1,164 research outputs found
Modelling of dielectric barrier discharge plasma actuators for direct numerical simulations
In recent years the development of devices known as plasma actuators has advanced the promise of controlling flows in new ways that increase lift, reduce drag and improve aerodynamic efficiencies; advances that may lead to safer, more efficient and quieter aircraft. The large number of parameters (location of the actuator, orientation, size, relative placement of the embedded and exposed electrodes, materials, applied voltage, frequency) affecting the performance of plasma actuators makes their development, testing and optimisation a very complicated task. Several approaches have been proposed for developing numerical models for plasma actuators. The discharge can be modelled by physics-based kinetic methods based on first principles, by semi-empirical phenomenological approaches and by PIV-based methods where the discharge is replaced by a steady-state body force. The latter approach receives a recent interest for its easy implementation in RANS and U-RANS solvers. Here, a forcing term extracted from experiments is implemented into our high-order Navier-Stokes solver (DNS) in order to evaluate its robustness and ability to mimic the effects of a surface dielectric barrier discharge. This experimental forcing term is compared to the numerical forcing term developed by Suzen & Huang (1, 2) with an emphasis on the importance of the wall-normal component of each model
Liquid Xenon Detectors for Positron Emission Tomography
PET is a functional imaging technique based on detection of annihilation
photons following beta decay producing positrons. In this paper, we present the
concept of a new PET system for preclinical applications consisting of a ring
of twelve time projection chambers filled with liquid xenon viewed by avalanche
photodiodes. Simultaneous measurement of ionization charge and scintillation
light leads to a significant improvement to spatial resolution, image quality,
and sensitivity. Simulated performance shows that an energy resolution of <10%
(FWHM) and a sensitivity of 15% are achievable. First tests with a prototype
TPC indicate position resolution <1 mm (FWHM).Comment: Paper presented at the International Nuclear Physics Conference,
Vancouver, Canada, 201
School violence, school differences and school discourses
This article highlights one strand of a study which investigated the concept of the violenceresilient school. In six inner-city secondary schools, data on violent incidents in school and violent crime in the neighbourhood were gathered, and compared with school practices to minimise violence, accessed through interviews. Some degree of association between the patterns of behaviour and school practices was found: schools with a wider range of wellconnected practices seemed to have less difficult behaviour. Interviews also showed that the different schools had different organisational discourses for construing school violence, its possible causes and the possible solutions. Differences in practices are best understood in connection with differences in these discourses. Some of the features of school discourses are outlined, including their range, their core metaphor and their silences. We suggest that organisational discourse is an important concept in explaining school effects and school differences, and that improvement attempts could have clearer regard to this concept
Weak ferromagnetism with very large canting in a chiral lattice: (pyrimidine)2FeCl2
The transition metal coordination compound (pyrimidine)2FeCl2 crystallizes in
a chiral lattice, space group I 4_1 2 2 (or I4_3 2 2). Combined magnetization,
Mossbauer spectroscopy and powder neutron diffraction studies reveal that it is
a canted antiferromagnet below T_N = 6.4 K with an unusually large canting of
the magnetic moments of 14 deg. from their general antiferromagnetic alignment,
one of the largest reported to date. This results in weak ferromagnetism with a
ferromagnetic component of 1 mu_B. The large canting is due to the interplay
between the antiferromagnetic exchange interaction and the local single-ion
anisotropy in the chiral lattice. The magnetically ordered structure of
(pyrimidine)2FeCl2, however, is not chiral. The implications of these findings
for the search of molecule based materials exhibiting chiral magnetic ordering
is discussed.Comment: 6 pages, 5 figure
Environmental assessment of the behavior of a BOF steel slag used in road construction : the PRECODD-ECLAIR research program
International audienceSteel production generate great amounts of by-products as steel slags. The use of Basic Oxygen Furnace slags (BOF slags) has been restrained due to insufficient volume stability, and due to the lack of environmental regulations. The purpose of the PRECODD-ECLAIR research program is to develop a behavior model based on a multi-scale physico-chemical, mechanical, hydrodynamic and ecotoxicological characterizations of a BOF slag used in a public works scenario. This paper aims at presenting the overall ECLAIR research program, the equipped experimental platform constructed using a BOF steel slag, and the first results of the slag characterization
