5,052 research outputs found
Global MHD simulations of stratified and turbulent protoplanetary discs. I. Model properties
We present the results of global 3-D MHD simulations of stratified and
turbulent protoplanetary disc models. The aim of this work is to develop thin
disc models capable of sustaining turbulence for long run times, which can be
used for on-going studies of planet formation in turbulent discs. The results
are obtained using two codes written in spherical coordinates: GLOBAL and
NIRVANA. Both are time--explicit and use finite differences along with the
Constrained Transport algorithm to evolve the equations of MHD. In the presence
of a weak toroidal magnetic field, a thin protoplanetary disc in hydrostatic
equilibrium is destabilised by the magnetorotational instability (MRI). When
the resolution is large enough (25 vertical grid cells per scale height), the
entire disc settles into a turbulent quasi steady-state after about 300 orbits.
Angular momentum is transported outward such that the standard alpha parameter
is roughly 4-6*10^{-3}. We find that the initial toroidal flux is expelled from
the disc midplane and that the disc behaves essentially as a quasi-zero net
flux disc for the remainder of the simulation. As in previous studies, the disc
develops a dual structure composed of an MRI--driven turbulent core around its
midplane, and a magnetised corona stable to the MRI near its surface. By
varying disc parameters and boundary conditions, we show that these basic
properties of the models are robust. The high resolution disc models we present
in this paper achieve a quasi--steady state and sustain turbulence for hundreds
of orbits. As such, they are ideally suited to the study of outstanding
problems in planet formation such as disc--planet interactions and dust
dynamics.Comment: 19 pages, 29 figures, accepted in Astronomy & Astrophysic
MHD simulations of the magnetorotational instability in a shearing box with zero net flux. I. The issue of convergence
We study the properties of MHD turbulence driven by the magnetorotational
instability (MRI) in accretion disks. We adopt the local shearing box model and
focus on the special case for which the initial magnetic flux threading the
disk vanishes. We employ the finite difference code ZEUS to evolve the ideal
MHD equations. Performing a set of numerical simulations in a fixed
computational domain with increasing resolution, we demonstrate that turbulent
activity decreases as resolution increases. We quantify the turbulent activity
by measuring the rate of angular momentum transport through evaluating the
standard alpha parameter. We find alpha=0.004 when (N_x,N_y,N_z)=(64,100,64),
alpha=0.002 when (N_x,N_y,N_z)=(128,200,128) and alpha=0.001 when
(N_x,N_y,N_z)=(256,400,256). This steady decline is an indication that
numerical dissipation, occurring at the grid scale is an important determinant
of the saturated form of the MHD turbulence. Analysing the results in Fourier
space, we demonstrate that this is due to the MRI forcing significant flow
energy all the way down to the grid dissipation scale. We also use our results
to study the properties of the numerical dissipation in ZEUS. Its amplitude is
characterised by the magnitude of an effective magnetic Reynolds number Re_M
which increases from 10^4 to 10^5 as the number of grid points is increased
from 64 to 256 per scale height. The simulations we have carried out do not
produce results that are independent of the numerical dissipation scale, even
at the highest resolution studied. Thus it is important to use physical
dissipation, both viscous and resistive, and to quantify contributions from
numerical effects, when performing numerical simulations of MHD turbulence with
zero net flux in accretion disks at the resolutions normally considered.Comment: 10 pages, 15 figures, accepted in A&A. Numerical results improved,
various numerical issues addressed (boundary conditions, box size, run
durations
Doing sociomaterial studies: the circuit of agency
In recent sociomaterialist, materialist and post-human theorizing which foregrounds the importance of objects and bodies, ideas of consciousness and intentionality are seen as potentially tainted either with Cartesian mind-body splits or with subjectivities that are too discursively constructed. At the same time, new theories of affect as something pre-personal and corporeal further marginalize the notion of human agency. But could the pendulum have swung too far in outlawing the human in favour of the pre-human and post-human? How can sociomaterial theories be reconciled with educators’ ongoing commitment to give their pupils voice and identify effective pedagogies for teaching digital media? This paper analyses data from a study of online multimodal writing practices in a London primary school to expand current theorizing about agency. It proposes the idea of a phenomenologically-inspired circuit of (sociomaterial) agency as a way to bring back the ‘human’ and incorporate the middle ranges of agency
Tight focal spots using azimuthally polarised light from a Fresnel cone
When focusing a light beam at high numerical aperture, the resulting electric
field profile in the focal plane depends on the transverse polarisation
profile, as interference between different parts of the beam needs to be taken
into account. It is well known that radial polarised light produces a
longitudinal polarisation component and can be focused below the conventional
diffraction limit for homogeneously polarised light, and azimuthally polarised
light that carries one unit of angular momentum can achieve even tighter focal
spots. This is of interest for example for enhancing resolution in scanning
microscopy. There are numerous ways to generate such polarisation structures,
however, setups can be expensive and usually rely on birefringent components,
hence prohibiting broadband operation. We have recently demonstrated a passive,
low-cost technique using a simple glass cone (Fresnel cone) to generate beams
with structured polarisation. We show here that the polarisation structure
generated by Fresnel cones focuses better than radial polarised light at all
numerical apertures. Furthermore, we investigate in detail the application of
polarised light structures for two-photon microscopy. Specifically we
demonstrate a method that allows us to generate the desired polarisation
structure at the back aperture of the microscope by pre-compensating any
detrimental phase shifts using a combination of waveplates
Attitudes to telecare among older people, professional care workers and informal carers: a preventative strategy or crisis management?
