1,199 research outputs found
Administrative Law - Airline Deregulation - Deregulatory Scheme Had No Effect on the Applicable Substantive Law to Determine Liability of Shipper for Lost Shipment of Goods
Administrative Law - Airline Deregulation - Deregulatory Scheme Had No Effect on the Applicable Substantive Law to Determine Liability of Shipper for Lost Shipment of Goods
A three-dimensional model for the radio emission of magnetic chemically peculiar stars
In this paper we present a three-dimensional numerical model for the radio
emission of Magnetic Chemically Peculiar stars, on the hypothesis that
energetic electrons emit by the gyrosynchrotron mechanism. For this class of
radio stars, characterized by a mainly dipolar magnetic field whose axis is
tilted with respect to the rotational axis, the geometry of the magnetosphere
and its deformation due to the stellar rotation are determined. The radio
emitting region is determined by the physical conditions of the magnetosphere
and of the stellar wind. Free-free absorption by the thermal plasma trapped in
the inner magnetosphere is also considered. Several free parameters are
involved in the model, such as the size of the emitting region, the energy
spectrum and the number density of the emitting electrons, and the
characteristics of the plasma in the inner magnetosphere. By solving the
equation of radiative transfer, along a path parallel to the line of sight, the
radio brightness distribution and the total flux density as a function of
stellar rotation are computed. As the model is applied to simulate the observed
5 GHz lightcurves of HD37479 and HD37017, several possible magnetosphere
configurations are found. After simulations at other frequencies, in spite of
the large number of parameters involved in the modeling, two solutions in the
case of HD37479 and only one solution in the case of HD37017 match the observed
spectral indices. The results of our simulations agree with the magnetically
confined wind-shock model in a rotating magnetosphere. The X-ray emission from
the inner magnetosphere is also computed, and found to be consistent with the
observations.Comment: 15 pages, 10 figures, A&A in pres
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Numerical and Experimental Analysis of Transient Flow in Roots Blower
The performance of rotary positive displacement machines highly depends on the operational clearances. It is widely believed that computational fluid dynamics (CFD) can help understanding internal leakage flows. Developments of grid generating tools for analysis of leakage flows by CFD in rotary positive displacement machines have not yet been fully validated. Roots blower is a good representative of positive displacement machines and as such is convenient for optical access in order to analyse internal flows. The experimental investigation of flow in optical roots blower by phase-locked PIV (Particle Image Velocimetry) performed in the Centre for Compressor Technology at City, University of London provided the velocity field suitable for validation of the simulation model. This paper shows the results of the three-dimensional CFD transient simulation model of a Roots blower with the dynamic numerical grids generated by SCORG and flow solution solved in ANSYS CFX flow solver to obtain internal flow patterns. The velocity fields obtained by simulation agree qualitatively with the experimental results and show the correct main flow features in the working chamber. There are some differences in the velocity magnitude and vortex distribution. The flow field in roots blower is highly turbulent and three-dimensional. The axial clearances should be included, and the axial grids should be refined in the simulation method. The paper outlines some directions for future simulation and experimental work. The work described in this paper is a part of the large project set to evaluate characteristics of the internal flow in rotary positive displacement machines and to characterize leakage flow
3D-modelling of the stellar auroral radio emission
The electron cyclotron maser is the coherent emission process that gives rise
to the radio lighthouse effect observed in the hot magnetic chemically peculiar
star CU Virginis. It has also been proposed to explain the highly circularly
polarized radio pulses observed on some ultra cool dwarfs, with spectral type
earlier than M7. Such kind of coherent events resemble the auroral radio
emission from the magnetized planets of the solar system. In this paper, we
present a tridimensional model able to simulate the timing and profile of the
pulses emitted by those stars characterized by a dipolar magnetic field by
following the hypothesis of the laminar source model, used to explain the
beaming of the terrestrial auroral kilometric radiation. This model proves to
be a powerful tool to understand the auroral radio-emission phenomenon,