Dynamics of Transformation from Segregation to Mixed Wealth Cities
We model the dynamics of the Schelling model for agents described simply by a
continuously distributed variable - wealth. Agents move to neighborhoods where
their wealth is not lesser than that of some proportion of their neighbors, the
threshold level. As in the case of the classic Schelling model where
segregation obtains between two races, we find here that wealth-based
segregation occurs and persists. However, introducing uncertainty into the
decision to move - that is, with some probability, if agents are allowed to
move even though the threshold level condition is contravened - we find that
even for small proportions of such disallowed moves, the dynamics no longer
yield segregation but instead sharply transition into a persistent mixed wealth
distribution. We investigate the nature of this sharp transformation between
segregated and mixed states, and find that it is because of a non-linear
relationship between allowed moves and disallowed moves. For small increases in
disallowed moves, there is a rapid corresponding increase in allowed moves, but
this tapers off as the fraction of disallowed moves increase further and
finally settles at a stable value, remaining invariant to any further increase
in disallowed moves. It is the overall effect of the dynamics in the initial
region (with small numbers of disallowed moves) that shifts the system away
from a state of segregation rapidly to a mixed wealth state.
The contravention of the tolerance condition could be interpreted as public
policy interventions like minimal levels of social housing or housing benefit
transfers to poorer households. Our finding therefore suggests that it might
require only very limited levels of such public intervention - just sufficient
to enable a small fraction of disallowed moves, because the dynamics generated
by such moves could spur the transformation from a segregated to mixed
equilibrium.Comment: 12 pages, 7 figure
Multi-particle effects in non-equilibrium electron tunnelling and field emission
We investigate energy resolved electric current from various correlated host
materials under out-of-equilibrium conditions. We find that, due to a combined
effect of electron-electron interactions, non-equilibrium and multi-particle
tunnelling, the energy resolved current is finite even above the Fermi edge of
the host material. In most cases, the current density possesses a singularity
at the Fermi level revealing novel manifestations of correlation effects in
electron tunnelling. By means of the Keldysh non-equilibrium technique, the
current density is calculated for one-dimensional interacting electron systems
and for two-dimensional systems, both in the pure limit and in the presence of
disorder. We then specialise to the field emission and provide a comprehensive
theoretical study of this effect in carbon nanotubes.Comment: 22 pages, 8 figures (eps files
Continuous symmetry of C60 fullerene and its derivatives
Conventionally, the Ih symmetry of fullerene C60 is accepted which is
supported by numerous calculations. However, this conclusion results from the
consideration of the molecule electron system, of its odd electrons in
particular, in a close-shell approximation without taking the electron spin
into account. Passing to the open-shell approximation has lead to both the
energy and the symmetry lowering up to Ci. Seemingly contradicting to a
high-symmetry pattern of experimental recording, particularly concerning the
molecule electronic spectra, the finding is considered in the current paper
from the continuous symmetry viewpoint. Exploiting both continuous symmetry
measure and continuous symmetry content, was shown that formal Ci symmetry of
the molecule is by 99.99% Ih. A similar continuous symmetry analysis of the
fullerene monoderivatives gives a reasonable explanation of a large variety of
their optical spectra patterns within the framework of the same C1 formal
symmetry exhibiting a strong stability of the C60 skeleton.Comment: 11 pages. 5 figures. 6 table
Polaron and bipolaron formation in the Hubbard-Holstein model: role of next-nearest neighbor electron hopping
The influence of next-nearest neighbor electron hopping, , on the
polaron and bipolaron formation in a square Hubbard-Holstein model is
investigated within a variational approach. The results for electron-phonon and
electron-electron correlation functions show that a negative value of
induces a strong anisotropy in the lattice distortions favoring
the formation of nearest neighbor intersite bipolaron. The role of
, electron-phonon and electron-electron interactions is briefly
discussed in view of the formation of charged striped domains.Comment: 4 figure
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