375 research outputs found
Comparison of high spatial resolution stereo-PIV measurements in a turbulent boundary layer with available DNS dataset
In the present contribution, the aptitude of Stereoscopic Particle Image Velocimetry (SPIV) and of Direct Numerical Simulations (DNS) to investigate coherent structures of near wall turbulence is evaluated. For this purpose, the general properties and constraints of the two techniques are first reviewed. Then, data obtained from stereo-PIV experiments in a boundary layer and DNS in a channel flow are considered. Some statistics of the velocity fields are computed, and the results obtained from the two approaches compared
Conversion of Cerrado into agricultural land in the south-western amazon: carbon stocks and soil fertility.
Mudanças de uso da terra e práticas de manejo modificam a dinâmica do C e a fertilidade do solo. Este estudo avaliou as implicações dos sistemas de cultivo (NT e CT) nos estoques de C e de nutrientes e identificou inter-relações entre estes estoques e outros atributos da fertilidade do solo em Latossolo após a mudança do uso da terra no cerrado amazônico. Os estoques de C e de nutrientes (P, K, Ca e Mg) ajustados pela massa equivalente do solo sob cerrado (CE), foram maiores principalmente sob NT. Após a adoção do NT, exceto em 2NT, os estoques de C foram maiores em relação às demais áreas avaliadas. Correlações entre estoques de C e de nutrientes revelaram algumas correlações positivas com Ca e Mg nas áreas sob NT, devido ao uso continuo de calcário, à maior quantidade de resíduos culturais e ao não revolvimento do solo, associado à correlações positivas com CTC, saturação por bases e pH. A correlação positiva (r = 0,91, p < 0,05) entre estoques de C e CTC em CE indica a importante contribuição da MOS na CT de solos tropicais, embora os sítios de troca estejam ocupados principalmente por H + Al. Estoques de P e K mostraram correlações positivas com estoques de C em CE (0,81 e 0,82; respectivamente), indicando a alta relação de P e K com a MOS em ecossistemas naturais. A alta variabilidade espacial associada à aplicação de fertilizantes (P e K) no sulco de plantio pode ter mascarado os resultados dos estoques. A principal fonte destes nutrientes para o solo foi o fertilizante aplicado e não a MOS
A temperature and magnetic field dependence Mössbauer study of ɛ-Fe2O3
ɛ-Fe2O3 was synthesized as nanoparticles by a pre-vacuum heat treatment of yttrium iron garnet (Y3Fe5O12) in a silica matrix at 300-C followed by sintering in air at 1,000-C for up to 10 h. It displays complex magnetic properties that are characterized by two transitions, one at 480 K from a paramagnet (P) to canted antiferromagnet (CAF1) and the second at ca. 120 K from the canted antiferromagnet (CAF1) to another canted antiferromagnet (CAF2). CAF2 has a smaller resultant magnetic moment (i.e. smaller canting angle) than CAF1. Analysis of the zero-field Mossbauer spectra at different temperatures shows an associated discontinuity of the hyperfine field around 120 K. In an applied field, the different magnetic sublattices were identified and the directions of their moments were assigned. The moments of the two sublattices are antiparallel and collinear at 160 K but are at right angle to each other at 4.2 K
High-mass star formation in the Southern Hemisphere sky
We report on a multi-wavelength (IR to cm) and multi-resolution (1 mas to 20
arcsec) exploration of high-mass star formation regions in the Galactic plane,
at longitudes observable from the Southern Hemisphere. Our source sample was
originally identified through methanol masers in the Galactic plane, which
exclusively trace high-mass star-forming regions. (Sub)millimetre continuum and
molecular line observations were carried out with SEST/SIMBA, JCMT/SCUBA and
ATNF/Mopra mm-wave telescopes and have allowed us to identify massive (
M) and luminous ( L) clumps in each star-forming
region. We have also constrained the SED with additional archival IR data, the
physical conditions (, , ) and the chemical composition of each
massive clump. Several types of objects were characterised based on the
ratio, the dust temperature and the molecular line
properties, ranging from class 0-like YSO clusters (,
T=30 K) to hot molecular clumps (, K).