allowing us to derive some general conclusions about the effects of the model's
free parameters on the features of the coherent pulses, and to learn more about
the detectability of such kind of pulsed radio emission.Comment: 11 pages, 8 figures; accepted for publication in MNRA
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Experimental Investigation of the Transient Flow in Roots Blower
Rotary positive displacement machines are common method to pump flow in various process industries. Their performance highly depends on the operational clearances. It is widely believed that computational fluid dynamics (CFD) can help understanding and reducing internal leakage flows. However, Developments of grid generating tools for use of CFD in rotary positive displacement machines have not yet been fully validated. Thereby arising a need to validate these models that help in better understanding of the leakage flows. Roots blower is a good representative of positive displacement machines and as such is convenient for optical access to analyse flows in in such machines. This paper describes the setup of the experimental test rig with the optical Roots blower in the Centre for Compressor Technology at City, University of London and the first results obtained using three different flow visualization methods. These are namely i) the high-speed camera (HC), ii) the continuous time resolved PIV (CPIV) and iii) the instantaneous PIV obtained with double pulse PIV laser and double shutter camera (IPIV). Test results from these three tests are compared and discussed in the paper. The CPIV test shows the movement of the vortex and the general shape of the flow field clearly but is not sufficient to calculate velocity vectors of high-velocity particles due to the limitation of the laser and camera. The IPIV test can produce quantitative velocity vector images of the internal flow but needs improvement to look into the leakage flow. The work described in this paper is a part of the large project set to evaluate characteristics of the internal flow in rotary positive displacement machines and to characterize leakage flows. The objective is to enable further improvements in 3D CFD analysis of leakage flows in rotary positive displacement machines and ultimately lead to the improvement in the performance of rotary positive displacement machines
Searching for OH maser emission towards the MIPSGAL compact Galactic bubbles
We conducted radio observations searching for OH 18-cm maser emission from a
sample of 169 unclassified MIPSGAL compact Galactic bubbles. These sources are
thought to be the circumstellar envelopes of different kinds of evolved stars.
Our observations were aimed at shedding light on the nature of MIPSGAL bubbles,
since their characterisation is a fundamental aid for the development of
accurate physical models of stellar and Galaxy evolution. The maser emission is
observatively linked to the last stages of the life of low- and
intermediate-mass stars, which may constitute a significant fraction of the
MIPSGAL bubbles. In particular OH masers are usually observed towards post-AGB
stars. Our observations were performed with the Green Bank Telescope and, for
each source, produced spectra around the four OH 18-cm transitions. The
observations were compared with archive interferometer data in order to exclude
possible contamination from nearby sources. The main result is that the OH
maser emission is not a common feature among the MIPSGAL bubbles, with only one
certain detection. We conclude that among the MIPSGAL bubbles the post-AGB
stars could be very rare
Large Binocular Telescope view of the atmosphere of GJ1214b
The atmospheric composition and vertical structure of the super-Earth GJ1214b
has been a subject of debate since its discovery in 2009. Recent studies have
indicated that high-altitude clouds might mask the lower layers. However, some
data points that were gathered at different times and facilities do not fit
this picture, probably because of a combination of stellar activity and
systematic errors. We observed two transits of GJ1214b with the Large Binocular
Camera, the dual-channel camera at the Large Binocular Telescope. For the first
time, we simultaneously measured the relative planetary radius
at blue and red optical wavelengths (), thus
constraining the Rayleigh scattering on GJ1214b after correcting for stellar
activity effects. To the same purpose, a long-term photometric follow-up of the
host star was carried out with WiFSIP at STELLA, revealing a rotational period
that is significantly longer than previously reported. Our new unbiased
estimates of yield a flat transmission spectrum extending to shorter
wavelengths, thus confirming the cloudy atmosphere scenario for GJ1214b.Comment: 11 pages, 5 figures, 3 tables. Published in A&A. Minor changes to
reflect the published versio
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