463 research outputs found
Granular packings with moving side walls
The effects of movement of the side walls of a confined granular packing are
studied by discrete element, molecular dynamics simulations. The dynamical
evolution of the stress is studied as a function of wall movement both in the
direction of gravity as well as opposite to it. For all wall velocities
explored, the stress in the final state of the system after wall movement is
fundamentally different from the original state obtained by pouring particles
into the container and letting them settle under the influence of gravity. The
original packing possesses a hydrostatic-like region at the top of the
container which crosses over to a depth-independent stress. As the walls are
moved in the direction opposite to gravity, the saturation stress first reaches
a minimum value independent of the wall velocity, then increases to a
steady-state value dependent on the wall-velocity. After wall movement ceases
and the packing reaches equilibrium, the stress profile fits the classic
Janssen form for high wall velocities, while it has some deviations for low
wall velocities. The wall movement greatly increases the number of
particle-wall and particle-particle forces at the Coulomb criterion. Varying
the wall velocity has only small effects on the particle structure of the final
packing so long as the walls travel a similar distance.Comment: 11 pages, 10 figures, some figures in colo
Preventing pain on injection of propofol: A comparison between lignocaine pre-treatment and lignocaine added to propofol
Publisher's copy made available with the permission of the publisherA randomized double-blind study compared two methods of preventing the pain from injection of propofol, lignocaine pre-treatment followed by propofol and lignocaine added to propofol. One hundred patients received a 4 ml solution intravenously with a venous tourniquet for 1 minute, followed by propofol mixed with 2 ml of solution. Patients were divided into two treatment groups of 50 patients each: 4 ml 1% lignocaine pre-treatment followed by propofol and 2 ml saline, or 4 ml saline followed by propofol and 2 ml 2% lignocaine. Pain was assessed with a 100 mm visual analogue scale after induction and in recovery. The incidence of injection pain was 8% in the propofol mixed with lignocaine group, and 28% in the lignocaine pre-treatment group. This difference is statistically significant (P=0.017). For those patients who had pain, the mean pain score was 26.5 on induction for the propofol with lignocaine group (n=4), while the mean score was 44.4 for the pre-treatment group (n=13). The difference was not statistically significant (P=0.25). None of the propofol mixed with lignocaine group recalled pain, while 13 of the pre-treatment group did so. Lignocaine pre-treatment does not improve the immediate or the recalled comfort of patients during propofol induction when compared to lignocaine added to propofol. It is recommended that lignocaine should be added to propofol for induction rather than given before induction.P. Lee, W. J. Russellhttp://www.aaic.net.au/Article.asp?D=200339
Onset of fluidization in vertically shaken granular material
When granular material is shaken vertically one observes convection, surface
fluidization, spontaneous heap formation and other effects. There is a
controversial discussion in literature whether there exists a threshold for the
Froude number below which these effects cannot be
observed anymore. By means of theoretical analysis and computer simulation we
find that there is no such single threshold. Instead we propose a modified
criterion which coincides with critical Froude number for small
driving frequency .Comment: 7 pages, 5 figure
The reversible polydisperse Parking Lot Model
We use a new version of the reversible Parking Lot Model to study the
compaction of vibrated polydisperse media. The particle sizes are distributed
according to a truncated power law. We introduce a self-consistent desorption
mechanism with a hierarchical initialization of the system. In this way, we
approach densities close to unity. The final density depends on the
polydispersity of the system as well as on the initialization and will reach a
maximum value for a certain exponent in the power law.Comment: 7 pages, Latex, 12 figure
The Wadi Faynan Project, Southern Jordan: a Preliminary Report on Geomorphology and Landscape Archaeology
Reproduced with permission of the publisher. © 1997 Council for British Research in the Levant. Details of the publication are available at: http://www.cbrl.org.uk/Publications/publications_default.shtmThe Wadi Faynan Project of the British Institute at Amman for Archaeology and History (BIAAH) has as its principal objective the provision of a detailed case study in the relationship between environmental change and human history in the arid zone, from prehistory to the present day. This report describes the preliminary findings of an initial campaign of fieldwork in geomorphology and landscape archaeology conducted by an inter-disciplinary team in 1996. A preliminary sequence of fluvial events has been established, represented by the Ghuwayr and Shayqar Beds dated to the Late Pleistocene, and the Faynan and Dana Beds dated to the Holocene. Methodologies have been trialed for recording, dating and interpreting the ancient field system assumed to be of Nabataean, Roman and Byzantine date; initial findings confirm its longevity of use and complexity of purpose. There are also indications that floodwater farming began in the Wadi Faynan in the Chalcolithic or Early Bronze Ag
Phenomenological glass model for vibratory granular compaction
A model for weakly excited granular media is derived by combining the free
volume argument of Nowak et al. [Phys. Rev. E 57, 1971 (1998)] and the
phenomenological model for supercooled liquids of Adam and Gibbs [J. Chem.
