65,371 research outputs found
Dynamics of axial separation in long rotating drums
We propose a continuum description for the axial separation of granular
materials in a long rotating drum. The model, operating with two local
variables, concentration difference and the dynamic angle of repose, describes
both initial transient traveling wave dynamics and long-term segregation of the
binary mixture. Segregation proceeds through ultra-slow logarithmic coarsening.Comment: 4 pages, 3 Postscript figures; submitted to PR
On Superalgebras of Matrices with Symmetry Properties
It is known that semi-magic square matrices form a 2-graded algebra or
superalgebra with the even and odd subspaces under centre-point reflection
symmetry as the two components. We show that other symmetries which have been
studied for square matrices give rise to similar superalgebra structures,
pointing to novel symmetry types in their complementary parts. In particular,
this provides a unifying framework for the composite `most perfect square'
symmetry and the related class of `reversible squares'; moreover, the
semi-magic square algebra is identified as part of a 2-gradation of the general
square matrix algebra. We derive explicit representation formulae for matrices
of all symmetry types considered, which can be used to construct all such
matrices.Comment: 25 page
Rainfall but not selective logging affect changes in abundance of tropical forest butterfly in Sabah, Borneo
We investigated the effects of rainfall on the distribution and abundance of the satyrine butterfly Ragadia makuta in selectively logged and unlogged forest on Borneo. In 1997-98, there was a severe El Nino-Southern Oscillation (ENSO) drought, and annual surveys over a 4-y period showed that abundance of R. makuta was greatly reduced during the drought, but that populations quickly recovered after it. Monthly surveys over a 12-mo period of typical rainfall showed that high rainfall in the month preceding surveys significantly reduced butterfly abundance. Butterfly abundance and distribution did not differ between selectively logged and unlogged areas in either monthly or annual surveys and there was no difference between selectively logged and unlogged areas in the pattern of post-drought recovery. These results indicate that the abundance of R. makuta was significantly reduced both after high rainfall and during severe drought, but that these impacts were short-lived and were not affected by habitat disturbance. ENSO droughts on Borneo naturally often lead to widespread forest fires and thus impacts of ENSO events for butterflies are more likely to be due to indirect effects of habitat loss, rather than direct effects of drought on butterfly population dynamics
Demographics and the long-term outlook for housing investment
John Hill and D'Ann Petersen measure the importance of projected shifts in the size and age distribution of the U.S. population for domestic housing investment. Their analysis runs through the year 2010 and provides separate estimates for single-family and multifamily investment. ; Hill and Petersen find that the contractionary effects of the population slowdown are already being felt in the housing industry and probably have been since the latter part of the 1980s. In Hill and Petersen's simulations, demographic shifts lower net housing investment by 17 percent from the late 1980s through the first half of the 1990s. Population factors then reduce net investment an additional 22 percent through the year 2005 before turning favorable. ; Hill and Petersen discuss the implications of their findings for construction jobs and housing prices. They suggest that the population slowdown need not produce an absolute contraction in housing employment. It will, however, reduce housing's share of national employment by as much as one-third. According to the authors, the changing demographics do not provide a compelling reason for average home prices to suffer a deep decline. They do suggest, however, that significant relative price adjustments may need to take place between different types of homes.Demography ; Housing
The optical counterpart of SAX J1808.4-3658, the transient bursting millisecond X-ray pulsar
A set of CCD images have been obtained during the decline of the X-ray
transient SAX J1808.4-3658 during April-June 1998. The optical counterpart has
been confirmed by several pieces of evidence. The optical flux shows a
modulation on several nights which is consistent with the established X-ray
binary orbit period of 2 hours. This optical variability is roughly in
antiphase with the weak X-ray modulation. The source mean magnitude of V=16.7
on April 18 declined rapidly after April 22. From May 2 onwards the magnitude
was more constant at around V=18.45 but by June 27 was below our sensitivity
limit. The optical decline precedes the rapid second phase of the X-ray
decrease by 3 +/- 1 days. The source has been identified on a 1974 UK Schmidt
plate at an estimated magnitude of ~20. The nature of the optical companion is
discussed.Comment: 5 pages, 3 figures; published in MNRAS, March 15th 199
Recommended from our members
Shopping for food: lessons from a London borough
Purpose – This paper aims to measure access to food in an inner London borough. Design/methodology/approach – There were six phases, which included designing food baskets, consultation with local residents and a shop survey. Recognising the cultural make-up of the borough food baskets and menus were developed for four key communities, namely: White British, Black Caribbean, Turkish, and Black African. Three areas were identified for the study and shopping hubs identified with a 500-metre radius from a central parade of shops. Findings – The findings paint an intricate web of interactions ranging from availability in shops to accessibility and affordability being key issues for some groups. It was found that in the areas studied there was availability of some key healthy items, namely fresh fruit and vegetables, but other items such as: fresh meat and poultry, fish, lower fat dairy foods, high fibre pasta and brown rice were not available. Access was found to be defined, by local people, as more extensive than just physical distance to/from shops – for many shopping was made more difficult by having to use taxis and inconvenient buses. Small shops were important in delivering healthy food options to communities in areas of deprivation and were judged to offer a better range and more appropriate food than the branches of the major supermarket chains. Research limitations/implications – The importance of monitoring the impact of shops and shop closures on healthy food availability is emphasised. From a policy perspective the findings suggest that approaches based on individual agency need to be balanced with upstream public health nutrition approaches in order to influence the options available. Originality/value – The paper is arguably the first to examine and dissect the issue of food availability and accessibility in the inner London borough in question, especially in the light of its proposed redevelopment for the London Olympics in 2012
Instrumentation for Millimeter-wave Magnetoelectrodynamic Investigations of Low-Dimensional Conductors and Superconductors
We describe instrumentation for conducting high sensitivity millimeter-wave
cavity perturbation measurements over a broad frequency range (40-200 GHz) and
in the presence of strong magnetic fields (up to 33 tesla). A Millimeter-wave
Vector Network Analyzer (MVNA) acts as a continuously tunable microwave source
and phase sensitive detector (8-350 GHz), enabling simultaneous measurements of
the complex cavity parameters (resonance frequency and Q-value) at a rapid
repetition rate (approx. 10 kHz). We discuss the principal of operation of the
MVNA and the construction of a probe for coupling the MVNA to various
cylindrical resonator configurations which can easily be inserted into a high
field magnet cryostat. We also present several experimental results which
demonstrate the potential of the instrument for studies of low-dimensional
conducting systems.Comment: 20 pages including fig
- …