode and to determine the amount of energy that flows along this path towards the roll mode. Since at sea waves are a superposition of waves with different wavelengths, we also pay attention to the properties of such a type of forcing containing stochastic elements. It is recommended that as a measure for the
occurrence of large deflections of the roll angle one should take the expected time for which a given large deflection may occur instead of the mean amplitude of the deflection
Microscopic theory of solvent mediated long range forces: influence of wetting
We show that a general density functional approach for calculating the force
between two big particles immersed in a solvent of smaller ones can describe
systems that exhibit fluid-fluid phase separation: the theory captures effects
of strong adsorption (wetting) and of critical fluctuations in the solvent. We
illustrate the approach for the Gaussian core model, a simple model of a
polymer mixture in solution and find extremely attractive, long ranged solvent
mediated potentials between the big particles for state points lying close to
the binodal, on the side where the solvent is poor in the species which is
favoured by the big particles.Comment: 7 pages, 3 figures, submitted to Europhysics Letter
Intrinsic point defects and volume swelling in ZrSiO4 under irradiation
The effects of high concentration of point defects in crystalline ZrSiO4 as
originated by exposure to radiation, have been simulated using first principles
density functional calculations. Structural relaxation and vibrational studies
were performed for a catalogue of intrinsic point defects, with different
charge states and concentrations. The experimental evidence of a large
anisotropic volume swelling in natural and artificially irradiated samples is
used to select the subset of defects that give similar lattice swelling for the
concentrations studied, namely interstitials of O and Si, and the anti-site
Zr(Si), Calculated vibrational spectra for the interstitials show additional
evidence for the presence of high concentrations of some of these defects in
irradiated zircon.Comment: 9 pages, 7 (color) figure
Phase separation in fluids exposed to spatially periodic external fields
We consider the liquid-vapor type phase transition for fluids confined within
spatially periodic external fields. For a fluid in d=3 dimensions, the periodic
field induces an additional phase, characterized by large density modulations
along the field direction. At the triple point, all three phases (modulated,
vapor, and liquid) coexist. At temperatures slightly above the triple point and
for low (high) values of the chemical potential, two-phase coexistence between
the modulated phase and the vapor (liquid) is observed. We study this
phenomenon using computer simulations and mean-field theory for the Ising
model. The theory shows that, in order for the modulated phase to arise, the
field wavelength must exceed a threshold value. We also find an extremely low
tension of the interface between the modulated phase and the vapor/liquid
phases. The tension is of the order 10^{-4} kB T per squared lattice spacing,
where kB is the Boltzmann constant, and T the temperature. In order to detect
such low tensions, a new simulation method is proposed. We also consider the
case of d=2 dimensions. The modulated phase then does not survive, leading to a
radically different phase diagram.Comment: 11 pages, 14 figure
Generation of defects and disorder from deeply quenching a liquid to form a solid
We show how deeply quenching a liquid to temperatures where it is linearly
unstable and the crystal is the equilibrium phase often produces crystalline
structures with defects and disorder. As the solid phase advances into the
liquid phase, the modulations in the density distribution created behind the
advancing solidification front do not necessarily have a wavelength that is the
same as the equilibrium crystal lattice spacing. This is because in a deep
enough quench the front propagation is governed by linear processes, but the
crystal lattice spacing is determined by nonlinear terms. The wavelength
mismatch can result in significant disorder behind the front that may or may
not persist in the latter stage dynamics. We support these observations by
presenting results from dynamical density functional theory calculations for
simple one- and two-component two-dimensional systems of soft core particles.Comment: 25 pages, 11 figure
Thermodynamically consistent description of the hydrodynamics of free surfaces covered by insoluble surfactants of high concentration
In this paper we propose several models that describe the dynamics of liquid
films which are covered by a high concentration layer of insoluble surfactant.
First, we briefly review the 'classical' hydrodynamic form of the coupled
evolution equations for the film height and surfactant concentration that are
well established for small concentrations. Then we re-formulate the basic model
as a gradient dynamics based on an underlying free energy functional that
accounts for wettability and capillarity. Based on this re-formulation in the
framework of nonequilibrium thermodynamics, we propose extensions of the basic
hydrodynamic model that account for (i) nonlinear equations of state, (ii)
surfactant-dependent wettability, (iii) surfactant phase transitions, and (iv)
substrate-mediated condensation. In passing, we discuss important differences
to most of the models found in the literature.Comment: 31 pages, 2 figure
Density functional approach for inhomogeneous star polymers
We propose microscopic density functional theory for inhomogeneous star
polymers. Our approach is based on fundamental measure theory for hard spheres,
and on Wertheim's first- and second-order perturbation theory for the
interparticle connectivity. For simplicity we consider a model in which all the
arms are of the same length, but our approach can be easily extended to the
case of stars with arms of arbitrary lengths.Comment: 4 pages, 3 figures, submitte
Analysis of circadian pattern reveals tissue-specific alternative transcription in leptin signaling pathway
*Background*
It has been previously reported that most mammalian genes display a circadian oscillation in their baseline expression. Consequently, the phase and amplitude of each component of a signal transduction cascade has downstream consequences. 

*Results*
We report our analysis of alternative transcripts in the leptin signaling pathway which is responsible for the systemic regulation of macronutrient storage and energy balance. We focused on the circadian expression pattern of a critical component of the leptin signaling system, suppressor of cytokine signaling 3 (SOCS3). On an Affymetrix GeneChip 430A2 microarray, this gene is represented by three probe sets targeting different regions within the 3’ end of the last exon. We demonstrate that in murine brown adipose tissue two downstream 3’ probe sets experience circadian baseline oscillation in counter-phase to the upstream probe set. Such differences in expression patterns are a telltale sign of alternative splicing within the last exon of SOCS3. In contrast, all three probe sets oscillated in a common phase in murine liver and white adipose tissue. This suggests that the regulation of SOCS3 expression in brown fat is tissue specific. Another component of the signaling pathway, Janus kinase (JAK), is directly regulated by SOCS and has alternative transcript probe sets oscillating in counter-phase in a white adipose tissue specific manner.
 
*Conclusion*
We hypothesize that differential oscillation of alternative transcripts may provide a mechanism to maintain steady levels of expression in spite of circadian baseline variation
- …