12,419 research outputs found
Effects of fluid inertia and turbulence on force coefficients for squeeze film dampers
The effects of fluid inertia and turbulence on the force coefficients of squeeze film dampers are investigated analytically. Both the convective and the temporal terms are included in the analysis of inertia effects. The analysis of turbulence is based on friction coefficients currently found in the literature for Poiseuille flow. The effect of fluid inertia on the magnitude of the radial direct inertia coefficient (i.e., to produce an apparent added mass at small eccentricity ratios, due to the temporal terms) is found to be completely reversed at large eccentricity ratios. The reversal is due entirely to the inclusion of the convective inertia terms in the analysis. Turbulence is found to produce a large effect on the direct damping coefficient at high eccentricity ratios. For the long or sealed squeeze film damper at high eccentricity ratios, the damping prediction with turbulence included is an order of magnitude higher than the laminar solution
Chemical Oscillations out of Chemical Noise
The dynamics of one species chemical kinetics is studied. Chemical reactions
are modelled by means of continuous time Markov processes whose probability
distribution obeys a suitable master equation. A large deviation theory is
formally introduced, which allows developing a Hamiltonian dynamical system
able to describe the system dynamics. Using this technique we are able to show
that the intrinsic fluctuations, originated in the discrete character of the
reagents, may sustain oscillations and chaotic trajectories which are
impossible when these fluctuations are disregarded. An important point is that
oscillations and chaos appear in systems whose mean-field dynamics has too low
a dimensionality for showing such a behavior. In this sense these phenomena are
purely induced by noise, which does not limit itself to shifting a bifurcation
threshold. On the other hand, they are large deviations of a short transient
nature which typically only appear after long waiting times. We also discuss
the implications of our results in understanding extinction events in
population dynamics models expressed by means of stoichiometric relations
Towards a Comprehensive Fueling-Controlled Theory on the Growth of Massive Black Holes and Host Spheroids
We study the relation between nuclear massive black holes and their host
spheroid gravitational potential. Using AMR numerical simulations, we analyze
how gas is transported in the nuclear (central kpc) regions of galaxies. We
study the gas fueling onto the inner accretion disk (sub-pc scale) and the star
formation in a massive nuclear disk like those generally found in
proto-spheroids (ULIRGs, SCUBA Galaxies). These sub-pc resolution simulation of
gas fueling that is mainly depleted by star formation naturally satisfy the
`M_BH - $M_virial' relation, with a scatter considerably less than the observed
one. We found a generalized version of Kennicutt-Schmidt Law for starbursts is
satisfied, in which the total gas depletion rate (dot{M}_gas = dot{M}_BH +
dot{M}_SF) is the one that scales as M_gas/t_orbital. We also found that the
`M_BH - sigma' relation is a byproduct of the `M_BH - M_virial' relation in the
fueling controlled scenario.Comment: 12 pages, figures, submited to ApJ, email: [email protected]
Diffusion of Hydrogen in Pd Assisted by Inelastic Ballistic Hot Electrons
Sykes {\it et al.} [Proc. Natl. Acad. Sci. {\bf 102}, 17907 (2005)] have
reported how electrons injected from a scanning tunneling microscope modify the
diffusion rates of H buried beneath Pd(111). A key point in that experiment is
the symmetry between positive and negative voltages for H extraction, which is
difficult to explain in view of the large asymmetry in Pd between the electron
and hole densities of states. Combining concepts from the theory of ballistic
electron microscopy and electron-phonon scattering we show that H diffusion is
driven by the -band electrons only, which explains the observed symmetry.Comment: 5 pages and 4 figure
Orbital-Free Molecular Dynamics Simulations of Melting in Na8 and Na20: Melting in Steps
The melting-like transitions of Na8 and Na20 are investigated by ab initio
constant energy molecular dynamics simulations, using a variant of the
Car-Parrinello method which employs an explicit electronic kinetic energy
functional of the density, thus avoiding the use of one-particle orbitals.
Several melting indicators are evaluated in order to determine the nature of
the various transitions, and compared with other simulations. Both Na8 and Na20
melt over a wide temperature range. For Na8, a transition is observed to begin
at approx. 110 K, between a rigid phase and a phase involving isomerizations
between the different permutational isomers of the ground state structure. The
``liquid'' phase is completely established at approx. 220 K. For Na20, two
transitions are observed: the first, at approx. 110 K, is associated with
isomerization transitions between those permutational isomers of the ground
state structure which are obtained by interchanging the positions of the
surface-like atoms; the second, at approx. 160 K, involves a structural
transition from the ground state isomer to a new set of isomers with the
surface molten. The cluster is completely ``liquid'' at approx. 220 K.Comment: Revised version, accepted for publication in J. Chem. Phys. The
changes include longer simulations for the Na20 microcluster, a more complete
comparison to previous theoretical results, and the discussion of some
technical details of the method applie
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First CRDS-measurements of water vapour continuum in the 940nm absorption band
Measurements of near-infrared water vapour continuum using continuous wave cavity ring down spectroscopy (cw-
CRDS) have been performed at around 10611.6 and 10685:2 cm1. The continuum absorption coefficients for N2-
broadening have been determined for two temperatures and wavenumbers.
These results represent the first near-IR continuum laboratory data determined within the complex spectral environment in the 940nm water vapour band and are in reasonable agreement with simulations using the semiempirical CKD formulation
A Scorza-Dragoni approach to second-order boundary value problems in abstract spaces
The existence and localization of strong (Carathéodory) solutions is proved for a second-order Floquet problem in a Banachspace. The result is obtained by combining a continuation principle together with a bounding (Liapunov-like) functions approach. Theapplication of the Scorza–Dragoni type technique allows us to use strictly localized transversality conditions
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