558 research outputs found
Experiments of a twenty cell PEFC operating under fault conditions with diode by-pass circuit for uninterrupted power delivery
International audienceThe work presents the results of experiments related to the electrical and dynamical behaviour of a 500W, twenty cell Polymer Electrolyte Fuel Cell (PEFC) stack operated under fault condition and connected to an anti-parallel diode acting as a by-pass. The stack is placed in an experimental set-up that reproduces the electrical coupling in series of two fuel cells. The results allow the evaluation of the by-pass diode solution in the case of specific degraded working modes such as the break of the gas reactant feeding. The experiments presented in this article constitute an extrapolation and a complementary investigation of the preliminary results already achieved on a two cell PEFC stack and which had demonstrated the capability of the reverse diode to electrically isolate a fuel cell stack under fault. The proposed experiments focus on the dynamic behaviour of the stack under degraded working modes and point out the key-role of the fuel cell stack impedance in the triggering of the anti-parallel diode switching
A Kato type Theorem for the inviscid limit of the Navier-Stokes equations with a moving rigid body
The issue of the inviscid limit for the incompressible Navier-Stokes
equations when a no-slip condition is prescribed on the boundary is a famous
open problem. A result by Tosio Kato says that convergence to the Euler
equations holds true in the energy space if and only if the energy dissipation
rate of the viscous flow in a boundary layer of width proportional to the
viscosity vanishes. Of course, if one considers the motion of a solid body in
an incompressible fluid, with a no-slip condition at the interface, the issue
of the inviscid limit is as least as difficult. However it is not clear if the
additional difficulties linked to the body's dynamic make this issue more
difficult or not. In this paper we consider the motion of a rigid body in an
incompressible fluid occupying the complementary set in the space and we prove
that a Kato type condition implies the convergence of the fluid velocity and of
the body velocity as well, what seems to indicate that an answer in the case of
a fixed boundary could also bring an answer to the case where there is a moving
body in the fluid
Well-posedness of boundary layer equations for time-dependent flow of non-Newtonian fluids
We consider the flow of an upper convected Maxwell fluid in the limit of high
Weissenberg and Reynolds number. In this limit, the no-slip condition cannot be
imposed on the solutions. We derive equations for the resulting boundary layer
and prove the well-posedness of these equations. A transformation to Lagrangian
coordinates is crucial in the argument
Possible large phase in psi(2S) -> 1-0- Decays
The strong and the electromagnetic amplitudes are analyzed on the basis of
the measurements of J/psi, psi(2S) -> 1-0- in e+e- experiments. The currently
available experimental information is revised with inclusion of the
contribution from e+e- -> gamma * -> 1-0- . The study shows that a large phase
around minus 90 degree between the strong and the electromagnetic amplitudes
could not be ruled out by the experimental data for psi(2S).Comment: 4 page
On the dynamics of WKB wave functions whose phase are weak KAM solutions of H-J equation
In the framework of toroidal Pseudodifferential operators on the flat torus
we begin by proving the closure under
composition for the class of Weyl operators with
simbols . Subsequently, we
consider when where and we exhibit the toroidal version of the
equation for the Wigner transform of the solution of the Schr\"odinger
equation. Moreover, we prove the convergence (in a weak sense) of the Wigner
transform of the solution of the Schr\"odinger equation to the solution of the
Liouville equation on written in the measure sense.
These results are applied to the study of some WKB type wave functions in the
Sobolev space with phase functions in the class
of Lipschitz continuous weak KAM solutions (of positive and negative type) of
the Hamilton-Jacobi equation for with , and to the study of the
backward and forward time propagation of the related Wigner measures supported
on the graph of
Displacements analysis of self-excited vibrations in turning
The actual research deals with determining by a new protocol the necessary
parameters considering a three-dimensional model to simulate in a realistic way
the turning process on machine tool. This paper is dedicated to the
experimental displacements analysis of the block tool / block workpiece with
self-excited vibrations. In connexion with turning process, the self-excited
vibrations domain is obtained starting from spectra of two accelerometers. The
existence of a displacements plane attached to the tool edge point is revealed.
