16,784 research outputs found
A theoretical model for the cross spectra between pressure and temperature downstream of a combustor
A theoretical model developed to calculate pressure-temperature cross spectra, pressure spectra, temperature spectra and pressure cross spectra in a ducted combustion system is presented. The model assumes the presence of a fluctuating-volumetric-heat-release-rate disk source and takes into account the spatial distribution of the steady-state volumetric-heat flux. Using the model, pressure, velocity, and temperature perturbation relationships can be obtained. The theoretical results show that, at a given air mass flow rate, the calculated pressure-temperature cross spectra phase angle at the combustor exit depends on the model selected for the steady-state volumetric-heat flux in the combustor. Using measurements of the phase angle, an appropriate source region model was selected. The model calculations are compared with the data. The comparison shows good agreement and indicates that with the use of this model the pressure-temperature cross spectra measurements provide useful information on the physical mechanisms active at the combustion noise source
Cross spectra between pressure and temperature in a constant-area duct downstream of a hydrogen-fueled combustor
Pressure temperature cross spectra are necessary in predicting noise propagation in regions of velocity gradients downstream of combustors if the effect of convective entropy disturbances is included. Pressure temperature cross spectra and coherences were measured at spatially separated points in a combustion rig fueled with hydrogen. Temperature-temperature and pressure-pressure cross spectra and coherences between the spatially separated points as well as temperature and pressure autospectra were measured. These test results were compared with previous results obtained in the same combustion rig using Jet A fuel in order to investigate their dependence on the type of combustion process. The phase relationships are not consistent with a simple source model that assumes that pressure and temperature are in phase at a point in the combustor and at all other points downstream are related to one another by only a time delay due to convection of temperature disturbances. Thus these test results indicate that a more complex model of the source is required
Cross spectra between temperature and pressure in a constant area duct downstream of a combustor
The feasibility of measuring pressure temperature cross spectra and coherence and temperature-temperature cross spectra and coherence at spatially separated points along with pressure and temperature auto-spectra in a combustion rig was investigated. The measurements were made near the inlet and exit of a 6.44 m long duct attached to a J-47 combustor. The fuel used was Jet A. The cross spectra and coherence measurements show the pressure and temperature fluctuations correlate best at low frequencies. At the inlet the phenomena controlling the phase relationship between pressure and temperature could not be identified. However, at the duct exit the phase angle of the pressure is related to the phase angle of the temperature by the convected flow time delay
Simultaneous optical and near-infrared linear spectropolarimetry of the earthshine
Aims: We aim to extend our current observational understanding of the
integrated planet Earth spectropolarimetry from the optical to the
near-infrared wavelengths. Major biomarkers like O and water vapor
are strong flux absorbents in the Earth atmosphere and some linear polarization
of the reflected stellar light is expected to occur at these wavelengths.
Methods: Simultaneous optical ( m) and near-infrared (
m) linear spectropolarimetric data of the earthshine were acquired by
observing the nightside of the waxing Moon. The data have sufficient spectral
resolution (2.51 nm in the optical, and 1.83 and 2.91 nm in the near-infrared)
to resolve major molecular species present in the Earth atmosphere.
Results: We find the highest values of linear polarization () at
the bluest wavelengths, which agrees with the literature. Linear polarization
intensity steadily decreases towards red wavelengths reaching a nearly flat
value beyond 0.8 m. In the near-infrared, we measured a polarization
degree of for the continuum. We report the detection of molecular
features due to O at m and HO at 0.6530.725
m, 0.7800.825 m, 0.93 and 1.12 m in the spectropolarimetric
data; most of them show high linear polarimetry degrees above the continuum. In
particular, the broad HO 1.12 m band displays a polarimetric
intensity as high as that of the blue optical. These features may become a
powerful tool to characterize Earth-like planets in polarized light.Comment: 4 pages, 3 figures. Accepted for publication as Letter in Astronomy
and Astrophysics on 23/01/201
Rotational modulation of the linear polarimetric variability of the cool dwarf TVLM 51346546
Aims: We aimed to monitor the optical linear polarimetric signal of the
magnetized, rapidly rotating M8.5 dwarf TVLM 51346546.
Methods: - and -band linear polarimetry images were collected with the
ALFOSC instrument of the 2.56-m Nordic Optical Telescope on two consecutive
nights covering about 0.5 and 4 rotation cycles in the and filters,
respectively. We also obtained simultaneous intensity curves by means of
differential photometry. The typical precision of the data is 0.46\%
(), 0.35\% () in the linear polarization degree and 9 mmag
(), 1.6 mmag () in the differential intensity curves.
