13,130 research outputs found
Large-scale anisotropy in stably stratified rotating flows
We present results from direct numerical simulations of the Boussinesq
equations in the presence of rotation and/or stratification, both in the
vertical direction. The runs are forced isotropically and randomly at small
scales and have spatial resolutions of up to grid points and Reynolds
numbers of . We first show that solutions with negative energy
flux and inverse cascades develop in rotating turbulence, whether or not
stratification is present. However, the purely stratified case is characterized
instead by an early-time, highly anisotropic transfer to large scales with
almost zero net isotropic energy flux. This is consistent with previous studies
that observed the development of vertically sheared horizontal winds, although
only at substantially later times. However, and unlike previous works, when
sufficient scale separation is allowed between the forcing scale and the domain
size, the total energy displays a perpendicular (horizontal) spectrum with
power law behavior compatible with , including in the
absence of rotation. In this latter purely stratified case, such a spectrum is
the result of a direct cascade of the energy contained in the large-scale
horizontal wind, as is evidenced by a strong positive flux of energy in the
parallel direction at all scales including the largest resolved scales
Evidence for Bolgiano-Obukhov scaling in rotating stratified turbulence using high-resolution direct numerical simulations
We report results on rotating stratified turbulence in the absence of
forcing, with large-scale isotropic initial conditions, using direct numerical
simulations computed on grids of up to 4096^3 points. The Reynolds and Froude
numbers are respectively equal to Re=5.4 x 10^4 and Fr=0.0242. The ratio of the
Brunt-V\"ais\"al\"a to the inertial wave frequency, N/f, is taken to be equal
to 4.95, a choice appropriate to model the dynamics of the southern abyssal
ocean at mid latitudes. This gives a global buoyancy Reynolds number
R_B=ReFr^2=32, a value sufficient for some isotropy to be recovered in the
small scales beyond the Ozmidov scale, but still moderate enough that the
intermediate scales where waves are prevalent are well resolved. We concentrate
on the large-scale dynamics, for which we find a spectrum compatible with the
Bolgiano-Obukhov scaling, and confirm that the Froude number based on a typical
vertical length scale is of order unity, with strong gradients in the vertical.
Two characteristic scales emerge from this computation, and are identified from
sharp variations in the spectral distribution of either total energy or
helicity. A spectral break is also observed at a scale at which the partition
of energy between the kinetic and potential modes changes abruptly, and beyond
which a Kolmogorov-like spectrum recovers. Large slanted layers are ubiquitous
in the flow in the velocity and temperature fields, with local overturning
events indicated by small Richardson numbers, and a small large-scale
enhancement of energy directly attributable to the effect of rotation is also
observed.Comment: 19 pages, 9 figures (including compound figures
Inverse cascades and resonant triads in rotating and stratified turbulence
Kraichnan’s seminal ideas on inverse cascades yielded new tools to study common phenomena in geophysical turbulent flows. In the atmosphere and the oceans, rotation and stratification result in a flow that can be approximated as two-dimensional at very large scales but which requires considering three-dimensional effects to fully describe turbulent transport processes and non-linear phenomena. Motions can thus be classified into two classes: fast modes consisting of inertia-gravity waves and slow quasi-geostrophic modes for which the Coriolis force and horizontal pressure gradients are close to balance. In this paper, we review previous results on the strength of the inverse cascade in rotating and stratified flows and then present new results on the effect of varying the strength of rotation and stratification (measured by the inverse Prandtl ratio N/f, of the Coriolis frequency to the Brunt-Väisäla frequency) on the amplitude of the waves and on the flow quasi-geostrophic behavior. We show that the inverse cascade is more efficient in the range of N/f for which resonant triads do not exist, /2≤N/f≤21/2≤N/f≤2. We then use the spatio-temporal spectrum to show that in this range slow modes dominate the dynamics, while the strength of the waves (and their relevance in the flow dynamics) is weaker.Fil: Oks, D.. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de FÃsica; ArgentinaFil: Mininni, Pablo Daniel. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de FÃsica de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de FÃsica de Buenos Aires; ArgentinaFil: Marino, R.. Universite Lyon 2; FranciaFil: Pouquet, A.. State University of Colorado Boulder; Estados Unido
