272 research outputs found
Forward and inverse cascades in decaying two-dimensional electron magnetohydrodynamic turbulence
Electron magnetohydrodynamic (EMHD) turbulence in two dimensions is studied
via high-resolution numerical simulations with a normal diffusivity. The
resulting energy spectra asymptotically approach a law with
increasing , the ratio of the nonlinear to linear timescales in the
governing equation. No evidence is found of a dissipative cutoff, consistent
with non-local spectral energy transfer. Dissipative cutoffs found in previous
studies are explained as artificial effects of hyperdiffusivity. Relatively
stationary structures are found to develop in time, rather than the variability
found in ordinary or MHD turbulence. Further, EMHD turbulence displays
scale-dependent anisotropy with reduced energy transfer in the direction
parallel to the uniform background field, consistent with previous studies.
Finally, the governing equation is found to yield an inverse cascade, at least
partially transferring magnetic energy from small to large scales.Comment: 16 pages, 6 figures, accepted by Physics of Plasmas. For high
resolution figures, please visit the PoP website or contact C.Warein
An unstable superfluid Stewartson layer in a differentially rotating neutron star
Experimental and numerical evidence is reviewed for the existence of a
Stewartson layer in spherical Couette flow at small Ekman and Rossby numbers
(\Ek \lsim 10^{-3}, \Ro \lsim 10^{-2}), the relevant hydrodynamic regime in
the superfluid outer core of a neutron star. Numerical simulations of a
superfluid Stewartson layer are presented for the first time, showing how the
layer is disrupted by nonaxisymmetric instabilities. The unstable ranges of
\Ek and \Ro are compared with estimates of these quantities in radio
pulsars that exhibit glitches. It is found that glitching pulsars lie on the
stable side of the instability boundary, allowing differential rotation to
build up before a glitch.Comment: 4 pages, 3 figures. Accepted for publication in ApJ Letter
Experimental study of super-rotation in a magnetostrophic spherical Couette flow
We report measurements of electric potentials at the surface of a spherical
container of liquid sodium in which a magnetized inner core is differentially
rotating. The azimuthal angular velocities inferred from these potentials
reveal a strong super-rotation of the liquid sodium in the equatorial region,
for small differential rotation. Super-rotation was observed in numerical
simulations by Dormy et al. [1]. We find that the latitudinal variation of the
electric potentials in our experiments differs markedly from the predictions of
a similar numerical model, suggesting that some of the assumptions used in the
model - steadiness, equatorial symmetry, and linear treatment for the evolution
of both the magnetic and velocity fields - are violated in the experiments. In
addition, radial velocity measurements, using ultrasonic Doppler velocimetry,
provide evidence of oscillatory motion near the outer sphere at low latitude:
it is viewed as the signature of an instability of the super-rotating region
Cascades in decaying three-dimensional electron magnetohydrodynamic turbulence
Decaying electron magnetohydrodynamic (EMHD) turbulence in three dimensions
is studied via high-resolution numerical simulations. The resulting energy
spectra asymptotically approach a k^{-2} law with increasing R_B, the ratio of
the nonlinear to linear timescales in the governing equation, consistent with
theoretical predictions. No evidence is found of a dissipative cutoff,
consistent with non-local spectral energy transfer and recent studies of 2D
EMHD turbulence. Dissipative cutoffs found in previous studies are explained as
artificial effects of hyperdiffusivity. In another similarity to 2D EMHD
turbulence, relatively stationary structures are found to develop in time,
rather than the variability found in ordinary or MHD turbulence. Further,
cascades of energy in 3D EMHD turbulence are found to be suppressed in all
directions under the influence of a uniform background field. Energy transfer
is further reduced in the direction parallel to the field, displaying scale
dependent anisotropy. Finally, the governing equation is found to yield a weak
inverse cascade, at least partially transferring magnetic energy from small to
large scales.Comment: 16 pages, 7 figures, accepted to Journal of Plasma Physics. High
resolution figures available from the autho
Numerical Simulations of Dynamos Generated in Spherical Couette Flows
We numerically investigate the efficiency of a spherical Couette flow at
generating a self-sustained magnetic field. No dynamo action occurs for
axisymmetric flow while we always found a dynamo when non-axisymmetric
hydrodynamical instabilities are excited. Without rotation of the outer sphere,
typical critical magnetic Reynolds numbers are of the order of a few
thousands. They increase as the mechanical forcing imposed by the inner core on
the flow increases (Reynolds number ). Namely, no dynamo is found if the
magnetic Prandtl number is less than a critical value .
