621 research outputs found
Understanding the effect of seams on the aerodynamics of an association football
The aerodynamic properties of an association football were measured using a wind tunnel arrangement. A third scale model of a generic football (with seams) was used in addition to a 'mini-football'. As the wind speed was increased, the drag coefficient decreased from 0.5 to 0.2, suggesting a transition from laminar to turbulent behaviour in the boundary layer. For spinning footballs, the Magnus effect was observed and it was found that reverse Magnus effects were possible at low Reynolds numbers. Measurements on spinning smooth spheres found that laminar behaviour led to a high drag coefficient for a large range of Reynolds numbers, and Magnus effects were inconsistent, but generally showed reverse Magnus behaviour at high Reynolds number and spin parameter. Trajectory simulations of free kicks demonstrated that a football that is struck in the centre will follow a near straight trajectory, dipping slightly before reaching the goal, whereas a football that is struck off centre will bend before reaching the goal, but will have a significantly longer flight time. The curving kick simulation was repeated for a smooth ball, which resulted in a longer flight time, due to increased drag, and the ball curving in the opposite direction, due to reverse Magnus effects. The presence of seams was found to encourage turbulent behaviour, resulting in reduced drag and more predictable Magnus behaviour for a conventional football, compared with a smooth ball. © IMechE 2005
Prediction of metabolisable energy value of broiler diets and water excretion from dietary chemical analyses
Collision induced cluster fragmentation: From fragment size distributions to the caloric curve
IPMInternational audienceWe report on a cluster fragmentation study involving collisions of high-energy (60 keV/amu) H3+(H2)m hydrogen cluster ions (m=9, 11) with atomic helium or fullerenes. The experimental characterisation of the cluster fragmentation not only by the average fragment size distribution but also by a statistical analysis of the fragmentation events has become possible owing to a recently developed multi-coincidence technique in which all the fragments of all collisions occurring in the experiment are mass analysed on an event-by-event basis. By selecting specific decay reactions we can start after the energizing collision with a microcanonical cluster ion ensemble of fixed excitation energy. From the respective fragment distributions for these selected decay reactions we derive corresponding temperatures of the decaying cluster ions. The relation between this temperature and the excitation energy (caloric curve) exhibits the typical prerequisites of a first order phase transition in a finite system, in the present case signalling the transition from a bound cluster type situation to the free gas phase
Electron-loss and target ionization cross sections for water vapor by 20-150 keV neutral atomic hydrogen impact
A complete set of cross sections is reported for the ionization of water
molecules by neutral atomic hydrogen impact at velocities of the order of the
Bragg peak. The measured relative cross sections are normalized by comparison
with proton impact results for the same target conditions and previous absolute
data. Event by event coincidence analysis of the product ions and the
projectile enables partial cross sections for target ionization and target plus
projectile ionization to be determined, as well as total cross sections for
electron loss reactions.Comment: To be published in Chemical Physics Letter
Attosecond emission from chromium plasma
International audienceWe present the first measurement of the attosecond emission generated from underdense plasma produced on a solid target. We generate high-order harmonics of a femtosecond Ti:sapphire laser focused in a weakly ionized underdense chromium plasma. Using the " Reconstruction of Attosecond Beating by Interference of Two-photon Transitions " (RABITT) technique, we show that the 11 th to the 19 th harmonic orders form in the time domain an attosecond pulse train with each pulse having 300 as duration, which is only 1.05 times the theoretical Fourier transform limit. Measurements reveal a very low positive group delay dispersion of 4200 as 2. Beside its fundamental interest, high-order harmonic generation in plasma plumes could thus provide an intense source of attosecond pulses for applications
Bond breaking in vibrationally excited methane on transition metal catalysts
The role of vibrational excitation of a single mode in the scattering of
methane is studied by wave packet simulations of oriented CH4 and CD4 molecules
from a flat surface. All nine internal vibrations are included. In the
translational energy range from 32 up to 128 kJ/mol we find that initial
vibrational excitations enhance the transfer of translational energy towards
vibrational energy and increase the accessibility of the entrance channel for
dissociation. Our simulations predict that initial vibrational excitations of
the asymmetrical stretch (nu_3) and especially the symmetrical stretch (nu_1)
modes will give the highest enhancement of the dissociation probability of
methane.Comment: 4 pages REVTeX, 2 figures (eps), to be published in Phys. Rev. B.
(See also arXiv:physics.chem-ph/0003031). Journal version at
http://publish.aps.org/abstract/PRB/v61/p1565
Asymptotics of Selberg-like integrals: The unitary case and Newton's interpolation formula
We investigate the asymptotic behavior of the Selberg-like integral ,
as for different scalings of the parameters and with .
Integrals of this type arise in the random matrix theory of electronic
scattering in chaotic cavities supporting channels in the two attached
leads. Making use of Newton's interpolation formula, we show that an asymptotic
limit exists and we compute it explicitly
Semiring and semimodule issues in MV-algebras
In this paper we propose a semiring-theoretic approach to MV-algebras based
on the connection between such algebras and idempotent semirings - such an
approach naturally imposing the introduction and study of a suitable
corresponding class of semimodules, called MV-semimodules.
We present several results addressed toward a semiring theory for
MV-algebras. In particular we show a representation of MV-algebras as a
subsemiring of the endomorphism semiring of a semilattice, the construction of
the Grothendieck group of a semiring and its functorial nature, and the effect
of Mundici categorical equivalence between MV-algebras and lattice-ordered
Abelian groups with a distinguished strong order unit upon the relationship
between MV-semimodules and semimodules over idempotent semifields.Comment: This version contains some corrections to some results at the end of
Section
Attosecond chirp-encoded dynamics of light nuclei Attosecond chirp-encoded dynamics of light nuclei
International audienceWe study the spectral phase of high-order harmonic emission as an observable for probing ultrafast nuclear dynamics after the ionization of a molecule. Using a strong-field approximation theory that includes nuclear dynamics, we relate the harmonic phase to the phase of the overlap integral of the nuclear wavefunctions of the initial neutral molecule and the molecular ion after an attosecond probe delay. We determine experimentally the group delay of the high harmonic emission from D 2 and H 2 molecules, which allows us to verify the relation between harmonic frequency and the attosecond delay. The small difference in the harmonic phase between H 2 and D 2 calculated theoretically is consistent with our experimental results
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