1,324 research outputs found
Measurement of angular momentum transport in turbulent flow between independently rotating cylinders
We present measurements of the angular momentum flux (torque) in
Taylor-Couette flow of water between independently rotating cylinders for all
regions of the \(\Omega_1, \Omega_2\) parameter space at high Reynolds
numbers, where \(\Omega_2\) is the inner (outer) cylinder angular
velocity. We find that the Rossby number Ro = \(\Omega_1 -
\Omega_2\)/\Omega_2 fully determines the state and torque as compared to
G(Ro = \infty) \equiv \Gi. The ratio G/\Gi is a linear function of
in four sections of the parameter space. For flows with
radially-increasing angular momentum, our measured torques greatly exceed those
of previous experiments [Ji \textit{et al.}, Nature, \textbf{444}, 343 (2006)],
but agree with the analysis of Richard and Zahn [Astron. Astrophys.,
\textbf{347}, 734 (1999)].Comment: 4 pages, 4 figures, to appear in Physical Review Letter
Collective patterns arising out of spatio-temporal chaos
We present a simple mathematical model in which a time averaged pattern
emerges out of spatio-temporal chaos as a result of the collective action of
chaotic fluctuations. Our evolution equation possesses spatial translational
symmetry under a periodic boundary condition. Thus the spatial inhomogeneity of
the statistical state arises through a spontaneous symmetry breaking. The
transition from a state of homogeneous spatio-temporal chaos to one exhibiting
spatial order is explained by introducing a collective viscosity which relates
the averaged pattern with a correlation of the fluctuations.Comment: 11 pages (Revtex) + 5 figures (postscript
Boolean Chaos
We observe deterministic chaos in a simple network of electronic logic gates
that are not regulated by a clocking signal. The resulting power spectrum is
ultra-wide-band, extending from dc to beyond 2 GHz. The observed behavior is
reproduced qualitatively using an autonomously updating Boolean model with
signal propagation times that depend on the recent history of the gates and
filtering of pulses of short duration, whose presence is confirmed
experimentally. Electronic Boolean chaos may find application as an
ultra-wide-band source of radio wavesComment: 10 pages and 4 figur
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]
Characterization of reconnecting vortices in superfluid helium
When two vortices cross, each of them breaks into two parts and exchanges
part of itself for part of the other. This process, called vortex reconnection,
occurs in classical as well as superfluids, and in magnetized plasmas and
superconductors. We present the first experimental observations of reconnection
between quantized vortices in superfluid helium. We do so by imaging
micron-sized solid hydrogen particles trapped on quantized vortex cores (Bewley
GP, Lathrop DP, Sreenivasan KR, 2006, Nature, 441:588), and by inferring the
occurrence of reconnection from the motions of groups of recoiling particles.
We show the distance separating particles on the just-reconnected vortex lines
grows as a power law in time. The average value of the scaling exponent is
approximately 1/2, consistent with the scale-invariant evolution of the
vortices
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]
Velocity Statistics Distinguish Quantum Turbulence from Classical Turbulence
By analyzing trajectories of solid hydrogen tracers, we find that the
distributions of velocity in decaying quantum turbulence in superfluid He
are strongly non-Gaussian with power-law tails. These features differ
from the near-Gaussian statistics of homogenous and isotropic turbulence of
classical fluids. We examine the dynamics of many events of reconnection
between quantized vortices and show by simple scaling arguments that they
produce the observed power-law tails.Comment: 4 pages, 4 figure
Perfect-fluid cylinders and walls - sources for the Levi-Civita space-time
The diagonal metric tensor whose components are functions of one spatial
coordinate is considered. Einstein's field equations for a perfect-fluid source
are reduced to quadratures once a generating function, equal to the product of
two of the metric components, is chosen. The solutions are either static fluid
cylinders or walls depending on whether or not one of the spatial coordinates
is periodic. Cylinder and wall sources are generated and matched to the vacuum
(Levi--Civita) space--time. A match to a cylinder source is achieved for
-\frac{1}{2}<\si<\frac{1}{2}, where \si is the mass per unit length in the
Newtonian limit \si\to 0, and a match to a wall source is possible for
|\si|>\frac{1}{2}, this case being without a Newtonian limit; the positive
(negative) values of \si correspond to a positive (negative) fluid density.
The range of \si for which a source has previously been matched to the
Levi--Civita metric is 0\leq\si<\frac{1}{2} for a cylinder source.Comment: 22 pages, LaTeX, one included figure. Revised version: three
(non-perfect-fluid) interior solutions are added, one of which falsifies the
original conjecture in Sec. 4, and the circular geodesics of the Levi-Civita
space-time are discussed in a footnot
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