106 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
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
Recommended from our members
NACA Technical Memorandums
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
Recommended from our members
NACA Technical Memorandums
Based upon a simplified representation of the mode of operation of the pulse-jet tube, the effect of the influences mentioned in the title were investigated and it will be shown that, for a jet tube with a form designed to be aerodynamically favorable, the ability to operate is at least questionable. This investigation will account for the important practical observation made by Paul Schmidt that the ratio of the effective valve cross-sectional area to the tube cross section may not be of any random magnitude and will explain why at too great flight speeds the jet tube ceases to operate. Chemical an thermodynamic processes (for example, constituents or mode of fuel-air-mixture formation or heat losses) are unimportant in this regard
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