582 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
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 Flattened Protostellar Envelope in Absorption around L1157
Deep Spitzer IRAC images of L1157 reveal many of the details of the outflow
and the circumstellar environment of this Class 0 protostar. In IRAC band 4, 8
microns, there is a flattened structure seen in absorption against the
background emission. The structure is perpendicular to the outflow and is
extended to a diameter of 2 arcminutes. This structure is the first clear
detection of a flattened circumstellar envelope or pseudo-disk around a Class 0
protostar. Such a flattened morphology is an expected outcome for many collapse
theories that include magnetic fields or rotation. We construct an extinction
model for a power-law density profile, but we do not constrain the density
power-law index.Comment: ApJL accepte
Enhanced decoherence for a neutral particle sliding on a metallic surface in vacuum
Bodies in relative motion, spatially separated in vacuum, experience a tiny
friction force known as quantum friction. This force has eluded experimental
detection so far due to its small magnitude and short range. Herein, we give
quantitative details so as to track traces of the quantum friction by measuring
coherences in the atom. We notice that the environmentally induced decoherence
can be decomposed into contributions of different signature: corrections
induced by the electromagnetic vacuum in presence of the dielectric sheet and
those induced by the motion of the particle. In this direction, we show that
non-contact friction enhances the decoherence of the moving atom. Further, its
effect can be enlarged by a thorough selection of the two-level particle and
the Drude-Lorentz parameters of the material. In this context, we suggest that
measuring decoherence times through velocity dependence of coherences could
indirectly demonstrate the existence of quantum friction.Comment: 10 pages, 9 figure
Removal and survival of fecal indicators in a constructed wetland after uasb pre-treatment
The experimentation plant, based on a sub-surface horizontal flow phytodepuration (SSHFP) unit with a pre-treatment by an upflow anaerobic sludge blanket (UASB) reactor, proved valuable in treating the sewage of a small rural community located in north Brazil. During a six-month trial, the plant achieved an average removal efficiency of 98.2% (1.74 log removal) for fecal coliforms (FC) and 96.0% (1.40 log removal) for Enterococci (EN), as well as 95.6% for BOD5, 91.0% for COD,00 and 95.4% for suspended solids (SS). The contribution of the UASB reactor to this overall performance was very significant as, alone, it achieved a yield of 62.7% for FC and 60% for EN, in addition to 65.2% for BOD5 and 65.0% for SS. EN was chosen, in addition to FC, because of its higher specificity and strong environmental persistence, leading to an increased risk to human health. In fact, the experimental results confirmed its lower removal efficiency compared to FC. The mechanical and biological mechanisms that led to such a removal efficiency of the two fecal indicators (FIs) are outlined in the article. The same mechanisms led to a good level of equivalence between the removal efficiency of the two FIs with the removal efficiency of SS and BOD5, for both the whole plant and the UASB reactor alone. The research demonstrated the close correlation between the concentrations of EN and FC for the plant effluent. This correlation can be explained by the following mathematical expression of the regression line Log EN = 0.2571 Log FC + 3.5301, with a coefficient of determination R-2 = 0.912. This implies that the concentration of the more specific indicator EN could be calculated, with acceptable approximation, from the simple analysis of FC and vice versa. The experimental plant brought important health benefits to the local population. In particular, there were no significant odor emissions; moreover, the risk of fecal pathogenic diseases was drastically reduced; finally, there was no proliferation of insects and other disease vectors, due to the absence of stagnant or semi-stagnant water exposed to the atmosphere
Analysis of the circumstellar environment of the B[e] star HD 45677 (FS CMa)
We studied the circumstellar environment of the B[e] star HD 45677 through
the analysis of the emission lines from ionized metals. We used the statistical
approach of the self absorption curve method (SAC) to derive physical
parameters of the line emitting region. The Fe II and Cr II double-peaked
emission line structure is explained by the presence of a thin absorption
component red shifted by ~3 km/s. This absorption component can be interpreted
geometricaly as being due to infalling material perpendicularly to the disk
seen nearly pole-on, as indicated by the emission line structure. The Cr II and
Fe II emission lines have a complex structure with two (narrow and broad)
components, of 45 and 180 km/s FWHM for the permitted lines and 25 and 100 km/s
FWHM for the forbidden ones, respectively. We argue that the narrow components
are principaly emitted by an optically thin disk seen nearly pole-on, in a
region whose minimum radius is estimated to be 4 10^12 cm, while the broad ones
are formed in a disk-linked wind.Comment: 14 pages, submitted to Astronomy and Astrophysic
Submillimeter CO emission from shock-heated gas in the L1157 outflow
We present the CO J=6-5, 4-3, and 3-2 spectra from the blueshifted gas of the
outflow driven by the low-mass class 0 protostar in the L1157 dark cloud.
Strong submillimeter CO emission lines with T_mb > 30 K have been detected at
63" (~0.13 pc) south from the protostar. It is remarkable that the blue wings
in the submillimeter lines are stronger by a factor of 3-4 than that of the CO
J=1-0 emission line. The CO line ratios suggest that the blueshifted lobe of
this outflow consists of moderately dense gas of n(H_2) = (1-3)x10^4 cm^-3
heated to T_kin = 50-170 K.It is also suggested that the kinetic temperature of
the outflowing gas increases from ~80 K near the protostar to ~170 K at the
shocked region in the lobe center, toward which the largest velocity dispersion
of the CO emission is observed. A remarkable correlation between the kinetic
temperature and velocity dispersion of the CO emission along the lobe provides
us with direct evidence that the molecular gas at the head of the jet-driven
bow shock is indeed heated kinematically. The lower temperature of ~80 K
measured at the other shocked region near the end of the lobe is explained if
this shock is in a later evolutionary stage, in which the gas has been cooled
mainly through radiation of the CO rotational lines.Comment: 10 pages, 4 PDF figures, APJL in pres
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