Results: Strong and variable linear polarization is detected in the and
filters, with values of maximum polarization ( = 1.300.35 \%)
similar for both bands. The intensity and the polarimetric curves present a
sinusoid-like pattern with a periodicity of 1.98 h, which we ascribe to
structures in TVLM 51346's surface synchronized with rotation. We found that
the peaks of the intensity and polarimetric curves occur with a phase
difference of 0.180.01, and that the maximum of the linear polarization
happens nearly half a period (0.590.03) after the radio pulse. We
discussed different scenarios to account for the observed properties of the
light curves.Comment: Accepted for publication in Astronomy and Astrophysic
A Statistical Survey of Peculiar L and T Dwarfs in SDSS, 2MASS, and WISE
We present the final results from a targeted search for brown dwarfs with
unusual near-infrared colors. From a positional cross-match of SDSS, 2MASS and
WISE, we have identified 144 candidate peculiar L and T dwarfs. Spectroscopy
confirms that 20 of the objects are peculiar or are candidate binaries. Nine of
the 420 objects in our sample are young (200 Myr; 2.1%) and another 8
(1.9%) are unusually red with no signatures of youth. With a spectroscopic
color of 2.58 0.11 mag, one of the new objects, the L6 dwarf
2MASS J03530419+0418193, is among the reddest field dwarfs currently known and
is one of the reddest objects with no signatures of youth known to date. We
have also discovered another potentially very low gravity object, the L1 dwarf
2MASS J00133470+1109403, and independently identified the young L7 dwarf 2MASS
J00440332+0228112, first reported by Schneider and collaborators. Our results
confirm that signatures of low gravity are no longer discernible in low to
moderate resolution spectra of objects older than 200 Myr. The 1.9% of
unusually red L dwarfs that do not show other signatures of youth could be
slightly older, up to 400 Myr. In this case a red color may be
more diagnostic of moderate youth than individual spectral features. However,
its is also possible that these objects are relatively metal-rich, and so have
an enhanced atmospheric dust content.Comment: 27 pages, 7 figures, 5 tables, accepted to A
Total syntheses of conformationally-locked difluorinated pentopyranose analogues and a pentopyranosyl phosphate mimetic
Trifluoroethanol has been elaborated, via a telescoped sequence involving a metalated difluoroenol, a difluoroallylic alcohol, [2,3]-Wittig rearrangement, and ultimately an RCM reaction and requiring minimal intermediate purification, to a number of cyclooctenone intermediates. Epoxidation of these intermediates followed by transannular ring opening or dihydroxylation, then transannular hemiacetalization delivers novel bicyclic analogues of pentopyranoses, which were elaborated (in one case) to an analogue of a glycosyl phosphate
Testing the existence of optical linear polarization in young brown dwarfs
Linear polarization can be used as a probe of the existence of atmospheric
condensates in ultracool dwarfs. Models predict that the observed linear
polarization increases withthe degree of oblateness, which is inversely
proportional to the surface gravity. We aimed to test the existence of optical
linear polarization in a sample of bright young brown dwarfs, with spectral
types between M6 and L2, observable from the Calar Alto Observatory, and
cataloged previously as low gravity objects using spectroscopy. Linear
polarimetric images were collected in I and R-band using CAFOS at the 2.2 m
telescope in Calar Alto Observatory (Spain). The flux ratio method was employed
to determine the linear polarization degrees. With a confidence of 3,
our data indicate that all targets have a linear polarimetry degree in average
below 0.69% in the I-band, and below 1.0% in the R-band, at the time they were
observed. We detected significant (i.e. P/ 3) linear polarization
for the young M6 dwarf 2MASS J04221413+1530525 in the R-band, with a degree of
= 0.81 0.17 %.Comment: Accepted for publication in MNRA
Spirituality and the Health of U. S. College Students: A Partial Replication and Extension
Spirituality has been linked with numerous positive effects, and the university experience is generally viewed as an important time for students to develop their spiritual identities. The purpose of this study was to partially replicate and extend the findings of Nelms et al. (2007) on the relationship between spirituality and the health of university students. In a survey study of 968 first-year undergraduate students in the western United States, we found that spirituality was associated with higher levels of physical health, emotional health, and exercise, and these relationships were generally different for white individuals and men than for minorities and women
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