2,862 research outputs found
On the relation between viscoelastic and magnetohydrodynamic flows and their instabilities
We demonstrate a close analogy between a viscoelastic medium and an
electrically conducting fluid containing a magnetic field. Specifically, the
dynamics of the Oldroyd-B fluid in the limit of large Deborah number
corresponds to that of a magnetohydrodynamic (MHD) fluid in the limit of large
magnetic Reynolds number. As a definite example of this analogy, we compare the
stability properties of differentially rotating viscoelastic and MHD flows. We
show that there is an instability of the Oldroyd-B fluid that is physically
distinct from both the inertial and elastic instabilities described previously
in the literature, but is directly equivalent to the magnetorotational
instability in MHD. It occurs even when the specific angular momentum increases
outwards, provided that the angular velocity decreases outwards; it derives
from the kinetic energy of the shear flow and does not depend on the curvature
of the streamlines. However, we argue that the elastic instability of
viscoelastic Couette flow has no direct equivalent in MHD.Comment: 21 pages, 3 figures, to be published in J. Fluid Mec
Destabilization by noise of tranverse perturbations to heteroclinic cycles: a simple model and an example from dynamo theory
We show that transverse perturbations from structurally stable heteroclinic cycles can be destabilized by surprisingly small amounts of noise, even when each individual fixed point of the cycle is stable to transverse modes. A condition that favours this process is that the linearization of the dynamics in the transverse direction be characterized by a non-normal matrix. The phenomenon is illustrated by a simple two-dimensional switching model and by a simulation of a convectively driven dynamo
A self-sustaining nonlinear dynamo process in Keplerian shear flows
A three-dimensional nonlinear dynamo process is identified in rotating plane
Couette flow in the Keplerian regime. It is analogous to the hydrodynamic
self-sustaining process in non-rotating shear flows and relies on the
magneto-rotational instability of a toroidal magnetic field. Steady nonlinear
solutions are computed numerically for a wide range of magnetic Reynolds
numbers but are restricted to low Reynolds numbers. This process may be
important to explain the sustenance of coherent fields and turbulent motions in
Keplerian accretion disks, where all its basic ingredients are present.Comment: 4 pages, 7 figures, accepted for publication in Physical Review
Letter
Constraining the Star Formation Histories of Spiral Bulges
Long-slit spectroscopic observations of line-strengths and kinematics made
along the minor axes of four spiral bulges are reported. Comparisons are made
between central line-strengths in spiral bulges and those in other
morphological types. The bulges are found to have central line-strengths
comparable with those of single stellar populations (SSPs) of approximately
solar abundance or above. Negative radial gradients are observed in
line-strengths, similar to those in elliptical galaxies. The bulge data are
consistent with correlations between Mg2, and central velocity dispersion
observed in elliptical galaxiess. In contrast to elliptical galaxies, central
line-strengths lie within the loci defining the range of and Mg2 achieved
by Worthey's (1994) solar abundance ratio, SSPs. The implication of solar
abundance ratios indicates differences in the star formation histories of
spiral bulges and elliptical galaxies. A ``single zone with in- fall'' model of
galactic chemical evolution, using Worthey's (1994) SSPs, is used to constrain
possible star formation histories in our sample. We show that , Mg2 and
Hbeta line-strengths observed in these bulges cannot be reproduced using
primordial collapse models of formation but can be reproduced by models with
extended in-fall of gas and star formation (2-17 Gyr) in the region modelled.
One galaxy (NGC 5689) shows a central population with luminosity weighted
average age of ~5 Gyr, supporting the idea of extended star formation.
Kinematic substructure, possibly associated with a central spike in
metallicity, is observed at the centre of the Sa galaxy NGC 3623.Comment: 14 pages. MNRAS latex file. Accepted for publication in MNRA
Pressure coefficients of Raman modes of carbon nanotubes resolved by chirality: Environmental effect on graphene sheet
Studies of the mechanical properties of single-walled carbon nanotubes are
hindered by the availability only of ensembles of tubes with a range of
diameters. Tunable Raman excitation spectroscopy picks out identifiable tubes.
Under high pressure, the radial breathing mode shows a strong environmental
effect shown here to be largely independent of the nature of the environment .
