350 research outputs found
Immersed boundary-finite element model of fluid-structure interaction in the aortic root
It has long been recognized that aortic root elasticity helps to ensure
efficient aortic valve closure, but our understanding of the functional
importance of the elasticity and geometry of the aortic root continues to
evolve as increasingly detailed in vivo imaging data become available. Herein,
we describe fluid-structure interaction models of the aortic root, including
the aortic valve leaflets, the sinuses of Valsalva, the aortic annulus, and the
sinotubular junction, that employ a version of Peskin's immersed boundary (IB)
method with a finite element (FE) description of the structural elasticity. We
develop both an idealized model of the root with three-fold symmetry of the
aortic sinuses and valve leaflets, and a more realistic model that accounts for
the differences in the sizes of the left, right, and noncoronary sinuses and
corresponding valve cusps. As in earlier work, we use fiber-based models of the
valve leaflets, but this study extends earlier IB models of the aortic root by
employing incompressible hyperelastic models of the mechanics of the sinuses
and ascending aorta using a constitutive law fit to experimental data from
human aortic root tissue. In vivo pressure loading is accounted for by a
backwards displacement method that determines the unloaded configurations of
the root models. Our models yield realistic cardiac output at physiological
pressures, with low transvalvular pressure differences during forward flow,
minimal regurgitation during valve closure, and realistic pressure loads when
the valve is closed during diastole. Further, results from high-resolution
computations demonstrate that IB models of the aortic valve are able to produce
essentially grid-converged dynamics at practical grid spacings for the
high-Reynolds number flows of the aortic root
Innovations in air sampling to detect plant pathogens
Many innovations in the development and use of air sampling devices have occurred in plant pathology since the first description of the Hirst spore trap. These include improvements in capture efficiency at relatively high air-volume collection rates, methods to enhance the ease of sample processing with downstream diagnostic methods and even full automation of sampling, diagnosis and wireless reporting of results. Other innovations have been to mount air samplers on mobile platforms such as UAVs and ground vehicles to allow sampling at different altitudes and locations in a short space of time to identify potential sources and population structure. Geographical Information Systems and the application to a network of samplers can allow a greater prediction of airborne inoculum and dispersal dynamics. This field of technology is now developing quickly as novel diagnostic methods allow increasingly rapid and accurate quantifications of airborne species and genetic traits. Sampling and interpretation of results, particularly action-thresholds, is improved by understanding components of air dispersal and dilution processes and can add greater precision in the application of crop protection products as part of integrated pest and disease management decisions. The applications of air samplers are likely to increase, with much greater adoption by growers or industry support workers to aid in crop protection decisions. The same devices are likely to improve information available for detection of allergens causing hay fever and asthma or provide valuable metadata for regional plant disease dynamics
Can forest management based on natural disturbances maintain ecological resilience?
Given the increasingly global stresses on forests, many ecologists argue that managers must maintain ecological resilience: the capacity of ecosystems to absorb disturbances without undergoing fundamental change. In this review we ask: Can the emerging paradigm of natural-disturbance-based management (NDBM) maintain ecological resilience in managed forests? Applying resilience theory requires careful articulation of the ecosystem state under consideration, the disturbances and stresses that affect the persistence of possible alternative states, and the spatial and temporal scales of management relevance. Implementing NDBM while maintaining resilience means recognizing that (i) biodiversity is important for long-term ecosystem persistence, (ii) natural disturbances play a critical role as a generator of structural and compositional heterogeneity at multiple scales, and (iii) traditional management tends to produce forests more homogeneous than those disturbed naturally and increases the likelihood of unexpected catastrophic change by constraining variation of key environmental processes. NDBM may maintain resilience if silvicultural strategies retain the structures and processes that perpetuate desired states while reducing those that enhance resilience of undesirable states. Such strategies require an understanding of harvesting impacts on slow ecosystem processes, such as seed-bank or nutrient dynamics, which in the long term can lead to ecological surprises by altering the forest's capacity to reorganize after disturbance
RS1, Custodial Isospin and Precision Tests
We study precision electroweak constraints within a RS1 model with gauge
fields and fermions in the bulk. The electroweak gauge symmetry is enhanced to
SU(2)_L \times SU(2)_R \times U(1)_{B-L}, thereby providing a custodial isospin
symmetry sufficient to suppress excessive contributions to the T parameter. We
then construct complete models, complying with all electroweak constraints, for
solving the hierarchy problem, without supersymmetry or large hierarchies in
the fundamental couplings. Using the AdS/CFT correspondence our models can be
interpreted as dual to a strongly coupled conformal Higgs sector with global
custodial symmetry, gauge and fermionic matter being fundamental fields
external to the CFT. This scenario has interesting collider signals, distinct
from other RS models in the literature.Comment: 32 pages, 6 figures, latex2e, minor changes, references adde
Evidence of Color Coherence Effects in W+jets Events from ppbar Collisions at sqrt(s) = 1.8 TeV
We report the results of a study of color coherence effects in ppbar
collisions based on data collected by the D0 detector during the 1994-1995 run
of the Fermilab Tevatron Collider, at a center of mass energy sqrt(s) = 1.8
TeV. Initial-to-final state color interference effects are studied by examining
particle distribution patterns in events with a W boson and at least one jet.
The data are compared to Monte Carlo simulations with different color coherence
implementations and to an analytic modified-leading-logarithm perturbative
calculation based on the local parton-hadron duality hypothesis.Comment: 13 pages, 6 figures. Submitted to Physics Letters
Species abundance distributions: moving beyond single prediction theories to integration within an ecological framework
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/75247/1/j.1461-0248.2007.01094.x.pd
Dynamics of spherically symmetric spacetimes: hydrodynamics and radiation
Using the 3+1 formalism of general relativity we obtain the equations
governing the dynamics of spherically symmetric spacetimes with arbitrary
sources. We then specialize for the case of perfect fluids accompanied by a
flow of interacting massless or massive particles (e.g. neutrinos) which are
described in terms of relativistic transport theory. We focus in three types of
coordinates: 1) isotropic gauge and maximal slicing, 2) radial gauge and polar
slicing, and 3) isotropic gauge and polar slicing.Comment: submitted to Phys. Rev. D, 46 pages, RevTex file, no figure
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