Preliminary high-angular resolution observations for a subset of the sample
with the ATNF/ATCA at 3 mm, the VLA at 15, 22 and 43 GHz and Gemini in MIR have
revealed that several (proto)stellar objects are embedded in the massive
clumps: massive protostars, hot cores and hyper-compact HII regions. We have
thus identified protoclusters of massive YSOs, which are the precursors of the
OB associations. This sample of Southern Hemisphere star-forming regions will
be extremely valuable for the scientific preparation of the ALMA and HSO
observations.Comment: 4 pages, 3 figures, conference proceeding
Maser emission from SiO isotopologues traces the innermost 100 AU around Radio Source I in Orion BN/KL
We have used the Very Large Array (VLA) at 7 mm wavelength to image five
rotational transitions (J=1-0) from three SiO isotopologues towards Orion
BN/KL: 28SiO v=0,1,2; and 29SiO and 30SiO v=0. For the first time, we have
mapped the 29SiO and 30SiO J=1-0 emission, established the maser nature of the
emission, and confirmed association with the deeply embedded high-mass young
stellar object commonly denoted radio Source I. The 28SiO v=0 maser emission
shows a bipolar structure that extends over ~700 AU along a northeast-southwest
axis, and we propose that it traces a bipolar outflow driven by Source I. The
high-brightness isotopic SiO maser emission imaged with a ~0.2 arcsec
resolution has a more compact distribution, generally similar to that of the
28SiO v=1,2 emission, and it probably traces bulk gas flows in a region of
diameter <100 AU centered on Source I. On small scales of <10 AU, however,
compact 29SiO/30SiO v=0 and 28SiO v=1,2 emission features may be offset from
one another in position and line-of-sight velocity.
From a radiative transfer analysis based on a large velocity gradient (LVG)
pumping model, we derive similar temperatures and densities for the optimum
excitation of both 29SiO/30SiO v=0 and 28SiO v=1,2 masers, significantly higher
than required for 28SiO v=0 maser excitation. In order to account for the
small-scale differences among the isotopologues (v=0) and the main species
(v=1,2), follow-up radiative transfer modeling that incorporates non-local line
overlap among transitions of all SiO isotopic species may be required.Comment: 10 pages, 3 figures, accepted for publication by Ap
Deuterated water in the solar-type protostars NGC 1333 IRAS 4A and IRAS 4B
Aims. The aim of this paper is to study deuterated water in the solar-type
protostars NGC1333 IRAS4A and IRAS4B, to compare their HDO abundance
distribution with other star-forming regions, and to constrain their HDO/H2O
ratios. Methods. Using the Herschel/HIFI instrument as well as ground-based
telescopes, we observed several HDO lines covering a large excitation range
(Eup/k=22-168 K) towards these protostars and an outflow position. Non-LTE
radiative transfer codes were then used to determine the HDO abundance profiles
in these sources. Results. The HDO fundamental line profiles show a very broad
component, tracing the molecular outflows, in addition to a narrower emission
component and a narrow absorbing component. In the protostellar envelope of
NGC1333 IRAS4A, the HDO inner (T>100 K) and outer (T<100 K) abundances with
respect to H2 are estimated at 7.5x10^{-9} and 1.2x10^{-11}, respectively,
whereas, in NGC1333 IRAS4B, they are 1.0x10^{-8} and 1.2x10^{-10},
respectively. Similarly to the low-mass protostar IRAS16293-2422, an absorbing
outer layer with an enhanced abundance of deuterated water is required to
reproduce the absorbing components seen in the fundamental lines at 465 and 894
GHz in both sources. This water-rich layer is probably extended enough to
encompass the two sources as well as parts of the outflows. In the outflows
emanating from NGC1333 IRAS4A, the HDO column density is estimated at about
(2-4)x10^{13} cm^{-2}, leading to an abundance of about (0.7-1.9)x10^{-9}. An
HDO/H2O ratio between 7x10^{-4} and 9x10^{-2} is derived in the outflows. In
the warm inner regions of these two sources, we estimate the HDO/H2O ratios at
about 1x10^{-4}-4x10^{-3}. This ratio seems higher (a few %) in the cold
envelope of IRAS4A, whose possible origin is discussed in relation to formation
processes of HDO and H2O.Comment: 16 pages, 13 figure
Master crossover behavior of parachor correlations for one-component fluids
The master asymptotic behavior of the usual parachor correlations, expressing
surface tension as a power law of the density difference
between coexisting liquid and vapor, is analyzed for a
series of pure compounds close to their liquid-vapor critical point, using only
four critical parameters , , and ,
for each fluid.
