10,513 research outputs found
Theoretical Study of Fluid Forces on a Centrifugal Impeller Rotating and Whirling in a Volute
Fluid forces on a rotating and whirling centrifugal impeller in a volute are analyzed with the assumption of a two-dimensional rotational, inviscid flow. For simplicity, the flow is assumed to be perfectly guided by the impeller vanes. The theory predicts the tangential and the radial force on the whirling impeller as functions of impeller geometry, volute spacing, and whirl ratio. A good qualitative agreement with experiment is found
Two-dimensional unsteady analysis of fluid forces on a whirling centrifugal impeller in a volute
Destabilizing fluid forces on a whirling centrifugal impeller rotating in a volute were observed. A quasisteady analysis neglecting shed vorticity or an unsteady analysis without a volute does not predict the existence of such destabilizing fluid forces on a whirling impeller. The effects of a volute and the shed vorticity are considered. We treat cases when an impeller with an infinite number of vanes rotates with a constant velocity omega and its center whirls with a constant eccentric radius epsilon and a constant whirling velocity psi. It is assumed that: (1) the number of the vanes is so large that the impeller can be treated as an actuator impeller in which the flow is perfectly guided; (2) flow is inviscid, incompressible and two dimensional; (3) the eccentricity epsilon is so small that unsteady components can be linearized; (4) vorticity is transported on a prescribed mean flow, the operating point is near design flow rate; and (5) the volute can be represented by a curved plate
Ballistic Localization in Quasi-1D Waveguides with Rough Surfaces
Structure of eigenstates in a periodic quasi-1D waveguide with a rough
surface is studied both analytically and numerically. We have found a large
number of "regular" eigenstates for any high energy. They result in a very slow
convergence to the classical limit in which the eigenstates are expected to be
completely ergodic. As a consequence, localization properties of eigenstates
originated from unperturbed transverse channels with low indexes, are strongly
localized (delocalized) in the momentum (coordinate) representation. These
eigenstates were found to have a quite unexpeted form that manifests a kind of
"repulsion" from the rough surface. Our results indicate that standard
statistical approaches for ballistic localization in such waveguides seem to be
unappropriate.Comment: 5 pages, 4 figure
A comparison of reflector antenna designs for wide-angle scanning
Conventional reflector antennas are typically designed for up to + or - 20 beamwidths scan. An attempt was made to stretch this scan range to some + or - 300 beamwidths. Six single and dual reflector antennas were compared. It is found that a symmetrical parabolic reflector with f/D = 2 and a single circular waveguide feed has the minimum scan loss (only 0.6 dB at Theta sub 0 = 8 deg, or a 114 beamwidths scan). The scan is achieved by tilting the parabolic reflector by an angle equal to the half-scan angle. The f/D may be shortened if a cluster 7 to 19 elements instead of one element is used for the feed. The cluster excitation is adjusted for each new beam scan direction to compensate for the imperfect field distribution over the reflector aperture. The antenna can be folded into a Cassegrain configuration except that, due to spillover and blockage considerations, the amount of folding achievable is small
A comparison of reflector antenna designs for wide-angle scanning
Conventional reflector antennas are typically designed for up to + or - 20 beamwidths scan. An attempt was made to stretch this scan range to some + or - 300 beamwidths. Six single and dual reflector antennas were compared. It is found that a symmetrical parabolic reflector with f/D = 2 and a single circular waveguide feed has the minimum scan loss (only 0.6 dB at Theta sub 0 = 8 deg, or a 114 beamwidths scan). The scan is achieved by tilting the parabolic reflector by an angle equal to the half-scan angle. The f/D may be shortened if a cluster 7 to 19 elements instead of one element is used for the feed. The cluster excitation is adjusted for each new beam scan direction to compensate for the imperfect field distribution over the reflector aperture. The antenna can be folded into a Cassegrain configuration except that, due to spillover and blockage considerations, the amount of folding achievable is small
Low optical polarisation at the core of the optically-thin jet of M87
We study the optical linear and circular polarisation in the optically-thin
regime of the core and jet of M87. Observations were acquired two days before
the Event Horizon Telescope (EHT) campaign in early April 2017. A high degree
( per cent) of linear polarisation (P) is detected in the
bright jet knots resolved at to
(-) from the centre, whereas the nucleus and inner jet
show P per cent. The position angle of the linear
polarisation shifts by degrees from each knot to the adjacent ones,
with the core angle perpendicular to the first knot. The nucleus was in a low
level of activity (P- per cent), and no emission was
detected from HST-1. No circular polarisation was detected either in the
nucleus or the jet above a level of P per cent,
discarding the conversion of P into P. A disordered
magnetic field configuration or a mix of unresolved knots polarised along axes
with different orientations could explain the low P. The latter
implies a smaller size of the core knots, in line with current interferometric
observations. Polarimetry with EHT can probe this scenario in the future. A
steep increase of both P and P with increasing
frequency is expected for the optically-thin domain, above the turnover point.