Phys. 43, 139 (1965)]. This is made possible by relating the granular
excitation parameter \Gamma, defined as the peak acceleration of the driving
pulse scaled by gravity, to a temperature-like parameter \eta(\Gamma). The
resulting master equation is formally identical to that of Bouchaud's trap
model for glasses [J. Phys. I 2, 1705 (1992)]. Analytic and simulation results
are shown to compare favourably with a range of known experimental behaviour.
This includes the logarithmic densification and power spectrum of fluctuations
under constant \eta, the annealing curve when \eta is varied cyclically in
time, and memory effects observed for a discontinuous shift in \eta. Finally,
we discuss the physical interpretation of the model parameters and suggest
further experiments for this class of systems.Comment: 2 references added; some figure labels tweaked. To appear in PR
Glassy dynamics in granular compaction: sand on random graphs
We discuss the use of a ferromagnetic spin model on a random graph to model
granular compaction. A multi-spin interaction is used to capture the
competition between local and global satisfaction of constraints characteristic
for geometric frustration. We define an athermal dynamics designed to model
repeated taps of a given strength. Amplitude cycling and the effect of
permanently constraining a subset of the spins at a given amplitude is
discussed. Finally we check the validity of Edwards' hypothesis for the
athermal tapping dynamics.Comment: 13 pages Revtex, minor changes, to appear in PR
Roughness of Sandpile Surfaces
We study the surface roughness of prototype models displaying self-organized
criticality (SOC) and their noncritical variants in one dimension. For SOC
systems, we find that two seemingly equivalent definitions of surface roughness
yields different asymptotic scaling exponents. Using approximate analytical
arguments and extensive numerical studies we conclude that this ambiguity is
due to the special scaling properties of the nonlinear steady state surface. We
also find that there is no such ambiguity for non-SOC models, although there
may be intermediate crossovers to different roughness values. Such crossovers
need to be distinguished from the true asymptotic behaviour, as in the case of
a noncritical disordered sandpile model studied in [10].Comment: 5 pages, 4 figures. Accepted for publication in Phys. Rev.
Tipburn resilience in lettuce (Lactuca spp.) – the importance of germplasm resources and production system‐specific assays
BACKGROUND
Tipburn is a physiological disorder of lettuce (Lactuca spp.). It causes discoloration and collapse of leaf margins, leading to unsaleable crops in both protected (glasshouse, hydroponic) and outdoor production systems. The occurrence of tipburn is hard to predict and is sensitive to environmental conditions. Phenotyping for tipburn resilience requires diverse germplasm resources and, to date, limited material has been investigated for this condition.
RESULTS
Using a Lactuca diversity fixed foundation set (DFFS) under glasshouse conditions, we identified a significant (P < 0.001) genotypic effect on tipburn resilience across both the entire population and across lines belonging to the cultivated species L. sativa alone. Latuca sativa lines exhibited significantly (P < 0.05) higher average tipburn severity than those belonging to the wild species L. saligna, L. serriola, and L. virosa but we were able to identify both cultivated and wild tipburn-resilient lines. Leaf morphology factors, which included pigmentation, width, and serration, also significantly (P < 0.05) influenced tipburn resilience. Using a recombinant inbred line (RIL) mapping population derived from two DFFS lines, different small-effect quantitative trait loci (QTLs) accounting for 12.3% and 25.2% of total tipburn variation were identified in glasshouse and field conditions, respectively.
CONCLUSIONS
These results reflect the advantages of phenotyping under production-system-specific conditions for the examination of environmentally sensitive traits and highlight genetic markers and germplasm resources for the development of tipburn resilient lines for use in both protected and outdoor lettuce production
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