4,304 research outputs found
Mean flow instabilities of two-dimensional convection in strong magnetic fields
The interaction of magnetic fields with convection is of great importance in astrophysics. Two well-known aspects of the interaction are the tendency of convection cells to become narrow in the perpendicular direction when the imposed field is strong, and the occurrence of streaming instabilities involving horizontal shears. Previous studies have found that the latter instability mechanism operates only when the cells are narrow, and so we investigate the occurrence of the streaming instability for large imposed fields, when the cells are naturally narrow near onset. The basic cellular solution can be treated in the asymptotic limit as a nonlinear eigenvalue problem. In the limit of large imposed field, the instability occurs for asymptotically small Prandtl number. The determination of the stability boundary turns out to be surprisingly complicated. At leading order, the linear stability problem is the linearisation of the same nonlinear eigenvalue problem, and as a result, it is necessary to go to higher order to obtain a stability criterion. We establish that the flow can only be unstable to a horizontal mean flow if the Prandtl number is smaller than order , where B0 is the imposed magnetic field, and that the mean flow is concentrated in a horizontal jet of width in the middle of the layer. The result applies to stress-free or no-slip boundary conditions at the top and bottom of the layer
Transdifferentiation of blood-derived human adult endothelial progenitor cells into functionally active cardiomyocytes
Background - Further to promoting angiogenesis, cell therapy may be an approach for cardiac regeneration. Recent studies suggest that progenitor cells can transdifferentiate into other lineages. However, the transdifferentiation potential of endothelial progenitor cells (EPCs) is unknown
A compendium of NASA Aerobee sounding rocket launchings for 1966
Compendium of Aerobee sounding rocket launchings for 196
Variational bound on energy dissipation in turbulent shear flow
We present numerical solutions to the extended Doering-Constantin variational
principle for upper bounds on the energy dissipation rate in plane Couette
flow, bridging the entire range from low to asymptotically high Reynolds
numbers. Our variational bound exhibits structure, namely a pronounced minimum
at intermediate Reynolds numbers, and recovers the Busse bound in the
asymptotic regime. The most notable feature is a bifurcation of the minimizing
wavenumbers, giving rise to simple scaling of the optimized variational
parameters, and of the upper bound, with the Reynolds number.Comment: 4 pages, RevTeX, 5 postscript figures are available as one .tar.gz
file from [email protected]
Detection of fixed points in spatiotemporal signals by clustering method
We present a method to determine fixed points in spatiotemporal signals. A
144-dimensioanl simulated signal, similar to a Kueppers-Lortz instability, is
analyzed and its fixed points are reconstructed.Comment: 3 pages, 3 figure
Variational bound on energy dissipation in plane Couette flow
We present numerical solutions to the extended Doering-Constantin variational
principle for upper bounds on the energy dissipation rate in turbulent plane
Couette flow. Using the compound matrix technique in order to reformulate this
principle's spectral constraint, we derive a system of equations that is
amenable to numerical treatment in the entire range from low to asymptotically
high Reynolds numbers. Our variational bound exhibits a minimum at intermediate
Reynolds numbers, and reproduces the Busse bound in the asymptotic regime. As a
consequence of a bifurcation of the minimizing wavenumbers, there exist two
length scales that determine the optimal upper bound: the effective width of
the variational profile's boundary segments, and the extension of their flat
interior part.Comment: 22 pages, RevTeX, 11 postscript figures are available as one
uuencoded .tar.gz file from [email protected]
Square patterns in Rayleigh-Benard convection with rotation about a vertical axis
We present experimental results for Rayleigh-Benard convection with rotation
about a vertical axis at dimensionless rotation rates in the range 0 to 250 and
upto 20% above the onset. Critical Rayleigh numbers and wavenumbers agree with
predictions of linear stability analysis. For rotation rates greater than 70
and close to onset, the patterns are cellular with local four-fold coordination
and differ from the theoretically expected Kuppers-Lortz unstable state. Stable
as well as intermittent defect-free square lattices exist over certain
parameter ranges. Over other ranges defects dynamically disrupt the lattice but
cellular flow and local four-fold coordination is maintained.Comment: ReVTeX, 4 pages, 7 eps figures include
Dynamics and thermodynamics of axisymmetric flows: I. Theory
We develop new variational principles to study stability and equilibrium of
axisymmetric flows. We show that there is an infinite number of steady state
solutions. We show that these steady states maximize a (non-universal)
-function. We derive relaxation equations which can be used as numerical
algorithm to construct stable stationary solutions of axisymmetric flows. In a
second part, we develop a thermodynamical approach to the equilibrium states at
some fixed coarse-grained scale. We show that the resulting distribution can be
divided in a universal part coming from the conservation of robust invariants
and one non-universal determined by the initial conditions through the fragile
invariants (for freely evolving systems) or by a prior distribution encoding
non-ideal effects such as viscosity, small-scale forcing and dissipation (for
forced systems). Finally, we derive a parameterization of inviscid mixing to
describe the dynamics of the system at the coarse-grained scale
Patterns of convection in rotating spherical shells
Patterns of convection in internally heated, self-gravitating rotating
spherical fluid shells are investigated through numerical simulations. While
turbulent states are of primary interest in planetary and stellar applications
the present paper emphasizes more regular dynamical features at Rayleigh
numbers not far above threshold which are similar to those which might be
observed in laboratory or space experiments. Amplitude vacillations and spatial
modulations of convection columns are common features at moderate and large
Prandtl numbers. In the low Prandtl number regime equatorially attached
convection evolves differently with increasing Rayleigh number and exhibits an
early transition into a chaotic state. Relationships of the dynamical features
to coherent structures in fully turbulent convection states are emphasized
Massilia norwichensis sp. nov., isolated from an air sample
A Gram-negative, rod-shaped and motile bacterial isolate, designated strain NS9T, isolated from
air of the Sainsbury Centre for Visual Arts in Norwich, UK, was subjected to a polyphasic
taxonomic study including phylogenetic analyses based on partial 16S rRNA, gyrB and lepA gene
sequences and phenotypic characterization. The 16S rRNA gene sequence of NS9T identified
Massilia haematophila CCUG 38318T, M. niastensis 5516S-1T (both 97.7% similarity), M.
aerilata 5516S-11T (97.4 %) and M. tieshanensis TS3T (97.4 %) as the next closest relatives. In
partial gyrB and lepA sequences, NS9T shared the highest similarities with M. haematophila
CCUG 38318T (94.5 %) and M. aerilata 5516-11T (94.3 %), respectively. These sequence data
demonstrate the affiliation of NS9T to the genus Massilia. The detection of the predominant
ubiquinone Q-8, a polar lipid profile consisting of the major compounds diphosphatidylglycerol,
phosphatidylethanolamine and phosphatidylglycerol and a polyamine pattern containing 2-
hydroxyputrescine and putrescine were in agreement with the assignment of strain NS9T to the
genus Massilia. Major fatty acids were summed feature 3 (C16 : 1v7c and/or iso-C15 : 0 2-OH),
C16 : 0, C18 : 1v7c and C10 : 0 3-OH. Dissimilarities in partial lepA and gyrB gene sequences as
well as results from DNA–DNA hybridizations demonstrate that strain NS9T is a representative of
an as-yet undescribed species of the genus Massilia that is also distinguished from its close
relatives based on physiological and biochemical traits. Hence, we describe a novel species, for
which we propose the name Massilia norwichensis sp. nov., with the type strain NS9T (5CCUG
65457T5LMG 28164T)
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