For the G-mode, the pressure coefficient varies with diameter consistent with
the thick-wall tube model. However, results show an unexpectedly strong
environmental effect on the pressure coefficients. Reappraisal of data for
graphene and graphite gives the G-mode Grueuneisen parameter gamma = 1.34 and
the shear deformation parameter beta = 1.34.Comment: Submitted to Physical Review
Oscillations and secondary bifurcations in nonlinear magnetoconvection
Complicated bifurcation structures that appear in nonlinear systems governed by partial differential equations (PDEs) can be explained by studying appropriate low-order amplitude equations. We demonstrate the power of this approach by considering compressible magnetoconvection. Numerical experiments reveal a transition from a regime with a subcritical Hopf bifurcation from the static solution, to one where finite-amplitude oscillations persist although there is no Hopf bifurcation from the static solution. This transition is associated with a codimension-two bifurcation with a pair of zero eigenvalues. We show that the bifurcation pattern found for the PDEs is indeed predicted by the second-order normal form equation (with cubic nonlinearities) for a Takens-Bogdanov bifurcation with Z2 symmetry. We then extend this equation by adding quintic nonlinearities and analyse the resulting system. Its predictions provide a qualitatively accurate description of solutions of the full PDEs over a wider range of parameter values. Replacing the reflecting (Z2) lateral boundary conditions with periodic [O(2)] boundaries allows stable travelling wave and modulated wave solutions to appear; they could be described by a third-order system
Observation of liquid–liquid phase transitions in ethane at 300 K
We have conducted Raman spectroscopy
experiments on liquid ethane (C2H6) at 300 K, obtaining a
large amount of data at very high resolution. This has enabled
the observation of Raman peaks expected but not previously
observed in liquid ethane and a detailed experimental study of
the liquid that was not previously possible. We have observed a
transition between rigid and nonrigid liquid states in liquid
ethane at ca. 250 MPa corresponding to the recently proposed
Frenkel line, a dynamic transition between rigid liquid
(liquidlike) and nonrigid liquid (gaslike) states beginning in
the subcritical region and extending to arbitrarily high pressure
and temperature. The observation of this transition in liquid
(subcritical) ethane allows a clear differentiation to be made
between the Frenkel line (beginning in the subcritical region at
higher density than the boiling line) and the Widom lines (emanating from the critical point and not existing in the subcritical
region). Furthermore, we observe a narrow transition at ca. 1000 MPa to a second rigid liquid state. We propose that this
corresponds to a state in which orientational order must exist to achieve the expected density and can view the transition in
analogy to the transition in the solid state away from the orientationally disordered phase I to the orientationally ordered phases
II and III
Vicious walkers, friendly walkers and Young tableaux II: With a wall
We derive new results for the number of star and watermelon configurations of
vicious walkers in the presence of an impenetrable wall by showing that these
follow from standard results in the theory of Young tableaux, and combinatorial
descriptions of symmetric functions. For the problem of -friendly walkers,
we derive exact asymptotics for the number of stars and watermelons both in the
absence of a wall and in the presence of a wall.Comment: 35 pages, AmS-LaTeX; Definitions of n-friendly walkers clarified; the
statement of Theorem 4 and its proof were correcte
The inverse cascade and nonlinear alpha-effect in simulations of isotropic helical hydromagnetic turbulence
A numerical model of isotropic homogeneous turbulence with helical forcing is
investigated. The resulting flow, which is essentially the prototype of the
alpha^2 dynamo of mean-field dynamo theory, produces strong dynamo action with
an additional large scale field on the scale of the box (at wavenumber k=1;
forcing is at k=5). This large scale field is nearly force-free and exceeds the
equipartition value. As the magnetic Reynolds number R_m increases, the
saturation field strength and the growth rate of the dynamo increase. However,
the time it takes to built up the large scale field from equipartition to its
final super-equipartition value increases with magnetic Reynolds number. The
large scale field generation can be identified as being due to nonlocal
interactions originating from the forcing scale, which is characteristic of the
alpha-effect. Both alpha and turbulent magnetic diffusivity eta_t are
determined simultaneously using numerical experiments where the mean-field is
modified artificially. Both quantities are quenched in a R_m-dependent fashion.
The evolution of the energy of the mean field matches that predicted by an
alpha^2 dynamo model with similar alpha and eta_t quenchings. For this model an
analytic solution is given which matches the results of the simulations. The
simulations are numerically robust in that the shape of the spectrum at large
scales is unchanged when changing the resolution from 30^3 to 120^3 meshpoints,
or when increasing the magnetic Prandtl number (viscosity/magnetic diffusivity)
from 1 to 100. Increasing the forcing wavenumber to 30 (i.e. increasing the
scale separation) makes the inverse cascade effect more pronounced, although it
remains otherwise qualitatively unchanged.Comment: 21 pages, 26 figures, ApJ (accepted
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