Vanishing conductivity of quantum solitons in polyacetylene
Quantum solitons or polarons are supposed to play a crucial role in the
electric conductivity of polyacetylene, in the intermediate doping regime. We
present an exact fully quantized calculation of the quantum soliton
conductivity in polyacetylene and show that it vanishes exactly. This is
obtained by applying a general method of soliton quantization, based on
order-disorder duality, to a Z(2)-symmetric complex extension of the TLM
dimerization effective field theory. We show that, in this theory,
polyacetylene solitons are sine-Gordon solitons in the phase of the complex
field.Comment: To appear in J. Phys. A: Math. Theor., 15 page
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
Observing multiple stellar populations with FORS2@VLT - Main sequence photometry in outer regions of NGC 6752, NGC 6397, and NGC 6121 (M 4)
We present the photometric analysis of the external regions of three Galactic
Globular Clusters: NGC 6121, NGC 6397 and NGC 6752. The main goal is the
characterization of the multiple stellar populations along the main sequence
(MS) and the study of the radial trend of the different populations hosted by
the target clusters. The data have been collected using FORS2 mounted at the
ESO/VLT@UT1 telescope in UBVI filters. From these data sets we extracted
high-accuracy photometry and constructed color-magnitude diagrams. We exploit
appropriate combination of colors and magnitudes which are powerful tools to
identify multiple stellar populations, like B versus U-B and V versus
c_{U,B,I}=(U-B)-(B-I) CMDs. We confirm previous findings of a split MS in NGC
6752 and NGC 6121. Apart from the extreme case of omega Centauri, this is the
first detection of multiple MS from ground-based photometry. For NGC 6752 and
NGC 6121 we compare the number ratio of the blue MS to the red MS in the
cluster outskirts with the fraction of first and second generation stars
measured in the central regions. There is no evidence for significant radial
trend. The MS of NGC 6397 is consistent with a simple stellar population. We
propose that the lack of multiple sequences is due both to observational errors
and to the limited sensitivity of U,B,V,I photometry to multiple stellar
populations in metal-poor GCs. Finally, we compute the helium abundance for the
stellar populations hosted by NGC 6121 and NGC 6752, finding a mild (Delta Y ~
0.02) difference between stars in the two sequences.Comment: 16 pages, 5 tables, 17 figures, accepted for pubblication in A&
Galaxy Evolution in Local Group Analogs. I. A GALEX study of nearby groups
Understanding the astrophysical processes acting within galaxy groups and
their effects on the evolution of the galaxy population is one of the crucial
topic of modern cosmology, as almost 60% of galaxies in the Local Universe are
found in groups. We imaged in the far (FUV 1539 A) and near ultraviolet (NUV
2316 A) with GALEX three nearby groups, namely LGG93, LGG127 and LGG225. We
obtained the UV galaxy surface photometry and, for LGG225, the only group
covered by the SDSS, the photometry in u, g, r, i, z bands. We discuss galaxy
morphologies looking for interaction signatures and we analyze the SED of
galaxies to infer their luminosity-weighted ages. The UV and optical photometry
was also used to perform a kinematical and dynamical analysis of each group and
to evaluate the stellar mass. A few member galaxies in LGG225 show a distorted
UV morphology due to ongoing interactions. (FUV-NUV) colors suggest that
spirals in LGG93 and LGG225 host stellar populations in their outskirts younger
than that of M31 and M33 in the LG or with less extinction. The irregular
interacting galaxy NGC3447A has a significantly younger stellar population (few
Myr old) than the average of the other irregular galaxies in LGG225 suggesting
that the encounter triggered star formation. The early-type members of LGG225,
NGC3457 and NGC3522, have masses of the order of a few 10^9 Mo, comparable to
the Local Group ellipticals. For the most massive spiral in LGG225, we estimate
a stellar mass of ~4x10 Mo, comparable to M33 in the LG. Ages of stellar
populations range from a few to ~7 Gyr for the galaxies in LGG225. The
kinematical and dynamical analysis indicates that LGG127 and LGG225 are in a
pre-virial collapse phase, i.e. still undergoing dynamical relaxation, while
LGG93 is likely virialized. (Abridged)Comment: 20 pages, 13 figures, accepted for publication in Astronomy and
Astrophysic
Inverter-converter automatic paralleling and protection
Electric control and protection circuits for parallel operation of inverter-converte
- …