... The main consequences of these theoretical estimations are discussed in
the light of engineering applications and process simulations where parachor
correlations constitute one of the most practical method for estimating surface
tension from density and capillary rise measurements
Dynamics of a ferromagnetic domain wall and the Barkhausen effect
We derive an equation of motion for the the dynamics of a ferromagnetic
domain wall driven by an external magnetic field through a disordered medium
and we study the associated depinning transition. The long-range dipolar
interactions set the upper critical dimension to be , so we suggest that
mean-field exponents describe the Barkhausen effect for three-dimensional soft
ferromagnetic materials. We analyze the scaling of the Barkhausen jumps as a
function of the field driving rate and the intensity of the demagnetizing
field, and find results in quantitative agreement with experiments on
crystalline and amorphous soft ferromagnetic alloys.Comment: 4 RevTex pages, 3 ps figures embedde
Spin-wave Scattering in the Effective Lagrangian Perspective
Nonrelativistic systems exhibiting collective magnetic behavior are analyzed
in the framework of effective Lagrangians. The method, formulating the dynamics
in terms of Goldstone bosons, allows to investigate the consequences of
spontaneous symmetry breaking from a unified point of view. Low energy theorems
concerning spin-wave scattering in ferro- and antiferromagnets are established,
emphasizing the simplicity of actual calculations. The present work includes
approximate symmetries and discusses the modification of the low energy
structure imposed by an external magnetic and an anisotropy field,
respectively. Throughout the paper, analogies between condensed matter physics
and Lorentz-invariant theories are pointed out, demonstrating the universal
feature of the effective Lagrangian technique.Comment: Published versio
Fluctuations of elastic interfaces in fluids: Theory and simulation
We study the dynamics of elastic interfaces-membranes-immersed in thermally
excited fluids. The work contains three components: the development of a
numerical method, a purely theoretical approach, and numerical simulation. In
developing a numerical method, we first discuss the dynamical coupling between
the interface and the surrounding fluids. An argument is then presented that
generalizes the single-relaxation time lattice-Boltzmann method for the
simulation of hydrodynamic interfaces to include the elastic properties of the
boundary. The implementation of the new method is outlined and it is tested by
simulating the static behavior of spherical bubbles and the dynamics of bending
waves. By means of the fluctuation-dissipation theorem we recover analytically
the equilibrium frequency power spectrum of thermally fluctuating membranes and
the correlation function of the excitations. Also, the non-equilibrium scaling
properties of the membrane roughening are deduced, leading us to formulate a
scaling law describing the interface growth, W^2(L,T)=L^3 g[t/L^(5/2)], where
W, L and T are the width of the interface, the linear size of the system and
the temperature respectively, and g is a scaling function. Finally, the
phenomenology of thermally fluctuating membranes is simulated and the frequency
power spectrum is recovered, confirming the decay of the correlation function
of the fluctuations. As a further numerical study of fluctuating elastic
interfaces, the non-equilibrium regime is reproduced by initializing the system
as an interface immersed in thermally pre-excited fluids.Comment: 15 pages, 11 figure
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