Oscillating quadrupolar dynamos are present in the vicinity of the dynamo
onset. Saturated magnetic fields obtained in supercritical regimes (either
or ) correspond to the equipartition between magnetic and
kinetic energies. A global rotation of the system (Ekman numbers ) yields to a slight decrease (factor 2) of the critical magnetic
Prandtl number, but we find a peculiar regime where dynamo action may be
obtained for relatively low magnetic Reynolds numbers (). In this
dynamical regime (Rossby number , spheres in opposite direction) at
a moderate Ekman number (), a enhanced shear layer around the inner
core might explain the decrease of the dynamo threshold. For lower
() this internal shear layer becomes unstable, leading to small
scales fluctuations, and the favorable dynamo regime is lost. We also model the
effect of ferromagnetic boundary conditions. Their presence have only a small
impact on the dynamo onset but clearly enhance the saturated magnetic field in
the ferromagnetic parts. Implications for experimental studies are discussed
Sheet-like and plume-like thermal flow in a spherical convection experiment performed under microgravity
We introduce, in spherical geometry, experiments on electro-hydrodynamic driven Rayleigh-BĂ©nard convection that have been performed for both temperature-independent (âGeoFlow I') and temperature-dependent fluid viscosity properties (âGeoFlow II') with a measured viscosity contrast up to 1.5. To set up a self-gravitating force field, we use a high-voltage potential between the inner and outer boundaries and a dielectric insulating liquid; the experiments were performed under microgravity conditions on the International Space Station. We further run numerical simulations in three-dimensional spherical geometry to reproduce the results obtained in the âGeoFlow' experiments. We use Wollaston prism shearing interferometry for flow visualization - an optical method producing fringe pattern images. The flow patterns differ between our two experiments. In âGeoFlow I', we see a sheet-like thermal flow. In this case convection patterns have been successfully reproduced by three-dimensional numerical simulations using two different and independently developed codes. In contrast, in âGeoFlow II', we obtain plume-like structures. Interestingly, numerical simulations do not yield this type of solution for the low viscosity contrast realized in the experiment. However, using a viscosity contrast of two orders of magnitude or higher, we can reproduce the patterns obtained in the âGeoFlow II' experiment, from which we conclude that nonlinear effects shift the effective viscosity rati
Full sphere hydrodynamic and dynamo benchmarks
Convection in planetary cores can generate fluid flow and magnetic fields, and a number of sophisticated codes exist to simulate the dynamic behaviour of such systems. We report on the first community activity to compare numerical results of computer codes designed to calculate fluid flow within a whole sphere. The flows are incompressible and rapidly rotating and the forcing of the flow is either due to thermal convection or due to moving boundaries. All problems defined have solutions that allow easy comparison, since they are either steady, slowly drifting or perfectly periodic. The first two benchmarks are defined based on uniform internal heating within the sphere under the Boussinesq approximation with boundary conditions that are uniform in temperature and stress-free for the flow. Benchmark 1 is purely hydrodynamic, and has a drifting solution. Benchmark 2 is a magnetohydrodynamic benchmark that can generate oscillatory, purely periodic, flows and magnetic fields. In contrast, Benchmark 3 is a hydrodynamic rotating bubble benchmark using no slip boundary conditions that has a stationary solution. Results from a variety of types of code are reported, including codes that are fully spectral (based on spherical harmonic expansions in angular coordinates and polynomial expansions in radius), mixed spectral and finite difference, finite volume, finite element and also a mixed Fourierâfinite element code. There is good agreement between codes. It is found that in Benchmarks 1 and 2, the approximation of a whole sphere problem by a domain that is a spherical shell (a sphere possessing an inner core) does not represent an adequate approximation to the system, since the results differ from whole sphere results
