33,085 research outputs found
Polydispersity Effects in the Dynamics and Stability of Bubbling Flows
The occurrence of swarms of small bubbles in a variety of industrial systems
enhances their performance. However, the effects that size polydispersity may
produce on the stability of kinematic waves, the gain factor, mean bubble
velocity, kinematic and dynamic wave velocities is, to our knowledge, not yet
well established. We found that size polydispersity enhances the stability of a
bubble column by a factor of about 23% as a function of frequency and for a
particular type of bubble column. In this way our model predicts effects that
might be verified experimentally but this, however, remain to be assessed. Our
results reinforce the point of view advocated in this work in the sense that a
description of a bubble column based on the concept of randomness of a bubble
cloud and average properties of the fluid motion, may be a useful approach that
has not been exploited in engineering systems.Comment: 11 pages, 2 figures, presented at the 3rd NEXT-SigmaPhi International
Conference, 13-18 August, 2005, Kolymbari, Cret
Momentum distributions of -particles from decaying low-lying C-resonances
The complex scaled hyperspherical adiabatic expansion method is used to
compute momentum and energy distributions of the three -particles
emerging from the decay of low-lying C-resonances. The large distance
continuum properties of the wave functions are crucial and must be accurately
calculated. We discuss separately decays of natural parity states: two ,
one , three , one , two , one , and one of each of
unnatural parity, , , , . The lowest natural parity state
of each decays predominantly sequentially via the Be ground
state whereas other states including unnatural parity states predominantly
decay directly to the continuum. We present Dalitz plots and systematic
detailed momentum correlations of the emerging -particles.Comment: 11 pages, 7 figures, accepted for publication in Physical Review
Non-gaussianity from the second-order cosmological perturbation
Several conserved and/or gauge invariant quantities described as the
second-order curvature perturbation have been given in the literature. We
revisit various scenarios for the generation of second-order non-gaussianity in
the primordial curvature perturbation \zeta, employing for the first time a
unified notation and focusing on the normalisation f_{NL} of the bispectrum.
When the classical curvature perturbation first appears a few Hubble times
after horizon exit, |f_{NL}| is much less than 1 and is, therefore, negligible.
Thereafter \zeta (and hence f_{NL}) is conserved as long as the pressure is a
unique function of energy density (adiabatic pressure). Non-adiabatic pressure
comes presumably only from the effect of fields, other than the one pointing
along the inflationary trajectory, which are light during inflation (`light
non-inflaton fields'). During single-component inflation f_{NL} is constant,
but multi-component inflation might generate |f_{NL}| \sim 1 or bigger.
Preheating can affect f_{NL} only in atypical scenarios where it involves light
non-inflaton fields. The curvaton scenario typically gives f_{NL} \ll -1 or
f_{NL} = +5/4. The inhomogeneous reheating scenario can give a wide range of
values for f_{NL}. Unless there is a detection, observation can eventually
provide a limit |f_{NL}| \lsim 1, at which level it will be crucial to
calculate the precise observational limit using second order theory.Comment: Latex file in Revtex style. 13 pages, 1 figure. v2: minor changes.
Discussion in Subsection VI-A enlarged. References added. Conclusions
unchanged. v3: minor typographic changes. Correlated and uncorrelated \chi^2
non-gaussianity concepts and consequences introduced. Section VI-A enlarged.
Small change in Table I. References updated and added. Conclusions unchanged.
Version to appear in Physical Review
Polyradical character and spin frustration in fullerene molecules: An ab initio non-collinear Hartree--Fock study
Most {\em ab initio} calculations on fullerene molecules have been carried
out based on the paradigm of the H\"uckel model. This is consistent with the
restricted nature of the independent-particle model underlying such
calculations, even in single-reference-based correlated approaches. On the
other hand, previous works on some of these molecules using model Hamiltonians
have clearly indicated the importance of short-range inter-atomic spin-spin
correlations. In this work, we consider {\em ab initio} non-collinear
Hartree--Fock (HF) solutions for representative fullerene systems: the bowl,
cage, ring, and pentagon isomers of C, and the larger C,
C, C, C, and C fullerene cages. In all cases but
the ring we find that the HF minimum corresponds to a truly non-collinear
solution with a torsional spin density wave. Optimized geometries at the
generalized HF (GHF) level lead to fully symmetric structures, even in those
cases where Jahn-Teller distortions have been previously considered. The nature
of the GHF solutions is consistent with the -electron space becoming
polyradical in nature: each -orbital remains effectively singly occupied.
The spin frustration, induced by the pentagon rings in an otherwise
anti-ferromagnetic background, is minimized at the HF level by aligning the
spins in non-collinear arrangements. The long-range magnetic ordering observed
is reminiscent of the character of broken symmetry HF solutions in polyacene
systems.Comment: 16 figure
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