61,129 research outputs found
Non-linear bioconvection in a deep suspension of gyrotactic swimming micro-organisms
The non-linear structure of deep, stochastic, gyrotactic bioconvection is explored. A linear analysis is reviewed and a weakly non-linear analysis justifies its application by revealing the supercritical nature of the bifurcation. An asymptotic expansion is used to derive systems of partial differential equations for long plume structures which vary slowly with depth. Steady state and travelling wave solutions are found for the first order system of partial differential equations and the second order system is manipulated to calculate the speed of vertically travelling pulses. Implications of the results and possibilities of experimental validation are discussed
Analytical approximations for the orientation distribution of small dipolar particles in steady shear flows
Analytic approximations are obtained to solutions of the steady Fokker-Planck equation describing the probability density functions for the orientation of dipolar particles in a steady, low-Reynolds-number shear flow and a uniform external field. Exact computer algebra is used to solve the equation in terms of a truncated spherical harmonic expansion. It is demonstrated that very low orders of approximation are required for spheres but that spheroids introduce resolution problems in certain flow regimes. Moments of the orientation probability density function are derived and applications to swimming cells in bioconvection are discussed. A separate asymptotic expansion is performed for the case in which spherical particles are in a flow with high vorticity, and the results are compared with the truncated spherical harmonic expansion. Agreement between the two methods is excellent
Bioactive glass engineered coatings for Ti6Al4V alloys: Influence of strontium substitution for calcium on sintering behaviour
NOTICE: this is the authorâs version of a work that was accepted for publication in JOURNAL OF NON-CRYSTALLINE SOLIDS. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in JOURNAL OF NON-CRYSTALLINE SOLIDS, [VOL 356, ISSUE 44-49, (2010), DOI 10.1016/j.jnoncrysol.2010.05.01
Efficient Searches for r-Process-Enhanced, Metal-Poor Stars
Neutron-capture-enhanced, metal-poor stars are of central importance to
developing an understanding of the operation of the r-process in the early
Galaxy, thought to be responsible for the formation of roughly half of all
elements beyond the iron peak. A handful of neutron-capture-rich, metal-poor
stars with [Fe/H] < -2.0 have already been identified, including the well known
r-process-enhanced stars CS 22892-052 and CS 31082-001. However, many questions
of fundamental interest can only be addressed with the assemblage of a much
larger sample of such stars, so that general properties can be distinguished.
We describe a new effort, HERES: The Hamburg/ESO R-Process-Enhanced Star
survey, nearing completion, which will identify on the order of 5-10 additional
highly r-process-enhanced, metal-poor stars, and in all likelihood, a similar
or greater number of mildly r-process-enhanced, metal-poor stars in the halo of
the Galaxy. HERES is based on rapid "snapshot" spectra of over 350 candidate
halo giants with [Fe/H] < -2.0, obtained at moderately high resolution, and
with moderate signal-to-noise ratios, using the UVES spectrograph on the
European VLT 8m telescope.Comment: Contributed paper to The Eighth Nuclei in the Cosmos conference, to
appear (in refereed form) in Nuclear Physics
Long lasting instabilities in granular mixtures
We have performed experiments of axial segregation in the Oyama's drum. We
have tested binary granular mixtures during very long times. The segregation
patterns have been captured by a CCD camera and spatio-temporal graphs are
created. We report the occurence of instabilities which can last several hours.
We stress that those instabilities originate from the competition between axial
and radial segregations. We put into evidence the occurence of giant
fluctuations in the fraction of grain species along the surface during the
unstable periods.Comment: 10 pages, 10 figures, (2002
Temperature-dependent molecular absorption cross sections for exoplanets and other atmospheres
Exoplanets, and in particular hot ones such as hot Jupiters, require a very
significant quantities of molecular spectroscopic data to model radiative
transport in their atmospheres or to interpret their spectra. This data is
commonly provided in the form of very extensive transition line lists. The size
of these line lists is such that constructing a single model may require the
consideration of several billion lines. We present a procedure to simplify this
process based on the use of cross sections. Line lists for water, H, HCN
/HNC and ammonia have been turned into cross sections on a fine enough grid to
preserve their spectroscopic features. Cross sections are provided at a fixed
range of temperatures and an interpolation procedure which can be used to
generate cross sections at arbitrary temperatures is described. A web-based
interface (www.exomol.com/xsec) has been developed to allow astronomers to
download cross sections at specified temperatures and spectral resolution.
Specific examples are presented for the key water molecule.Comment: Icarus (submitted
The Effect of Flow Regulation on Channel Geomorphic Unit (CGU) Composition in the SoÄa River, Slovenia.
This paper sets out to examine the effects of flow regulation on the size, spatial distribution and connectivity of physical habitats or channel geomorphic units (CGU) of an upland river system in Slovenia. A river channel survey was completed along three reaches (totalling 14.3km) of the SoÄa River in order compare an unregulated stretch (reach 1), with two regulated reaches with lower discharges, i.e. a bypassed section that experiences significant flow reduction (reach 2), and a stretch regulated by dam operation (reach 3). CGUâs were classified according to a modified version of the Hawkins et al (1993) system, and mapped on foot and from a boat using a combination of visual assessment and physical measurement. Mapping-grade GPS was used to locate CGU boundaries to sub-metre accuracy, and the application of GIS (MapInfo) enabled the description and analysis of the longitudinal distribution of CGUâs along each reach.
Results demonstrated significant differences in the CGU composition between the unregulated and regulated reaches. The unregulated stretch (reach 1) was dominated by the glides (55%) with relatively fast-flowing and turbulent features (runs, riffles and rapids) making up the rest of the reach. The dominant feature of both of the regulated reaches were the slow flowing pool CGUâs occupying 44% of the bypassed section (reach 2), and 76% of the dam regulated section (reach 3), with glides, runs, riffles and rapids forming the remainder of the CGUâs.
Physical measurements highlighted the extent to which the reduced discharge in the regulated reaches was dewatering the channel and reducing the size of the CGUâs. The average CGU size in the unregulated stretch (reach 1) was 58m2 of water area per m of channel length (m2/m) compared to 18.42m2/m in the reach 2, and 29.22m2/m in reach 3. CGUâs tended to be shorter, and hence there was greater habitat division or fragmenta-tion evident in the two regulated reaches, particularly the bypassed section. For example, there were on aver-age 6.81 CGUâs per km in the unregulated reach (reach 1), 18.12 CGU/km in reach 2, and 8.08 CGU/km in reach 3.
This study suggests that in the SoÄa River under the flow conditions present during the survey, flow regula-tion alters the dominant types of CGUâs present (to slower flowing and less turbulent features), significantly reduces the size of CGUâs, and affects the longitudinal distribution of types by reducing habitat connectivity and creating greater habitat fragmentation
- âŠ