6,343 research outputs found
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 âreal-worldâ 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
SPH Simulations of Galactic Gaseous Disk with Bar: Distribution and Kinematic Structure of Molecular Clouds toward the Galactic Center
We have performed Smoothed Particle Hydrodynamic (SPH) simulations to study
the response of molecular clouds in the Galactic disk to a rotating bar and
their subsequent evolution in the Galactic Center (GC) region. The Galactic
potential in our models is contributed by three axisymmetric components
(massive halo, exponential disk, compact bulge) and a non-axisymmetric bar.
These components are assumed to be invariant in time in the frame corotating
with the bar. Some noticeable features such as an elliptical outer ring, spiral
arms, a gas-depletion region, and a central concentration have been developed
due to the influence of the bar. The rotating bar induces non-circular motions
of the SPH particles, but hydrodynamic collisions tend to suppress the random
components of the velocity. The velocity field of the SPH particles is
consistent with the kinematics of molecular clouds observed in HCN (1-0)
transition; these clouds are thought to be very dense clouds. However, the l-v
diagram of the clouds traced by CO is quite different from that of our SPH
simulation, being more similar to that obtained from simulations using
collisionless particles. The diagram of a mixture of collisional and
collisionless particles gives better reproduction of the kinematic structures
of the GC clouds observed in the CO line. The fact that the kinematics of HCN
clouds can be reproduced by the SPH particles suggests that the dense clouds in
the GC are formed via cloud collisions induced by rotating bar.Comment: 31 pages, 10 pigures, accepted for publication in Ap
A model for the orientational ordering of the plant microtubule cortical array
The plant microtubule cortical array is a striking feature of all growing
plant cells. It consists of a more or less homogeneously distributed array of
highly aligned microtubules connected to the inner side of the plasma membrane
and oriented transversely to the cell growth axis. Here we formulate a
continuum model to describe the origin of orientational order in such confined
arrays of dynamical microtubules. The model is based on recent experimental
observations that show that a growing cortical microtubule can interact through
angle dependent collisions with pre-existing microtubules that can lead either
to co-alignment of the growth, retraction through catastrophe induction or
crossing over the encountered microtubule. We identify a single control
parameter, which is fully determined by the nucleation rate and intrinsic
dynamics of individual microtubules. We solve the model analytically in the
stationary isotropic phase, discuss the limits of stability of this isotropic
phase, and explicitly solve for the ordered stationary states in a simplified
version of the model.Comment: 15 pages, 5 figure
The properties of the Galactic bar implied by gas kinematics in the inner Milky Way
Longitude-velocity (l-V) diagrams of H I and CO gas in the inner Milky Way
have long been known to be inconsistent with circular motion in an axisymmetric
potential. Several lines of evidence suggest that the Galaxy is barred, and gas
flow in a barred potential could be consistent with the observed ``forbidden''
velocities and other features in the data. We compare the H I observations to
l-V diagrams synthesized from 2-D fluid dynamical simulations of gas flows in a
family of barred potentials. The gas flow pattern is very sensitive to the
parameters of the assumed potential, which allows us to discriminate among
models. We present a model that reproduces the outer contour of the H I l-V
diagram reasonably well; this model has a strong bar with a semimajor axis of
3.6 kpc, an axis ratio of approximately 3:1, an inner Lindblad resonance (ILR),
and a pattern speed of 42 km/s/kpc, and matches the data best when viewed from
34\deg to the bar major axis. The behavior of the models, combined with the
constraint that the shocks in the Milky Way bar should resemble those in
external barred galaxies, leads us to conclude that wide ranges of parameter
space are incompatible with the observations. In particular we suggest that the
bar must be fairly strong, must have an ILR, and cannot be too end-on, with the
bar major axis at 35\deg +/- 5\deg to the line of sight. The H I data exhibit
larger forbidden velocities over a wider longitude range than are seen in
molecular gas; this important difference is the reason our favored model
differs so significantly from other recently proposed models.Comment: 23 pages, 14 figures, 1 table, uses emulateapj and psfig, 640 kb.
Submitted to Ap
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