This work describes the methodology to recover the four Stokes parameters using
a wave-plate polarimeter.Comment: Accepted for publication in MNRAS. 10 pages, 8 figure
Mid-Infrared Images of Luminous Infrared Galaxies in a Merging Sequence
We report mid-infrared observations of several luminous infrared galaxies
(LIGs) carried out with the Infrared Space Observatory. Our sample was chosen
to represent different phases of a merger sequence of galaxy-galaxy interaction
with special emphasis on early/intermediate stages of merging. The mid-infrared
emission of these LIGs shows extended structures for the early and intermediate
mergers, indicating that most of the mid-infrared luminosities are not from a
central active galactic nucleus (AGN). Both the infrared hardness (indicated by
the IRAS 12, 25, and 60 \micron flux density ratios) and the peak-to-total
flux density ratios of these LIGs increase as projected separation of these
interacting galaxies become smaller, consistent with increasing star formation
activities that are concentrated to a smaller area as the merging process
advances. These observations provide among the first observational constraint
of largely theoretically based scenarios.Comment: 10 pages, 3 figures, please refer to ApJ Letters for the final
versio
Cavity-enhanced room-temperature magnetometry using absorption by nitrogen-vacancy centers in diamond
We demonstrate a cavity-enhanced room-temperature magnetic field sensor based
on nitrogen-vacancy centers in diamond. Magnetic resonance is detected using
absorption of light resonant with the 1042 nm spin-singlet transition. The
diamond is placed in an external optical cavity to enhance the absorption, and
significant absorption is observed even at room temperature. We demonstrate a
magnetic field sensitivity of 2.5 nT/sqrt(Hz), and project a photon
shot-noise-limited sensitivity of 70 pT/sqrt(Hz) for a few mW of infrared
light, and a quantum projection-noise-limited sensitivity of 250 fT/sqrt(Hz)
for the sensing volume of 90 um x 90 um 200 um.Comment: main text 5 pages, supplementary material 3 page
Application of Near Infrared Reflectance Spectroscopy (NIRS) to Forage Evaluation in Uruguay
Near infrared reflectance spectroscopy (NIRS) were used to assess the chemical composition of a wide range of forages. Nearly five hundred herbage samples covering a wide range of temperate pastures species and mixtures, previously analysed by conventional wet chemistry were scanned in reflectance with a NIRS 6500 (NIRSystems, Silver Spring, USA). The coefficients of determination in calibration (R2 ) and the errors in cross validation (SECV) were 0.98 (5.8), 0.94 (22.2), 0.97 (5.6), 0.94 (6.9) and 0.89 (19.7) for crude protein, in vitro organic matter digestibility, dry matter, ash and acid detergent fiber, in g kg –1 on a dry weight respectively. The high correlation between NIRS and chemical analysis found in this study showed the potential use of NIRS for prediction forage quality
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