This plane proves to be inclined compared to the machines tool axes. We
establish that the tool tip point describes an ellipse. This ellipse is very
small and can be considered as a small straight line segment for the stable
cutting process (without vibrations). In unstable mode (with vibrations) the
ellipse of displacements is really more visible. A difference in phase occurs
between the tool tip displacements on the radial direction and on the cutting
one. The feed motion direction and the cutting one are almost in phase. The
values of the long and small ellipse axes (and their ratio) shows that these
sizes are increasing with the feed rate value. The axis that goes through the
stiffness center and the tool tip represents the maximum stiffness direction.
The maximum (resp. minimum) stiffness axis of the tool is perpendicular to the
large (resp. small) ellipse displacements axis. FFT analysis of the
accelerometers signals allows to reach several important parameters and
establish coherent correlations between tool tip displacements and the static -
elastic characteristics of the machine tool components tested
Semi-inclusive B Decays and Direct CP Violation in QCD Factorization
We have systematically investigated the semi-inclusive B decays B->MX, which
are manifestations of the quark decay b->Mq, within the framework of
QCD-improved factorization. These decays are theoretically clean and have
distinctive experimental signatures. We focus on a class of these that do not
require any form factor information and therefore may be especially suitable
for extracting information on the angles and of the unitarity
triangle. The nonfactorizable effects, such as vertex-type and penguin-type
corrections to the two-body b decay and hard spectator corrections to the
3-body decay are calculable in the heavy quark limit. QCD factorization is
applicable when the emitted meson is a light meson or a charmonium. We discuss
the issue of the CPT constraint on partial rate asymmetries. The strong phase
coming from final-state rescattering due to hard gluon exchange between the
final states can induce large rate asymmetries for tree-dominated
color-suppressed modes . The nonfactorizable
hard spectator interactions in the 3-body decay, though phase-space suppressed,
are extremely important for the tree-dominated modes
, and the
penguin-dominated mode . In fact, they are dominated by the
hard spectator corrections. Our result for is in
agreement with experiment. The semi-inclusive decay modes: , ,
and are the most promising ones in searching for
direct CP violation. In fact, they have branching ratios of order
and CP rate asymmetries of order .Comment: 28 page
Controlling spin relaxation with a cavity
Spontaneous emission of radiation is one of the fundamental mechanisms by
which an excited quantum system returns to equilibrium. For spins, however,
spontaneous emission is generally negligible compared to other non-radiative
relaxation processes because of the weak coupling between the magnetic dipole
and the electromagnetic field. In 1946, Purcell realized that the spontaneous
emission rate can be strongly enhanced by placing the quantum system in a
resonant cavity -an effect which has since been used extensively to control the
lifetime of atoms and semiconducting heterostructures coupled to microwave or
optical cavities, underpinning single-photon sources. Here we report the first
application of these ideas to spins in solids. By coupling donor spins in
silicon to a superconducting microwave cavity of high quality factor and small
mode volume, we reach for the first time the regime where spontaneous emission
constitutes the dominant spin relaxation mechanism. The relaxation rate is
increased by three orders of magnitude when the spins are tuned to the cavity
resonance, showing that energy relaxation can be engineered and controlled
on-demand. Our results provide a novel and general way to initialise spin
systems into their ground state, with applications in magnetic resonance and
quantum information processing. They also demonstrate that, contrary to popular
belief, the coupling between the magnetic dipole of a spin and the
electromagnetic field can be enhanced up to the point where quantum
fluctuations have a dramatic effect on the spin dynamics; as such our work
represents an important step towards the coherent magnetic coupling of
individual spins to microwave photons.Comment: 8 pages, 6 figures, 1 tabl
Combining CP Asymmetries in Decays
We prove an approximate relation, to leading order in dominant terms, between
CP-violating rate differences in and
. We show how data from these two processes may be
combined in order to enhance the significance of a nonzero result.Comment: 9 pages, latex, no figures, submitted to Phys. Rev. Letters, revise
- âŠ