Symmetric and asymmetric action integration during cooperative object manipulation in virtual environments
Cooperation between multiple users in a virtual environment (VE) can take place at one of three levels. These
are defined as where users can perceive each other (Level 1), individually change the scene (Level 2), or
simultaneously act on and manipulate the same object (Level 3). Despite representing the highest level of
cooperation, multi-user object manipulation has rarely been studied. This paper describes a behavioral
experiment in which the piano movers' problem (maneuvering a large object through a restricted space) was
used to investigate object manipulation by pairs of participants in a VE. Participants' interactions with the object
were integrated together either symmetrically or asymmetrically. The former only allowed the common
component of participants' actions to take place, but the latter used the mean. Symmetric action integration was
superior for sections of the task when both participants had to perform similar actions, but if participants had to
move in different ways (e.g., one maneuvering themselves through a narrow opening while the other traveled
down a wide corridor) then asymmetric integration was superior. With both forms of integration, the extent to
which participants coordinated their actions was poor and this led to a substantial cooperation overhead (the
reduction in performance caused by having to cooperate with another person)
Cetuximab plus gemcitabine/oxaliplatin (GEMOXCET) in first-line metastatic pancreatic cancer: a multicentre phase II study
Targeting the epidermal growth factor receptor pathway in pancreatic cancer seems to be an attractive therapeutic approach. This study assessed the efficacy of cetuximab plus the combination of gemcitabine/oxaliplatin in metastatic pancreatic cancer. Eligible subjects had histological or cytological diagnosis of metastatic pancreatic adenocarcinoma. The primary end point was response according to RECIST. Patients received cetuximab 400âmgâmâ2 at first infusion followed by weekly 250âmgâmâ2 combined with gemcitabine 1000âmgâmâ2 as a 100âmin infusion on day 1 and oxaliplatin 100âmgâmâ2 as a 2-h infusion on day 2 every 2 weeks. Between January 2005 and August 2006, a total of 64 patients (22 women (34%), 42 men (66%); median age 64 years (range 31â78)) were enrolled at seven study centres. On October 2007, a total of 17 patients were alive. Sixty-two patients were evaluable for baseline and 61 for assessment of response to treatment in an intention-to-treat analysis. Six patients had an incomplete drug combination within the first cycle of the treatment plan (n=4 hypersensitivity reactions to the first cetuximab infusion, n=2 refused to continue therapy). Reported grade 3/4 toxicities (% of patients) were leukopaenia 15%, anaemia 8%, thrombocytopaenia 10%, diarrhoea 7%, nausea 18%, infection 18% and allergy 7%. Cetuximab-attributable skin reactions occurred as follows: grade 0: 20%, grade 1: 41%, grade 2: 30% and grade 3: 10%. The intention-to-treat analysis of 61 evaluable patients showed an overall response rate of 33%, including 1 (2%) complete and 19 (31%) partial remissions. There were 31% patients with stable and 36% with progressive disease or discontinuation of the therapy before re-staging. The presence of a grade 2 or higher skin rash was associated with a higher likelihood of achieving objective response. Median time to progression was 118 days, with a median overall survival of 213 days. A clinical benefit response was noted in 24 of the evaluable 61 patients (39%). The addition of cetuximab to the combination of gemcitabine and oxaliplatin is well tolerated but does not increase response or survival in patients with metastatic pancreatic cancer
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