This paper reports findings from an attitudinal survey towards telecare that emerged from twenty-two focus groups comprising ninety-two older people, fifty-five professional stakeholders and thirty-nine carers. These were convened in three different regions of England as a precursor to telecare service development. The results from this study suggest that informants’ views were shaped by prior knowledge of conventional health and social care delivery in their locality and the implication is that expectations and requirements in respect of telecare services in general are likely to be informed by wider perceptions about the extent to which community care should operate as a preventative strategy or as a mechanism for crisis management
MHD simulations of the magnetorotational instability in a shearing box with zero net flux. II. The effect of transport coefficients
We study the influence of the choice of transport coefficients (viscosity and
resistivity) on MHD turbulence driven by the magnetorotational instability
(MRI) in accretion disks. We follow the methodology described in paper I: we
adopt an unstratified shearing box model and focus on the case where the net
vertical magnetic flux threading the box vanishes. For the most part we use the
finite difference code ZEUS, including explicit transport coefficients in the
calculations. However, we also compare our results with those obtained using
other algorithms (NIRVANA, the PENCIL code and a spectral code) to demonstrate
both the convergence of our results and their independence of the numerical
scheme. We find that small scale dissipation affects the saturated state of MHD
turbulence. In agreement with recent similar numerical simulations done in the
presence of a net vertical magnetic flux, we find that turbulent activity
(measured by the rate of angular momentum transport) is an increasing function
of the magnetic Prandtl number Pm for all values of the Reynolds number Re that
we investigated. We also found that turbulence disappears when the Prandtl
number falls below a critical value Pm_c that is apparently a decreasing
function of Re. For the limited region of parameter space that can be probed
with current computational resources, we always obtained Pm_c>1. We conclude
that the magnitudes of the transport coefficients are important in determining
the properties of MHD turbulence in numerical simulations in the shearing box
with zero net flux, at least for Reynolds numbers and magnetic Prandtl numbers
that are such that transport is not dominated by numerical effects and thus can
be probed using current computational resources.Comment: 10 pages, 13 figures, accepted in A&A. Numerical results improved,
minor changes in the tex
Effects of spaced retrieval on memory and quality of life in older adults with probable Alzheimer\u27s disease
This study was designed to explore the effects of spaced-retrieval training on memory and quality of life in older adults with probable Alzheimer’s disease. A total nineteen older adults with probable Alzheimer’s disease (AD) participated in these experiments. Experiment one was designed to determine whether the spacing effect is contributing to the success of the spaced-retrieval intervention. Participants were trained to recall a name-face association using either the adjusted spaced-retrieval method or a fixed interval retrieval method. The results showed a more consistent performance profile for the spaced retrieval group in comparison to the fixed interval group, providing evidence that the spacing effect is contributing to the gains in memory associated with spaced retrieval. Experiment two explored the effect of supplemental training sessions, ‘booster sessions’, on long term retention of the name-face association at a six month retest after the initial spaced retrieval training. The findings show booster sessions enhanced the long term effectiveness of the intervention, particularly during the first retest session. Experiment three examined the flexibility of the spaced retrieval method for use with familiar name-face associations. Subjects were trained on a familiar name-face association using the same methodology previously used for a non-familiar name-face association. Results indicated that the intervention could be useful in training familiar name-face associations. In addition to these specific experiments, quality of life was measured for all participants in each experiment prior to and after receiving the spaced retrieval intervention. Results provide preliminary evidence of a link between performance on the spaced retrieval task and improved rankings of quality of life
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