2,335 research outputs found
Streamwise-traveling waves of spanwise wall velocity for turbulent drag reduction
Waves of spanwise velocity imposed at the walls of a plane turbulent channel
flow are studied by Direct Numerical Simulations. We consider sinusoidal waves
of spanwise velocity which vary in time and are modulated in space along the
streamwise direction. The phase speed may be null, positive or negative, so
that the waves may be either stationary or traveling forward or backward in the
direction of the mean flow. Such a forcing includes as particular cases two
known techniques for reducing friction drag: the oscillating wall technique (a
traveling wave with infinite phase speed) and the recently proposed steady
distribution of spanwise velocity (a wave with zero phase speed).
The traveling waves alter the friction drag significantly. Waves which slowly
travel forward produce a large reduction of drag, that can relaminarize the
flow at low values of the Reynolds number. Faster waves yield a totally
different outcome, i.e. drag increase. Even faster waves produce a drag
reduction effect again. Backward-traveling waves instead lead to drag reduction
at any speed.
The traveling waves, when they reduce drag, operate in similar fashion to the
oscillating wall, with an improved energetic efficiency. Drag increase is
observed when the waves travel at a speed comparable with that of the
convecting near-wall turbulence structures. A diagram illustrating the
different flow behaviors is presented
Oxygen control and improved denitrification efficiency by means of a post-anoxic reactor
The presence of dissolved oxygen (DO) in biological denitrification reactors
determines inhibition effects on the denitrification rate. The article shows the results of an
experimental study to control the DO concentration in the pre-denitrification stage by a
post-anoxic reactor. The results demonstrate that the post-anoxic reactor is very effective in
improving the nitrogen removal efficiency because it causes a considerable reduction of the
DO content in the mixed liquor recycle sent to the pre-denitrification reactor. This reduction
is influenced by both the retention time and the F:M ratio (referred to the denitrification and
the oxidation-nitrification volume). In fact, a retention time and a F:M ratio equal to 1.5 h
and 0.130 kgBOD5 kgMLVSS−1·day−1, respectively, allow to limit DO in the post-anoxic reactor
at 0.31 mgO2·L−1. Such concentration determines a DO concentration of 0.11 mgO2·L−1 in the
pre-denitrification reactor and, consequently, a denitrification efficiency of 91%. Moreover,
the contribution of the endogenous denitrification to the whole denitrification efficiency was
found negligible. The paper contributes to the progress in nitrogen removal from sewage,
a fundamental issue for a sustainable management of water resources
The 2006 hot phase of Romano's star (GR 290) in M33
Understanding the nature of the instabilities of LBVs is important to
understand the late evolutionary stages of very massive stars.
We investigate the long term, S Dor-type variability of the luminous blue
variable GR290 (Romano's star) in M33, and its 2006 minimum phase.
New spectroscopic and photometric data taken in November and December 2006
were employed in conjunction with already published data on GR290 to derive the
physical structure of GR290 in different phases and the time scale of the
variability.
We find that by the end of 2006, GR 290 had reached the deepest visual
minimum so far recorded. Its present spectrum resembles closely that of the
Of/WN9 stars, and is the hottest so far recorded in this star (and in any LBV
as well), while its visual brightness decreased by about 1.4 mag.
This first spectroscopic record of GR290 during a minimum phase confirms
that, similarly to AG Car and other LBVs, the star is subject to ample S
Dor-type variations, being hotter at minimum, suggesting that the variations
take place at constant bolometric luminosity.Comment: 4 figures, 1 table, accepted for publication in A&A Letter
Hard X-ray emission from Eta Carinae
Context : If relativistic particle acceleration takes place in colliding-wind
binaries, hard X-rays and gamma-rays are expected through inverse Compton
emission, but to date these have never been unambiguously detected.
Aims : To detect this emission, observations of Eta Carinae were performed
with INTEGRAL, leveraging its high spatial resolution.
Methods : Deep hard X-ray images of the region of Eta Car were constructed in
several energy bands.
Results : The hard X-ray emission previously detected by BeppoSax around Eta
Car originates from at least 3 different point sources. The emission of Eta Car
itself can be isolated for the first time, and its spectrum unambiguously
analyzed. The X-ray emission of Eta Car in the 22-100 keV energy range is very
hard (photon index around 1) and its luminosity is 7E33 erg/s.
