1,890 research outputs found
Simulations of the Boundary Layer Between a White Dwarf and its Accretion Disk
Using a 2.5D time-dependent axisymmetric numerical code we recently
developed, we solve the full compressible Navier-Stokes equations (including an
alpha-viscosity prescription) to determine the structure of the boundary layer
between the white dwarf and the accretion disk in non-magnetic cataclysmic
varia ble systems. In this preliminary work, our numerical approach does not
include radiation. In the energy equation, we either take the dissipation
function (Phi) into account or we assumed that the energy is instantly radiated
away (Phi). For a slowly rotating non magnetized accreting white dwarf, the
accretion disk e xtends all the way to the stellar surface. There, the matter
impacts and spread s towards the poles as new matter continuously piles up
behind it. We carried out numerical simulations for different values of the
alpha viscosity parameter (alpha), corresponding to different mass accretion
rates. In the high viscosity cases (alpha=0.1), the spreading boundary layer
sets off a gravity wave in the s urface matter. The accretion flow moves
supersonically over the cusp making it s usceptible to the rapid development of
gravity wave and/or Kelvin-Helmholtz shea ring instabilities. This BL is
optically thick and extends more than 30 degrees to either side of the disk
plane after only 3/4 of a Keplerian rotation period (19s). In the low viscosity
cases (alpha=0.001), the spreading boundary layer does not set off gravity
waves and it is optically thin.Comment: final version, ApJ, in pres
A Survey of Far Ultraviolet Spectroscopic Explorer Observations of Cataclysmic Variables
During its lifetime, the Far Ultraviolet Spectroscopic Explorer (FUSE) was
used to observe 99 cataclysmic variables in 211 separate observations. Here, we
present a survey of the moderate resolution (R\simeq10,000), far-ultraviolet
(905 - 1188 {\deg}A), time-averaged FUSE spectra of cataclysmic variables
(CVs). The FUSE spectra are morphologically diverse. They show contributions
from the accretion disk, the disk chromosphere, disk outflows, and the white
dwarf, but the relative contribution of each component varies widely as a
function of CV subtype, orbital period and evolutionary state, inclination,
mass accretion rate, and magnetic field strength of the white dwarf. The data
reveal information about the structure, temperature, density and mass flow
rates of the disk and disk winds, the temperature of the white dwarf and the
effects of ongoing accretion on its structure, and the long-term response of
the systems to disk outbursts. The complete atlas of time-averaged FUSE spectra
of CVs are available at the Multimission Archive at Space Telescope Science
Institute as a High Level Science Product.Comment: ApJS, in press. The extra panels in the figure sets for Figures 1 and
2 are included at the end of the manuscrip
An Exploratory Study of RFID Implementation in the Supply Chain
The purpose of this paper is to investigate the barriers and motivations for adopting radio frequency identification (RFID), the level of RFID implementation, the processes RFID is utilized in, and issues in the deployment of RFID. A survey instrument was developed based on a literature review. The survey was then distributed to the members of the Association for Operations Management Rhode Island and Boston chapters. The results were then analyzed. It was found that the majority of the surveyed firms are not considering RFID implementation. Lack of a business case and lack of understanding were cited as their main concerns. For firms considering RFID implementation and firms that had implemented RFID, better inventory management, obtaining competitive advantage, and cost reduction were the three most important motivations for adopting RFID. Financial concerns and the lack of a business case were the most prevalent issues. In addition, product tracking (pallets, cases, and items) in shipping was the most cited RFID application. It was also found that considering firms are facing less pressure from customers to adopt RFID and reported a much higher degree of apprehension regarding potential issues than implementing firms reported for actual difficulties faced. One of the limitations is the small sample size (n = 49) which may limit the generalizability of the results. By identifying barriers, motivations, and issues in the implementation of RFID, this study further educates practitioners on the challenges and opportunities of RFID, as well as providing direction to academicians for further research on this area
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