171 research outputs found
New frictional resistance law for smooth plates
From measurements in the free boundary layer of a plate the laws governing the velocity distribution and a new resistance law are derived which, by increasing Reynolds number Re(sub x) afford lower resistance values than the logarithmic law. The transverse velocities, the shearing stress, and the mixing path profiles were also defined
Experimental evidence for magnetorotational instability in a helical magnetic field
A recent paper [R. Hollerbach and G. Rudiger, Phys. Rev. Lett. 95, 124501
(2005)] has shown that the threshold for the onset of the magnetorotational
instability (MRI) in a Taylor-Couette flow is dramatically reduced if both
axial and azimuthal magnetic fields are imposed. In agreement with this
prediction, we present results of a Taylor-Couette experiment with the liquid
metal alloy GaInSn, showing evidence for the existence of the MRI at Reynolds
numbers of order 1000 and Hartmann numbers of order 10.Comment: 4 pages, 4 figure
A Gas-poor Planetesimal Capture Model for the Formation of Giant Planet Satellite Systems
Assuming that an unknown mechanism (e.g., gas turbulence) removes most of the
subnebula gas disk in a timescale shorter than that for satellite formation, we
develop a model for the formation of regular (and possibly at least some of the
irregular) satellites around giant planets in a gas-poor environment. In this
model, which follows along the lines of the work of Safronov et al. (1986),
heliocentric planetesimals collide within the planet's Hill sphere and generate
a circumplanetary disk of prograde and retrograde satellitesimals extending as
far out as . At first, the net angular momentum of this
proto-satellite swarm is small, and collisions among satellitesimals leads to
loss of mass from the outer disk, and delivers mass to the inner disk (where
regular satellites form) in a timescale years. This mass loss
may be offset by continued collisional capture of sufficiently small km
interlopers resulting from the disruption of planetesimals in the feeding zone
of the giant planet. As the planet's feeding zone is cleared in a timescale
years, enough angular momentum may be delivered to the
proto-satellite swarm to account for the angular momentum of the regular
satellites of Jupiter and Saturn.(abridged)Comment: 45 pages, 11 figures, 3 appendices, uses rgfmacro.tex, accepted for
publication to Icaru
Hydrodynamic turbulence cannot transport angular momentum effectively in astrophysical disks
The most efficient energy sources known in the Universe are accretion disks.
Those around black holes convert 5 -- 40 per cent of rest-mass energy to
radiation. Like water circling a drain, inflowing mass must lose angular
momentum, presumably by vigorous turbulence in disks, which are essentially
inviscid. The origin of the turbulence is unclear. Hot disks of electrically
conducting plasma can become turbulent by way of the linear magnetorotational
instability. Cool disks, such as the planet-forming disks of protostars, may be
too poorly ionized for the magnetorotational instability to occur, hence
essentially unmagnetized and linearly stable. Nonlinear hydrodynamic
instability often occurs in linearly stable flows (for example, pipe flows) at
sufficiently large Reynolds numbers. Although planet-forming disks have extreme
Reynolds numbers, Keplerian rotation enhances their linear hydrodynamic
stability, so the question of whether they can be turbulent and thereby
transport angular momentum effectively is controversial. Here we report a
laboratory experiment, demonstrating that non-magnetic quasi-Keplerian flows at
Reynolds numbers up to millions are essentially steady. Scaled to accretion
disks, rates of angular momentum transport lie far below astrophysical
requirements. By ruling out purely hydrodynamic turbulence, our results
indirectly support the magnetorotational instability as the likely cause of
turbulence, even in cool disks.Comment: 12 pages and 4 figures. To be published in Nature on November 16,
2006, available at
http://www.nature.com/nature/journal/v444/n7117/abs/nature05323.htm
Magnetohydrodynamic experiments on cosmic magnetic fields
It is widely known that cosmic magnetic fields, i.e. the fields of planets,
stars, and galaxies, are produced by the hydromagnetic dynamo effect in moving
electrically conducting fluids. It is less well known that cosmic magnetic
fields play also an active role in cosmic structure formation by enabling
outward transport of angular momentum in accretion disks via the
magnetorotational instability (MRI). Considerable theoretical and computational
progress has been made in understanding both processes. In addition to this,
the last ten years have seen tremendous efforts in studying both effects in
liquid metal experiments. In 1999, magnetic field self-excitation was observed
in the large scale liquid sodium facilities in Riga and Karlsruhe. Recently,
self-excitation was also obtained in the French "von Karman sodium" (VKS)
experiment. An MRI-like mode was found on the background of a turbulent
spherical Couette flow at the University of Maryland. Evidence for MRI as the
first instability of an hydrodynamically stable flow was obtained in the
"Potsdam Rossendorf Magnetic Instability Experiment" (PROMISE). In this review,
the history of dynamo and MRI related experiments is delineated, and some
directions of future work are discussed.Comment: 25 pages, 26 figures, to appear in ZAM
Stability and instability of hydromagnetic Taylor–Couette flows
Decades ago S. Lundquist, S. Chandrasekhar, P. H. Roberts and R. J. Tayler first posed questions about the stability of Taylor–Couette flows of conducting material under the influence of large-scale magnetic fields. These and many new questions can now be answered numerically where the nonlinear simulations even provide the instability-induced values of several transport coefficients. The cylindrical containers are axially unbounded and penetrated by magnetic background fields with axial and/or azimuthal components. The influence of the magnetic Prandtl number Pm on the onset of the instabilities is shown to be substantial. The potential flow subject to axial fields becomes unstable against axisymmetric perturbations for a certain supercritical value of the averaged Reynolds number Rm¯=√Re⋅Rm (with Re the Reynolds number of rotation, Rm its magnetic Reynolds number). Rotation profiles as flat as the quasi-Keplerian rotation law scale similarly but only for Pm≫1 while for Pm≪1 the instability instead sets in for supercritical Rm at an optimal value of the magnetic field. Among the considered instabilities of azimuthal fields, those of the Chandrasekhar-type, where the background field and the background flow have identical radial profiles, are particularly interesting. They are unstable against nonaxisymmetric perturbations if at least one of the diffusivities is non-zero. For Pm≪1 the onset of the instability scales with Re while it scales with Rm¯ for Pm≫1. Even superrotation can be destabilized by azimuthal and current-free magnetic fields; this recently discovered nonaxisymmetric instability is of a double-diffusive character, thus excluding Pm=1. It scales with Re for Pm→0 and with Rm for Pm→∞.
The presented results allow the construction of several new experiments with liquid metals as the conducting fluid. Some of them are described here and their results will be discussed together with relevant diversifications of the magnetic instability theory including nonlinear numerical studies of the kinetic and magnetic energies, the azimuthal spectra and the influence of the Hall effect
ChemInform Abstract: METAL COMPLEXES WITH MACROCYCLIC LIGANDS. X. ON THE METAL COMPLEXATION KINETICS OF TWO N4-MACROCYCLES CONTAINING A PYRIDINE RING
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