3,892 research outputs found
The angular momentum transport by standard MRI in quasi-Kepler cylindric Taylor-Couette flows
The instability of a quasi-Kepler flow in dissipative Taylor-Couette systems
under the presence of an homogeneous axial magnetic field is considered with
focus to the excitation of nonaxisymmetric modes and the resulting angular
momentum transport. The excitation of nonaxisymmetric modes requires higher
rotation rates than the excitation of the axisymmetric mode and this the more
the higher the azimuthal mode number m. We find that the weak-field branch in
the instability map of the nonaxisymmetric modes has always a positive slope
(in opposition to the axisymmetric modes) so that for given magnetic field the
modes with m>0 always have an upper limit of the supercritical Reynolds number.
In order to excite a nonaxisymmetric mode at 1 AU in a Kepler disk a minimum
field strength of about 1 Gauss is necessary. For weaker magnetic field the
nonaxisymmetric modes decay. The angular momentum transport of the
nonaxisymmetric modes is always positive and depends linearly on the Lundquist
number of the background field. The molecular viscosity and the basic rotation
rate do not influence the related {\alpha}-parameter. We did not find any
indication that the MRI decays for small magnetic Prandtl number as found by
use of shearing-box codes. At 1 AU in a Kepler disk and a field strength of
about 1 Gauss the {\alpha} proves to be (only) of order 0.005
A Scaling Law to Predict the Finite-Length Performance of Spatially-Coupled LDPC Codes
Spatially-coupled LDPC codes are known to have excellent asymptotic
properties. Much less is known regarding their finite-length performance. We
propose a scaling law to predict the error probability of finite-length
spatially-coupled ensembles when transmission takes place over the binary
erasure channel. We discuss how the parameters of the scaling law are connected
to fundamental quantities appearing in the asymptotic analysis of these
ensembles and we verify that the predictions of the scaling law fit well to the
data derived from simulations over a wide range of parameters. The ultimate
goal of this line of research is to develop analytic tools for the design of
spatially-coupled LDPC codes under practical constraints
Helicity and alpha-effect by current-driven instabilities of helical magnetic fields
Helical magnetic background fields with adjustable pitch angle are imposed on
a conducting fluid in a differentially rotating cylindrical container. The
small-scale kinetic and current helicities are calculated for various field
geometries, and shown to have the opposite sign as the helicity of the
large-scale field. These helicities and also the corresponding -effect
scale with the current helicity of the background field. The -tensor is
highly anisotropic as the components and have
opposite signs. The amplitudes of the azimuthal -effect computed with
the cylindrical 3D MHD code are so small that the operation of an
dynamo on the basis of the current-driven, kink-type
instabilities of toroidal fields is highly questionable. In any case the low
value of the -effect would lead to very long growth times of a dynamo
in the radiation zone of the Sun and early-type stars of the order of
mega-years.Comment: 6 pages, 7 figures, submitted to MNRA
Feeding ecology of sprat (Sprattus sprattus L.) and sardine (Sardina pilchardus W.) larvae in the German Bight, North Sea
The abundance of the sardine in the North Sea suddenly increased after 1995. Since 2002, the sardine has been spawning regularly in the German Bight, and all its life stages can be found in the area. The larval feeding ecology of two small pelagic clupeiform species with very similar life histories was investigated, the particular aim being to determine signs of food overlap. The distribution and feeding of sprat and sardine larvae were investigated during late spring 2003 on two transects covering a wide range of environmental conditions in the German Bight. Larvae co-occurred at all the stations investigated. Sprat and sardine larvae shared a wide range of prey types. Gut fullness and feeding success were similar in both species; however, potential food competition is avoided to some extent by different habitat preferences
Managerial Ownership Dynamics and Firm Value
From 1988 to 2003, the average change in managerial ownership is significantly negative every year for American firms. The probability of large decreases in ownership is strongly increasing in contemporaneous and past stock returns but the probability of large increases in ownership through managerial purchases of shares is not. The relation between changes in Tobin's q and past and contemporaneous changes in ownership depends critically on controlling for past stock returns. When controlling for past stock returns, past large decreases in managerial ownership are unrelated to current changes in Tobin's q but there is some evidence that past large increases in managerial ownership are positively related to current changes in Tobin's q. Because managers sell shares when a firm's stock is performing well, large contemporaneous decreases in managerial ownership are associated with increases in Tobin's q. We argue that our evidence is mostly inconsistent with existing theories and propose a managerial discretion theory of ownership consistent with our evidence.
