32 research outputs found
The instability of a liquid layer heated from the side when the upper surface is open to air
When a liquid layer is heated from the side, a monocellular flow develops immediately, no matter
how small the temperature difference is. If the temperature gradient between the side walls is
increased, this flow becomes unstable. Laser Doppler velocimetry measurements are reported here
in an attempt to describe the main features of both the basic flow and the instability modes. It is
found that before the appearance of traveling waves ~the most dangerous mode as predicted by the
theory!, stable rolls with their axes perpendicular to the temperature gradient, span over the whole
liquid layer, starting from the hot side, even if the aspect ratio ~the length of the layer divided by its
thickness! is very high. This unexpected situation modifies the basic flow. A further increase of the
temperature gradient leads to the appearance of a time periodic motion. © 1996 American Institute
of Physics
Amplitude equations for Rayleigh-Benard convective rolls far from threshold
An extension of the amplitude method is proposed. An iterative algorithm is developed to build an amplitude equation model that is shown to provide precise quantitative results even far from the linear instability threshold. The method is applied to the study of stationary Rayleigh-Benard thermoconvective rolls in the nonlinear regime. In particular, the generation of second and third spatial harmonics is analyzed. Comparison with experimental results and direct numerical calculations is also made and a very good agreement is found.Peer reviewe
Why are flare ribbons associated with the spines of magnetic null points generically elongated?
Coronal magnetic null points exist in abundance as demonstrated by
extrapolations of the coronal field, and have been inferred to be important for
a broad range of energetic events. These null points and their associated
separatrix and spine field lines represent discontinuities of the field line
mapping, making them preferential locations for reconnection. This field line
mapping also exhibits strong gradients adjacent to the separatrix (fan) and
spine field lines, that can be analysed using the `squashing factor', . In
this paper we make a detailed analysis of the distribution of in the
presence of magnetic nulls. While is formally infinite on both the spine
and fan of the null, the decay of away from these structures is shown in
general to depend strongly on the null-point structure. For the generic case of
a non-radially-symmetric null, decays most slowly away from the spine/fan
in the direction in which increases most slowly. In particular,
this demonstrates that the extended, elliptical high- halo around the spine
footpoints observed by Masson et al. (Astrophys. J., 700, 559, 2009) is a
generic feature. This extension of the halos around the spine/fan
footpoints is important for diagnosing the regions of the photosphere that are
magnetically connected to any current layer that forms at the null. In light of
this, we discuss how our results can be used to interpret the geometry of
observed flare ribbons in `circular ribbon flares', in which typically a
coronal null is implicated. We conclude that both the physics in the vicinity
of the null and how this is related to the extension of away from the
spine/fan can be used in tandem to understand observational signatures of
reconnection at coronal null points.Comment: Pre-print version of article accepted for publication in Solar
Physic
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Neuron-oligodendrocyte potassium shuttling at nodes of Ranvier protects against inflammatory demyelination
Multiple sclerosis (MS) is a progressive inflammatory-demyelinating disease of the central nervous system. Increasing evidence suggests that vulnerable neurons in MS exhibit fatal metabolic exhaustion over time, a phenomenon hypothesized to be caused by chronic hyperexcitability. Axonal Kv7 (outward rectifying) and oligodendroglial Kir4.1 (inward rectifying) potassium channels have important roles in regulating neuronal excitability at and around nodes of Ranvier. Here, we studied the spatial and functional relationship between neuronal Kv7 and oligodendroglial Kir4.1 channels and assessed the transcriptional and functional signatures of cortical and retinal projection neurons under physiological and inflammatory-demyelinating conditions. We found that both channels became dysregulated in MS and experimental autoimmune encephalomyelitis (EAE) with Kir4.1 channels being chronically downregulated and Kv7 channel subunits being transiently upregulated during inflammatory demyelination. Further, we observed that pharmacological Kv7 channel opening with retigabine reduced neuronal hyperexcitability in human and EAE neurons, improved clinical EAE signs and rescued neuronal pathology in oligodendrocyte-Kir4.1-deficient mice. In summary, our findings indicate that neuron-oligodendrocyte compensatory interactions promote resilience through Kv7 and Kir4.1 channels and suggest pharmacological activation of nodal Kv7 channels as a neuroprotective strategy against inflammatory demyelination
Confinement effect on the dynamics of non-equilibrium concentration fluctuations far from the onset of convection
A preliminary experimental investigation of the stability of flows with an imposed temperature gradient
The instability of a liquid layer heated from the side when the upper surface is open to air
When a liquid layer is heated from the side, a monocellular flow develops immediately, no matter
how small the temperature difference is. If the temperature gradient between the side walls is
increased, this flow becomes unstable. Laser Doppler velocimetry measurements are reported here
in an attempt to describe the main features of both the basic flow and the instability modes. It is
found that before the appearance of traveling waves ~the most dangerous mode as predicted by the
theory!, stable rolls with their axes perpendicular to the temperature gradient, span over the whole
liquid layer, starting from the hot side, even if the aspect ratio ~the length of the layer divided by its
thickness! is very high. This unexpected situation modifies the basic flow. A further increase of the
temperature gradient leads to the appearance of a time periodic motion. © 1996 American Institute
of Physics