2,533 research outputs found
Universal neural field computation
Turing machines and G\"odel numbers are important pillars of the theory of
computation. Thus, any computational architecture needs to show how it could
relate to Turing machines and how stable implementations of Turing computation
are possible. In this chapter, we implement universal Turing computation in a
neural field environment. To this end, we employ the canonical symbologram
representation of a Turing machine obtained from a G\"odel encoding of its
symbolic repertoire and generalized shifts. The resulting nonlinear dynamical
automaton (NDA) is a piecewise affine-linear map acting on the unit square that
is partitioned into rectangular domains. Instead of looking at point dynamics
in phase space, we then consider functional dynamics of probability
distributions functions (p.d.f.s) over phase space. This is generally described
by a Frobenius-Perron integral transformation that can be regarded as a neural
field equation over the unit square as feature space of a dynamic field theory
(DFT). Solving the Frobenius-Perron equation yields that uniform p.d.f.s with
rectangular support are mapped onto uniform p.d.f.s with rectangular support,
again. We call the resulting representation \emph{dynamic field automaton}.Comment: 21 pages; 6 figures. arXiv admin note: text overlap with
arXiv:1204.546
Fingering convection induced by atomic diffusion in stars: 3D numerical computations and applications to stellar models
Iron-rich layers are known to form in the stellar subsurface through a
combination of gravitational settling and radiative levitation. Their presence,
nature and detailed structure can affect the excitation process of various
stellar pulsation modes, and must therefore be modeled carefully in order to
better interpret Kepler asteroseismic data. In this paper, we study the
interplay between atomic diffusion and fingering convection in A-type stars,
and its role in the establishment and evolution of iron accumulation layers. To
do so, we use a combination of three-dimensional idealized numerical
simulations of fingering convection, and one-dimensional realistic stellar
models. Using the three-dimensional simulations, we first validate the mixing
prescription for fingering convection recently proposed by Brown et al. (2013),
and identify what system parameters (total mass of iron, iron diffusivity,
thermal diffusivity, etc.) play a role in the overall evolution of the layer.
We then implement the Brown et al. (2013) prescription in the Toulouse-Geneva
Evolution code to study the evolution of the iron abundance profile beneath the
stellar surface. We find, as first discussed by Th\'eado et al. (2009), that
when the concurrent settling of helium is ignored, this accumulation rapidly
causes an inversion in the mean molecular weight profile, which then drives
fingering convection. The latter mixes iron with the surrounding material very
efficiently, and the resulting iron layer is very weak. However, taking helium
settling into account partially stabilizes the iron profile against fingering
convection, and a large iron overabundance can accumulate. The opacity also
increases significantly as a result, and in some cases ultimately triggers
dynamical convection.Comment: 38 pages, 16 figures, submitted to Ap
The Formation and Coarsening of the Concertina Pattern
The concertina is a magnetization pattern in elongated thin-film elements of
a soft material. It is a ubiquitous domain pattern that occurs in the process
of magnetization reversal in direction of the long axis of the small element.
Van den Berg argued that this pattern grows out of the flux closure domains as
the external field is reduced. Based on experimental observations and theory,
we argue that in sufficiently elongated thin-film elements, the concertina
pattern rather bifurcates from an oscillatory buckling mode. Using a reduced
model derived by asymptotic analysis and investigated by numerical simulation,
we quantitatively predict the average period of the concertina pattern and
qualitatively predict its hysteresis. In particular, we argue that the
experimentally observed coarsening of the concertina pattern is due to
secondary bifurcations related to an Eckhaus instability. We also link the
concertina pattern to the magnetization ripple and discuss the effect of a weak
(crystalline or induced) anisotropy
Threshold Curve for the Excitability of Bidimensional Spiking Neurons
International audienceWe shed light on the threshold for spike initiation in two-dimensional neuron models. A threshold criterion that depends on both the membrane voltage and the recovery variable is proposed. This approach provides a simple and unified framework that accounts for numerous voltage threshold properties including adaptation, variability and time-dependent dynamics. In addition, neural features such as accommodation, inhibition-induced spike, and post-inhibitory (-excitatory) facilitation are the direct consequences of the existence of a threshold curve. Implications for neural modeling are also discussed
Experimental assessment of drag reduction by traveling waves in a turbulent pipe flow
We experimentally assess the capabilities of an active, open-loop technique
for drag reduction in turbulent wall flows recently introduced by Quadrio et
al. [J. Fluid Mech., v.627, 161, (2009)]. The technique consists in generating
streamwise-modulated waves of spanwise velocity at the wall, that travel in the
streamwise direction.
A proof-of-principle experiment has been devised to measure the reduction of
turbulent friction in a pipe flow, in which the wall is subdivided into thin
slabs that rotate independently in the azimuthal direction. Different speeds of
nearby slabs provide, although in a discrete setting, the desired streamwise
variation of transverse velocity.
Our experiment confirms the available DNS results, and in particular
demonstrates the possibility of achieving large reductions of friction in the
turbulent regime. Reductions up to 33% are obtained for slowly
forward-traveling waves; backward-traveling waves invariably yield drag
reduction, whereas a substantial drop of drag reduction occurs for waves
traveling forward with a phase speed comparable to the convection speed of
near-wall turbulent structures.
A Fourier analysis is employed to show that the first harmonics introduced by
the discrete spatial waveform that approximates the sinusoidal wave are
responsible for significant effects that are indeed observed in the
experimental measurements. Practical issues related to the physical
implementation of this control scheme and its energetic efficiency are briefly
discussed.Comment: Article accepted by Phys. Fluids. After it is published, it will be
found at http://pof.aip.or
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