Stratorotational instability in Taylor-Couette flow heated from above
We investigate the instability and nonlinear saturation of
temperature-stratified Taylor-Couette flows in a finite height cylindrical gap
and calculate angular-momentum transport in the nonlinear regime. The model is
based on an incompressible fluid in Boussinesq approximation with a positive
axial temperature gradient applied. While both ingredients itself, the
differential rotation as well as the stratification due to the temperature
gradient, are stable, together the system becomes subject of the
stratorotational instability and nonaxisymmetric flow pattern evolve. This flow
configuration transports angular momentum outwards and will therefor be
relevant for astrophysical applications. The belonging viscosity
coefficient is of the order of unity if the results are adapted to the size of
an accretion disc. The strength of the stratification, the fluids Prandtl
number and the boundary conditions applied in the simulations are well-suited
too for a laboratory experiment using water and a small temperature gradient
below five Kelvin. With such a rather easy realizable set-up the SRI and its
angular momentum transport could be measured in an experiment.Comment: 10 pages, 6 figures, revised version appeared in J. Fluid Mech.
(2009), vol. 623, pp. 375--38
Natriuretic peptide receptors on rat thymocytes: Inhibition of proliferation by atrial natriuretic peptide.
Because the thymus expresses the natriuretic peptides (NP) as well
as their respective receptors, an involvement of NP in the physiology
of this organ has been suggested. To evaluate functional aspects of NP
in the thymus, we looked for thymic cells bearing NP receptors (Npr).
Furthermore, the regulation of Npr expression by activation of cells
and the influence of NP on the proliferation of thymocytes was studied.
Expression of receptor messenger RNAs CmRNAs) was examined
by PCR and Northern blot. Existence of functional Npr was confirmed
by measurement of cGMP, the second messenger of NP. Proliferation
of thymocytes upon concanavalin A (Con A) stimulation was analyzed by incorporation of [“Hlthymidine. We report here that thymocytes express mRNAs for the three Npr, namely Npra, Nprb, and Nprc and
that activation of Npra and Nprb increases cGMP levels. Stimulation
of thymocytes with Con A (1 pg/ml, 48 h) resulted in an increase of
mRNA coding for Npra, the receptor specific for atria1 natriuretic
peptide (ANP) and brain natriuretic peptide. Nprb and Nprc receptor
expression was not altered under these conditions. In agreement with
these data only ANP, but not the C-type natriuretic peptide, elicited
increased cGMP response in Con A-stimulated cells. ANP inhibited
also the proliferation of Con A stimulated thymocytes, whereas C-type
natriuretic peptide did not show this effect. These results suggest that
ANP affects the complex mechanisms of thymocyte proliferation and
differentiation
Nonaxisymmetric MHD instabilities of Chandrasekhar states in Taylor-Couette geometry
We consider axially periodic Taylor-Couette geometry with insulating boundary
conditions. The imposed basic states are so-called Chandrasekhar states, where
the azimuthal flow and magnetic field have the same radial
profiles. Mainly three particular profiles are considered: the Rayleigh limit,
quasi-Keplerian, and solid-body rotation. In each case we begin by computing
linear instability curves and their dependence on the magnetic Prandtl number
Pm. For the azimuthal wavenumber m=1 modes, the instability curves always scale
with the Reynolds number and the Hartmann number. For sufficiently small Pm
these modes therefore only become unstable for magnetic Mach numbers less than
unity, and are thus not relevant for most astrophysical applications. However,
modes with m>10 can behave very differently. For sufficiently flat profiles,
they scale with the magnetic Reynolds number and the Lundquist number, thereby
allowing instability also for the large magnetic Mach numbers of astrophysical
objects. We further compute fully nonlinear, three-dimensional equilibration of
these instabilities, and investigate how the energy is distributed among the
azimuthal (m) and axial (k) wavenumbers. In comparison spectra become steeper
for large m, reflecting the smoothing action of shear. On the other hand
kinetic and magnetic energy spectra exhibit similar behavior: if several
azimuthal modes are already linearly unstable they are relatively flat, but for
the rigidly rotating case where m=1 is the only unstable mode they are so steep
that neither Kolmogorov nor Iroshnikov-Kraichnan spectra fit the results. The
total magnetic energy exceeds the kinetic energy only for large magnetic
Reynolds numbers Rm>100.Comment: 12 pages, 14 figures, submitted to Ap
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