Conclusions : The observed emission is in agreement with the predictions of
inverse Compton models, and corresponds to about 0.1% of the energy available
in the wind collision. Eta Car is expected to be detected in the GeV energy
range.Comment: 5 pages with 2 figures. Accepted as a Letter in Astronomy &
Astrophysic
A new lattice Boltzmann model for interface reactions between immiscible fluids
In this paper, we describe a lattice Boltzmann model to simulate chemical reactions taking place at the interface between two immiscible fluids. The phase-field approach is used to identify the interface and its orientation, the concentration of reactant at the interface is then calculated iteratively to impose the correct reactive flux condition. The main advantages of the model is that interfaces are considered part of the bulk dynamics with the corrective reactive flux introduced as a source/sink term in the collision step, and, as a consequence, the model’s implementation and performance is independent of the interface geometry and orientation. Results obtained with the proposed model are compared to analytical solution for three different benchmark tests (stationary flat boundary, moving flat boundary and dissolving droplet). We find an excellent agreement between analytical and numerical solutions in all cases. Finally, we present a simulation coupling the Shan Chen multiphase model and the interface reactive model to simulate the dissolution of a collection of immiscible droplets with different sizes rising by buoyancy in a stagnant fluid
Streamwise oscillation of spanwise velocity at the wall of a channel for turbulent drag reduction
Steady forcing at the wall of a channel flow is studied via DNS to assess its
ability of yielding reductions of turbulent friction drag. The wall forcing
consists of a stationary distribution of spanwise velocity that alternates in
the streamwise direction. The idea behind the forcing builds upon the existing
technique of the spanwise wall oscillation, and exploits the convective nature
of the flow to achieve an unsteady interaction with turbulence.
The analysis takes advantage of the equivalent laminar flow, that is solved
analytically to show that the energetic cost of the forcing is unaffected by
turbulence. In a turbulent flow, the alternate forcing is found to behave
similarly to the oscillating wall; in particular an optimal wavelength is found
that yields a maximal reduction of turbulent drag. The energetic performance is
significantly improved, with more than 50% of maximum friction saving at large
intensities of the forcing, and a net energetic saving of 23% for smaller
intensities.
Such a steady, wall-based forcing may pave the way to passively interacting
with the turbulent flow to achieve drag reduction through a suitable
distribution of roughness, designed to excite a selected streamwise wavelength
Revisiting the psychologization of religiosity
Este trabajo analiza los procesos de psicologizaciónde la religiosidad en un ámbito identificado con las clases medias de Buenos Aires. Con foco en versiones reavivadas del catolicismo y en experiencias vinculadas con la llamada Nueva Era, analiza comparativamente la especificidad de categorías y prácticas psi en esos ámbitos. Dando prioridad a la cultura letrada urbana por sobre la denominaciones religiosas, propone un análisis contrastivo con los modos diferenciales de construcción de la persona psicológica en espacios seculares de la cultura letrada e, incluso, en la religiosidad de los sectores populares.This paper analyzes the processes of psychologization of religiosity in an area identified with the middle classes of Buenos Aires. Focusing on revivalist versions of Catholicism and experiences related to the so-called New Age spirituality, this paper comparatively analyzes the specificity of psychological categories and practices in these contexts. Prioritizing urban learned culture over the religious denominations, this work proposes a contrastive analysis with other modes of psychological personhood settled in secular contexts of learned culture, and even in the religiosity of popular classes.Fil: Viotti, Nicolas. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Facultad Latinoamericana de Ciencias Sociales; Argentin
Image analysis procedure for studying Back-Diffusion phenomena from low-permeability layers in laboratory tests
In this study, the long-term tailing derived from the storage process of contaminants in low-permeability zones is investigated. The release from these areas in the groundwater can be considered a long-term source that often undermines remediation efforts. An Image Analysis technique is used to analyze the process and evaluate the concentrations of a tracer at different points of the test section. Furthermore, the diffusive flux from the low-permeability lenses is determined. To validate the proposed technique, the results are compared with samples, and the diffusive fluxes resulting from the low-permeability zones of the reconstructed aquifer are compared with a theoretical approach
The origin of the supersoft X-ray--optical/UV flux anticorrelation in the symbiotic binary AG Draconis
AG Draconis produces a strong supersoft X-ray emission.The X-ray and
optical/UV fluxes are in a strict anticorrelation throughout the active and
quiescent phases. The aim of this contribution is to identify the source of the
X-ray emission and reveal the nature of the observed flux anticorrelation. For
this purpose we model the X-ray and UV observations with XMM-Newton, far-UV
spectroscopy from FUSE, low- and high-resolution IUE spectra and
optical/near-IR spectroscopic and/or photometric observations. Our analysis
showed that the supersoft X-ray emission is produced by the white dwarf
photosphere. The X-ray and far-UV fluxes make it possible to determine its
temperature unambiguously. The supersoft X-ray--optical/UV flux anticorrelation
is caused by the variable wind from the hot star. The enhanced hot star wind
gives rise to the optical bursts by reprocessing high-energy photons from the
Lyman continuum to the optical/UV.Comment: 9 pages, 2 figures, 3 tables. Submitted to AA on 25/11/2008, revised
on 27/05/200
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