1,451 research outputs found
An investigation on the mechanics of homogeneous expansion in gas-fluidized beds
The Richardson and Zaki (1954, Sedimentation and fluidization. Trans. Inst. Chem. Eng. 32, pp. 35–53.) equation has been used extensively to investigate the expansion profiles of homogeneous gas-fluidized beds. The experimental value of the parameter n appearing in the equation indicates how significantly interparticle forces affect the expansion of these beds, revealing the relative importance of these forces with respect to the fluid dynamic ones. In this work, we modeled the stable expansion of gas-fluidized beds of different diameter, accounting for enduring contacts among particles and wall effects. We solved the model numerically to obtain the bed expansion profiles, back-calculating from them the values of the parameter n. For all the cases considered, we observed that the values of n are higher than those obtained by purely fluid dynamic correlations, such as those advanced by Richardson and Zaki, and Rowe (1987, A convenient empirical equation for estimation of the Richardson and Zaki exponent. Chem. Eng. Sci. 42, pp. 2795.). This effect was more pronounced in beds of smaller diameter. To validate our model, we carried out fluidization and defluidization experiments, analyzing the results by means of the Richardson and Zaki equation. We obtained a reasonable agreement between numerical and experimental findings; this suggests that enduring contacts among particles, which are manifestations of cohesiveness, affect homogeneous bed expansion. This effect is amplified by wall friction
Catching Spiral - S0 transition in groups. Insights from SPH simulations with chemo-photometric implementation
We are investigating the co-evolution of galaxies within groups combining
multi-wavelength photometric and 2D kinematical observations. Here we focus on
S0s showing star formation in ring/arm-like structures. We use smooth particle
hydrodynamical simulations (SPH) with chemo-photometric implementation which
provide dynamical and morphological information together with the spectral
energy distribution (SED) at each evolutionary stage. As test cases, we
simulate the evolution of two such S0s: NGC 1533 and NGC 3626. The merging of
two halos with mass ratio 2:1, initially just composed of dark matter (DM) and
gas, well match their observed SEDs, their surface brightness profiles and
their overall kinematics. The residual star formation today "rejuvenating" the
ring/arm like structures in these S0s is then a mere consequence of a major
merger, i.e. this is a phase during the merger episode. The peculiar
kinematical features, e.g. gas-stars counter rotation in NGC 3626, depends on
the halos initial impact parameters. Furthermore, our simulations allow to
follow, in a fully consistent way, the transition of these S0s through the
green valley in the NUV-r vs. Mr colour magnitude diagram, which they cross in
about 3-5 Gyr, before reaching their current position in the red sequence. We
conclude that a viable mechanism driving the evolution of S0s in groups is of
gravitational origin.Comment: 30 pages, 6 figures; accepted for publication in Advances in Space
Research, Special Issue: Ultraviolet Astrophysic
Galaxy evolution in groups. USGC U268 and USGC U376 in the Leo cloud
With the aim of investigating galaxy evolution in nearby galaxy groups, we
analysed the spectral energy distribution of 24 galaxies, members of two groups
in the Leo cloud, USGC U268 and USGC U376. We estimated the ages and stellar
masses of the galaxies by fitting their total apparent magnitudes from
far-ultraviolet to near-infrared with population synthesis models. The
comparison of the results for a subsample of galaxies with smooth particle
hydrodynamic (SPH) simulations with chemo-photometric implementation, shows
that in most cases the estimated stellar masses obtained with the two different
approaches are in good agreement. The kinematical and dynamical analysis
indicates that USGC U268 is in a pre-virial collapse phase while USGC U376 is
likely in a more evolved phase towards virialization.Comment: 16 pages, 6 figures; accepted for publication in Advances in Space
Research, Special Issue: Ultraviolet Astrophysic
On Semiclassical Limits of String States
We explore the relation between classical and quantum states in both open and
closed (super)strings discussing the relevance of coherent states as a
semiclassical approximation. For the closed string sector a gauge-fixing of the
residual world-sheet rigid translation symmetry of the light-cone gauge is
needed for the construction to be possible. The circular target-space loop
example is worked out explicitly.Comment: 12 page
A Model for the Spectral Energy Distribution of the Ultraluminous Galaxy IRAS F10214+4724
If indeed early type galaxies used up most of their gas to form stars in a
time short compared to their collapse time and if a roughly constant fraction
of metals is locked up in dust grains, these galaxies may easily become opaque
to starlight and emit most of their luminosity in the far-IR. The corresponding
spectral energy distribution matches remarkably well the observed continuum
spectrum of the ultraluminous galaxy IRAS F from UV to sub-mm
wavelengths, i.e. over almost four decades in frequency, for a galactic age
\lsim 1\,Gyr. The bolometric luminosity in this model is \simeq 2.7\times
10^{14}\Lsol (, ), i.e. somewhat lower than implied by previous models. In the present
framework, the bolometric luminosity of the galaxy is expected to decrease by a
factor \gsim 30 during the subsequent evolution.Comment: TEX, 6 pages, 3 figures upon reques
Bar instability in cosmological halos
Aims: We want to investigate the growth of bar instability in stellar disks
embedded in a suitable dark matter halo evolving in a fully consistent
cosmological framework.
Methods: We perform seven cosmological simulations to emphasise the role of
both the disk-to-halo mass ratio and of the Toomre parameter, Q, on the
evolution of the disk.We also compare our fully cosmological cases with
corresponding isolated simulations where the same halo, is extracted from the
cosmological scenario and evolved in physical coordinates.
Results: A long living bar, lasting about 10 Gyr, appears in all our
simulations. In particular, disks expected to be stable according to classical
criteria, form indeed weak bars. We argue that such a result is due to the
dynamical properties of our cosmological halo which is far from stability and
isotropy, typical of the classical halos used in literature; it is dynamically
active, endowed of substructures and infall.
Conclusions: At least for mild self-gravitating disks, the study of the bar
instability using isolated isotropic halos, in gravitational equilibrium, can
lead to misleading results. Furthermore, the cosmological framework is needed
for quantitatively investigating such an instability.Comment: Astronomy & Astrophysics, accepted, 19 pages, 21 figure
New quadrature-based moment method for the mixing of inert polydisperse fluidized powders in commercial CFD codes
To describe the behavior of polydisperse multiphase systems in an Eulerian framework, we solved the population balance equation (PBE), letting it account only for particle size dependencies. To integrate the PBE within a commercial computational fluid dynamics code, we formulated and implemented a novel version of the quadrature method of moments (QMOM). This no longer assumes that the particles move with the same velocity, allowing the latter to be size-dependent. To verify and test the model, we simulated the mixing of inert polydisperse fluidized suspensions initially segregated, validating the results experimentally. Because the accuracy of QMOM increases with the number of moments tracked, we ran three classes of simulations, preserving the first four, six, and eight integer moments of the particle density function. We found that in some cases the numerics corrupts the higher-order moments and a corrective algorithm, designed to restore the validity of the moment set, has to be implemented
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