3,750 research outputs found
Crucial role of sidewalls in velocity distributions in quasi-2D granular gases
Our experiments and three-dimensional molecular dynamics simulations of
particles confined to a vertical monolayer by closely spaced frictional walls
(sidewalls) yield velocity distributions with non-Gaussian tails and a peak
near zero velocity. Simulations with frictionless sidewalls are not peaked.
Thus interactions between particles and their container are an important
determinant of the shape of the distribution and should be considered when
evaluating experiments on a tightly constrained monolayer of particles.Comment: 4 pages, 4 figures, Added reference, model explanation charified,
other minor change
X-ray diffraction from shock-loaded polycrystals
X-ray diffraction was demonstrated from shock-compressed polycrystalline
metal on nanosecond time scales. Laser ablation was used to induce shock waves
in polycrystalline foils of Be, 25 to 125 microns thick. A second laser pulse
was used to generate a plasma x-ray source by irradiation of a Ti foil. The
x-ray source was collimated to produce a beam of controllable diameter, and the
beam was directed at the Be sample. X-rays were diffracted from the sample, and
detected using films and x-ray streak cameras. The diffraction angle was
observed to change with shock pressure. The diffraction angles were consistent
with the uniaxial (elastic) and isotropic (plastic) compressions expected for
the loading conditions used. Polycrystalline diffraction will be used to
measure the response of the crystal lattice to high shock pressures and through
phase changes
A numerical investigation on the vortex formation and flow separation of the oscillatory flow in jet pumps
A two-dimensional computational fluid dynamics model is used to predict the
oscillatory flow through a tapered cylindrical tube section (jet pump) placed
in a larger outer tube. Due to the shape of the jet pump, there will exist an
asymmetry in the hydrodynamic end effects which will cause a time-averaged
pressure drop to occur that can be used to cancel Gedeon streaming in a
closed-loop thermoacoustic device. The performance of two jet pump geometries
with different taper angles is investigated. A specific time-domain impedance
boundary condition is implemented in order to simulate traveling acoustic wave
conditions. It is shown that by scaling the acoustic displacement amplitude to
the jet pump dimensions, similar minor losses are observed independent of the
jet pump geometry. Four different flow regimes are distinguished and the
observed flow phenomena are related to the jet pump performance. The simulated
jet pump performance is compared to an existing quasi-steady approximation
which is shown to only be valid for small displacement amplitudes compared to
the jet pump length.Comment: The following article has been accepted by the Journal of the
Acoustical Society of America. After it is published, it will be found at:
http://scitation.aip.org/JAS
Jet pumps for thermoacoustic applications: design guidelines based on a numerical parameter study
The oscillatory flow through tapered cylindrical tube sections (jet pumps) is
characterized by a numerical parameter study. The shape of a jet pump results
in asymmetric hydrodynamic end effects which cause a time-averaged pressure
drop to occur under oscillatory flow conditions. Hence, jet pumps are used as
streaming suppressors in closed-loop thermoacoustic devices. A two-dimensional
axisymmetric computational fluid dynamics model is used to calculate the
performance of a large number of conical jet pump geometries in terms of
time-averaged pressure drop and acoustic power dissipation. The investigated
geometrical parameters include the jet pump length, taper angle, waist diameter
and waist curvature. In correspondence with previous work, four flow regimes
are observed which characterize the jet pump performance and dimensionless
parameters are introduced to scale the performance of the various jet pump
geometries. The simulation results are compared to an existing quasi-steady
theory and it is shown that this theory is only applicable in a small operation
region. Based on the scaling parameters, an optimum operation region is defined
and design guidelines are proposed which can be directly used for future jet
pump design.Comment: The following article has been accepted by the Journal of the
Acoustical Society of America. After it is published, it will be found at
http://scitation.aip.org/JAS
Deviations from the mean field predictions for the phase behaviour of random copolymers melts
We investigate the phase behaviour of random copolymers melts via large scale
Monte Carlo simulations. We observe macrophase separation into A and B--rich
phases as predicted by mean field theory only for systems with a very large
correlation lambda of blocks along the polymer chains, far away from the
Lifshitz point. For smaller values of lambda, we find that a locally
segregated, disordered microemulsion--like structure gradually forms as the
temperature decreases. As we increase the number of blocks in the polymers, the
region of macrophase separation further shrinks. The results of our Monte Carlo
simulation are in agreement with a Ginzburg criterium, which suggests that mean
field theory becomes worse as the number of blocks in polymers increases.Comment: 6 pages, 4 figures, Late
Reducing car use amongst older drivers
Our work comprised a pilot study exploring potential means to support older people to reduce their car use. This group is under-represented in behaviour change research in transport, which often focuses on delaying the take up of driving or other critical stages in the life course such as having children. Indeed, research on older drivers is largely dominated by work exploring the potential negative impacts on their physical and mental health of driving cessation. Nonetheless, given the demands of the climate emergency and the scale of the requirement to reduce car use implied in any credible decarbonisation pathway, all sections of society will have to change their travel behaviour, at least to some extent. It is our contention that research into how this can be achieved for older drivers is not only a necessary component of informing wider car use reduction behaviour change strategies, but also that older age groups have a crucial role to play in signalling the need for change to others
First-order phase transition in a 2D random-field Ising model with conflicting dynamics
The effects of locally random magnetic fields are considered in a
nonequilibrium Ising model defined on a square lattice with nearest-neighbors
interactions. In order to generate the random magnetic fields, we have
considered random variables that change randomly with time according to
a double-gaussian probability distribution, which consists of two single
gaussian distributions, centered at and , with the same width
. This distribution is very general, and can recover in appropriate
limits the bimodal distribution () and the single gaussian one
(). We performed Monte Carlo simulations in lattices with linear sizes in
the range . The system exhibits ferromagnetic and paramagnetic
steady states. Our results suggest the occurence of first-order phase
transitions between the above-mentioned phases at low temperatures and large
random-field intensities , for some small values of the width .
By means of finite size scaling, we estimate the critical exponents in the
low-field region, where we have continuous phase transitions. In addition, we
show a sketch of the phase diagram of the model for some values of .Comment: 13 pages, 9 figures, accepted for publication in JSTA
Temperature dependence of binary and ternary recombination of H3+ ions with electron
We study binary and the recently discovered process of ternary He-assisted
recombination of H3+ ions with electrons in a low temperature afterglow plasma.
The experiments are carried out over a broad range of pressures and
temperatures of an afterglow plasma in a helium buffer gas. Binary and
He-assisted ternary recombination are observed and the corresponding
recombination rate coefficients are extracted for temperatures from 77 K to 330
K. We describe the observed ternary recombination as a two-step mechanism:
First, a rotationally-excited long-lived neutral molecule H3* is formed in
electron-H3+ collisions. Second, the H3* molecule collides with a helium atom
that leads to the formation of a very long-lived Rydberg state with high
orbital momentum. We present calculations of the lifetimes of H3* and of the
ternary recombination rate coefficients for para and ortho-H3+. The
calculations show a large difference between the ternary recombination rate
coefficients of ortho- and para-H3+ at temperatures below 300 K. The measured
binary and ternary rate coefficients are in reasonable agreement with the
calculated values.Comment: 15 page
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