95,153 research outputs found
Natural and laser-induced cavitation in corn stems: On the mechanisms of acoustic emissions
Water in plant xylem is often superheated, and therefore in a meta-stable
state. Under certain conditions, it may suddenly turn from the liquid to the
vapor state. This cavitation process produces acoustic emissions. We report the
measurement of ultrasonic acoustic emissions (UAE) produced by natural and
induced cavitation in corn stems. We induced cavitation and UAE in vivo, in
well controlled and reproducible experiments, by irradiating the bare stem of
the plants with a continuous-wave laser beam. By tracing the source of UAE, we
were able to detect absorption and frequency filtering of the UAE propagating
through the stem. This technique allows the unique possibility of studying
localized embolism of plant conduits, and thus to test hypotheses on the
hydraulic architecture of plants. Based on our results, we postulate that the
source of UAE is a transient "cavity oscillation" triggered by the disruptive
effect of cavitation inception.Comment: 8 pages, 5 figure
Linking microsimulators of bus stops and traffic operations: the case of PASSION and BusSIGSIM
The aim of this article is to explore the linkage of two microsimulators developed at theUniversity College London. At present, these models deal independently with buses ateither bus stops or traffic networks. First, both microsimulators are described in somedetail. The generic way in which both models can be connected is then proposed. As aresult of this analysis, the main issues for a comprehensive introduction of public transportvehicles (buses) into microscopic traffic simulators are highlighted. One practical outcomeof this study is that the improvement in the representation of buses in microscopic trafficsimulators will allow the engineers to take into account traffic management measures thatotherwise will not be assessed
Study of passenger-bus-traffic interactions on bus stop operations
Buses are the unsung heroes of public transport in modern cities around the world in high,medium and low income countries. However, the bus system is usually cobbled by poordesign which has resulted from poor understanding of how a busThis paper examines theimpacts of the interactions between buses, passengers and traffic on bus operations, especiallyin relation to delays and capacity at bus stops. First, the principles of bus stop operations arepresented. Issues like the stages of bus stop operations, the classification of times spent at busstops, causes of delays, and the interaction between bus flow and stop delays are analysed.This leads to the necessity of microscopic simulation to study stops operations. Then, anillustration of the sort of understanding that can be achieved with a simulation modem isshown. Simulation experiments regarding arrival patterns of buses and passengers, boardingtimes, difficulties for buses to leave the stop, and vehicle capacity are presented. Resultsindicate that it is important not to underestimate the real situation found at bus stops, asdesigning for ideal conditions will be insufficient if the reality is different
Geometric Algebras and Extensors
This is the first paper in a series (of four) designed to show how to use
geometric algebras of multivectors and extensors to a novel presentation of
some topics of differential geometry which are important for a deeper
understanding of geometrical theories of the gravitational field. In this first
paper we introduce the key algebraic tools for the development of our program,
namely the euclidean geometrical algebra of multivectors Cl(V,G_{E}) and the
theory of its deformations leading to metric geometric algebras Cl(V,G) and
some special types of extensors. Those tools permit obtaining, the remarkable
golden formula relating calculations in Cl(V,G) with easier ones in Cl(V,G_{E})
(e.g., a noticeable relation between the Hodge star operators associated to G
and G_{E}). Several useful examples are worked in details fo the purpose of
transmitting the "tricks of the trade".Comment: This paper (to appear in Int. J. Geom. Meth. Mod. Phys. 4 (6) 2007)
is an improved version of material appearing in math.DG/0501556,
math.DG/0501557, math.DG/050155
Measurement errors with low-cost citizen science radiometers
The KdUINO is a Do-It-Yourself buoy with low-cost radiometers that
measure a parameter related to water transparency, the diffuse attenuation coefficient
integrated into all the photosynthetically active radiation. In this contribution,
we analyze the measurement errors of a novel low-cost multispectral radiometer
that is used with the KdUINO.Peer Reviewe
Ediacaran Obduction of a Fore-Arc Ophiolite in SW Iberia: A Turning Point in the Evolving Geodynamic Setting of Peri- Gondwana
The Calzadilla Ophiolite is an ensemble of mafic and ultramafic rocks that represents the
transition between lower crust and upper mantle of a Cadomian (peri-Gondwanan) fore arc. Mapping and
structural analysis of the ophiolite demonstrates that it was obducted in latest Ediacaran times, because the
Ediacaran-Early Cambrian sedimentary series (Malcocinado Formation) discordantly covers it. The ophiolite
and emplacement-related structures are affected by Variscan deformation (Devonian-Carboniferous),
which includes SW verging overturned folds (D1) and thrusts (D2), upright folds (D3), extensional faults (D4),
and later faults (D5). These phases of deformation are explained in the context of Variscan tectonics as the
result of the progressive collision between Gondwana and Laurussia. Qualitative unstraining of Variscan
deformation reveals the primary geometry of Ediacaran-Cambrian structures and uncovers the generation of
east verging thrusts as responsible for the primary obduction of the Calzadilla Ophiolite. Restoration of
planar and linear structures associated with this event indicates an Ediacaran, east directed obduction of the
ophiolite, that is, emplacement of the Cadomian fore arc onto inner sections of the northern margin of
Gondwana. According to regional data, the obduction separates two extension-dominated stages in the
tectonic evolution of the African margin of northern Gondwana preserved in southern Europe. Preobduction
extension brought about the onset and widening of fore-arc and back-arc basins in the external part
of the continent, while postobduction extension facilitated the formation of extensional migmatitic domes,
an oceanward migration of back-arc spreading centers across peri-Gondwana, and the eventual opening
of a major basin such as the Rheic Ocean
Exciton condensates in semiconductor quantum wells emit coherent light
We show that a quasi-two dimensional condensate of optically active excitons
emits coherent light even in the absence of population inversion. This allows
an unambiguous and clear experimental detection of the condensed phase. We
prove that, due to the exciton-photon coupling, quantum and thermal
fluctuations do not destroy condensation at finite temperature. Suitable
conditions to achieve condensation are temperatures of a few K for typical
exciton densities, and the use of a pulsed, and preferably circularly
polarized, laser.Comment: 5 pages, no figure
A cluster expansion approach to renormalization group transformations
The renormalization group (RG) approach is largely responsible for the
considerable success which has been achieved in developing a quantitative
theory of phase transitions. This work treats the rigorous definition of the RG
map for classical Ising-type lattice systems in the infinite volume limit at
high temperature. A cluster expansion is used to justify the existence of the
partial derivatives of the renormalized interaction with respect to the
original interaction. This expansion is derived from the formal expressions,
but it is itself well-defined and convergent. Suppose in addition that the
original interaction is finite-range and translation-invariant. We will show
that the matrix of partial derivatives in this case displays an approximate
band property. This in turn gives an upper bound for the RG linearization.Comment: 13 page
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