17,780 research outputs found
Boundary-layer turbulence in experiments of quasi-Keplerian flows
Most flows in nature and engineering are turbulent because of their large
velocities and spatial scales. Laboratory experiments of rotating
quasi-Keplerian flows, for which the angular velocity decreases radially but
the angular momentum increases, are however laminar at Reynolds numbers
exceeding one million. This is in apparent contradiction to direct numerical
simulations showing that in these experiments turbulence transition is
triggered by the axial boundaries. We here show numerically that as the
Reynolds number increases turbulence becomes progressively confined to the
boundary layers and the flow in the bulk fully relaminarizes. Our findings
support that turbulence is unlikely to occur in isothermal constant density
quasi-Keplerian flows.Comment: 16 pages, 8 figures. Accepted for publication in Journal of Fluid
Mechanic
A Direct Reputation Model for VO Formation
We show that reputation is a basic ingredient in the Virtual Organisation (VO) formation process. Agents can use their experiences gained in direct past interactions to model other’s reputation and deciding on either join a VO or determining who is the most suitable set of partners. Reputation values are computed using a reinforcement learning algorithm, so agents can learn and adapt their reputation models of their partners according to their recent behaviour. Our approach is especially powerful if the agent participates in a VO in which the members can change their behaviour to exploit their partners. The reputation model presented in this paper deals with the questions of deception and fraud that have been ignored in current models of VO formation
Extended Derivative Dispersion Relations
It is shown that, for a wide class of functions with physical interest as
forward scattering amplitudes, integral dispersion relations can be replaced by
derivative forms without any high-energy approximation. The applicability of
these extended derivative relations, in the investigation of forward
proton-proton and antiproton-proton elastic scattering, is exemplified by means
of a Pomeron-Reggeon model with totally nondegenerate trajectories.Comment: 7 pages, 1 figure, contribution to "Sense of Beauty in Physics",
Miniconference in Honor of Adriano Di Giacomo on his 70th Birthday, Pisa,
Italy, Jan. 26-27, 200
Derivative dispersion relations above the physical threshold
We discuss some formal and practical aspects related to the replacement of
Integral Dispersion Relations (IDR) by derivative forms, without high-energy
approximations. We first demonstrate that, for a class of functions with
physical interest as forward scattering amplitudes, this replacement can be
analytically performed, leading to novel Extended Derivative Dispersion
Relations (EDDR), which, in principle, are valid for any energy above the
physical threshold. We then verify the equivalence between the IDR and EDDR by
means of a popular parametrization for total cross sections from proton-proton
and antiproton-proton scattering and compare the results with those obtained
through other representations for the derivative relations. Critical aspects on
the limitations of the whole analysis, from both formal and practical points of
view, are also discussed in some detail.Comment: Final version, published in Brazilian Journal of Physics, V. 37, 358
(2007
\phi K^{+}K^{-} production in electron-positron annihilation
In this work we study the e^{+}e^{-}\to\phi K^{+}K^{-} reaction. The leading
order electromagnetic contributions to this process involve the \gamma*\phi\
K^{+}K^{-} vertex function with a highly virtual photon. We calculate this
function at low energies using R\chi PT supplemented with the anomalous term
for the VV'P interactions. Tree level contributions involve the kaon form
factors and the K*K transition form factors. We improve this result, valid for
low photon virtualities, replacing the lowest order terms in the kaon form
factors and K*K transition form factors by the form factors as obtained in
U\chi PT in the former case and the ones extracted from recent data on
e^{+}e^{-}\to KK* in the latter case. We calculate rescattering effects which
involve meson-meson amplitudes. The corresponding result is improved using the
unitarized meson-meson amplitudes containing the scalar poles instead of the
lowest order terms. Using the BABAR value for BR(X\to \phi f_{0})\Gamma (X\to
e^{+} e^{-}), we calculate the contribution from intermediate X(2175). A good
description of data is obtained in the case of destructive interference between
this contribution and the previous ones, but more accurate data on the
isovector K*K transition form factor is required in order to exclude
contributions from an intermediate isovector resonance to e^{+}e^{-}\to \phi\
K^{+}K^{-} around 2.2 GeV.Comment: 21 pages, 17 figures. Revised version to appear in Phys. Rev. D.
Contributions of intermediate X(2175) included. Extraction of form factors
update
Columnar defects acting as passive internal field detectors
We have studied the angular dependence of the irreversible magnetization of
several YBaCuO and 2H-NbSe single crystals with columnar
defects tilted off the c-axis. At high magnetic fields, the irreversible
magnetization exhibits a well known maximum when the applied
field is parallel to the tracks. As the field is decreased below , the peak shifts away from the tracks' direction toward either the
c-axis or the ab-planes. We demonstrate that this shift results from the
misalignment between the external and internal field directions due to the
competition between anisotropy and geometry effects.Comment: 5 figure
Charge transfer during individual collisions in ice growing by riming
The charging of a target by riming in the wind was studied in the temperature range of (-10, -18 C). For each temperature, charge transfers of both signs are observed and, according to the environmental conditions, one of them prevails. The charge is more positive as the liquid water concentration is increased at any particular temperature. It is found that even at the low impact velocities used (5 m/s) there is abundant evidence of fragmentation following the collision
Cage-size control of guest vibration and thermal conductivity in Sr8Ga16Si30-xGex
We present a systematic study of thermal conductivity, specific heat,
electrical resistivity, thermopower and x-ray diffraction measurements
performed on single-crystalline samples of the pseudoquaternary type-I
clathrate system Sr8Ga16Si30-xGex, in the full range of 0 < x < 30. All the
samples show metallic behavior with n-type majority carriers. However, the
thermal conductivity and specific heat strongly depend on x. Upon increasing x
from 0 to 30, the lattice parameter increases by 3%, from 10.446 to 10.726 A,
and the localized vibrational energies of the Sr guest ions in the
tetrakaidekahedron (dodecahedron) cages decrease from 59 (120) K to 35 (90) K.
Furthermore, the lattice thermal conductivity at low temperatures is largely
suppressed. In fact, a crystalline peak found at 15 K for x = 0 gradually
decreases and disappears for x > 20, evolving into the anomalous glass-like
behavior observed for x = 30. It is found that the increase of the free space
for the Sr guest motion directly correlates with a continuous transition from
on-center harmonic vibration to off-center anharmonic vibration, with
consequent increase in the coupling strength between the guest's low-energy
modes and the cage's acoustic phonon modes.Comment: 7 pages, 7 figures, submitted to PR
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