9,014 research outputs found
DSIF station schedules
System manages Deep Space Instrumentation Facilities /DSIF/ equipment construction and modification planning. Versatile program applies to such tasks as employee time and task schedules, pay schedules, operations schedules, and plant and equipment procurement, construction, modification or service
Opening up the open spaces through space syntax
In many parts of the world, housing and neighbourhood developments of the latetwentieth century have been shaped by the belief that it is important to createcommunal open spaces that can provide a physical focus for local communities.This has given rise to a range of modern housing typologies that are based on anethos of building community through design, using concepts such as a ?privacygradient?, the separation of pedestrian and vehicular movement and the notion of?enclosure?, amongst others, to shape the public open space of the estate. Thispaper examines the contribution of such public spaces to community awareness intwenty-eight English city-centre housing estates with differing housingmorphologies, built between the middle of the nineteenth century and the earlytwenty-first century. As a manifestation of the complex social relationships thattake place within the domain of public space, we suggest that the above conceptsand their physical realisation need to be complemented by careful consideration ofsocial notions such as the constitutedness of the spaces, co-presence andsurveillance. The methodology used is based on a detailed analysis of the builtforms and spatial layouts of the estates, combined with a ?space syntax? analysis ofthe interconnectedness and accessibility of the public realm within each estate,representations that show the extent of visual fields from key public open spacesand an appraisal of the types of physical boundaries surrounding each of theseopen spaces and the openings that give onto them. The juxtaposition of thesedifferent analyses provides insight into the interfaces maintained spatially betweenresidents and non-residents and the possibilities that are thereby created formutual awareness and encounter. In many parts of the world, housing and neighbourhood developments of the latetwentieth century have been shaped by the belief that it is important to createcommunal open spaces that can provide a physical focus for local communities.This has given rise to a range of modern housing typologies that are based on anethos of building community through design, using concepts such as a ?privacygradient?, the separation of pedestrian and vehicular movement and the notion of?enclosure?, amongst others, to shape the public open space of the estate. Thispaper examines the contribution of such public spaces to community awareness intwenty-eight English city-centre housing estates with differing housingmorphologies, built between the middle of the nineteenth century and the earlytwenty-first century. As a manifestation of the complex social relationships thattake place within the domain of public space, we suggest that the above conceptsand their physical realisation need to be complemented by careful consideration ofsocial notions such as the constitutedness of the spaces, co-presence andsurveillance. The methodology used is based on a detailed analysis of the builtforms and spatial layouts of the estates, combined with a ?space syntax? analysis ofthe interconnectedness and accessibility of the public realm within each estate,representations that show the extent of visual fields from key public open spacesand an appraisal of the types of physical boundaries surrounding each of theseopen spaces and the openings that give onto them. The juxtaposition of thesedifferent analyses provides insight into the interfaces maintained spatially betweenresidents and non-residents and the possibilities that are thereby created formutual awareness and encounter
Catastrophic Phase Transitions and Early Warnings in a Spatial Ecological Model
Gradual changes in exploitation, nutrient loading, etc. produce shifts
between alternative stable states (ASS) in ecosystems which, quite often, are
not smooth but abrupt or catastrophic. Early warnings of such catastrophic
regime shifts are fundamental for designing management protocols for
ecosystems. Here we study the spatial version of a popular ecological model,
involving a logistically growing single species subject to exploitation, which
is known to exhibit ASS. Spatial heterogeneity is introduced by a carrying
capacity parameter varying from cell to cell in a regular lattice. Transport of
biomass among cells is included in the form of diffusion. We investigate
whether different quantities from statistical mechanics -like the variance, the
two-point correlation function and the patchiness- may serve as early warnings
of catastrophic phase transitions between the ASS. In particular, we find that
the patch-size distribution follows a power law when the system is close to the
catastrophic transition. We also provide links between spatial and temporal
indicators and analyze how the interplay between diffusion and spatial
heterogeneity may affect the earliness of each of the observables. We find that
possible remedial procedures, which can be followed after these early signals,
are more effective as the diffusion becomes lower. Finally, we comment on
similarities and differences between these catastrophic shifts and paradigmatic
thermodynamic phase transitions like the liquid-vapour change of state for a
fluid like water
Singularity theory study of overdetermination in models for L-H transitions
Two dynamical models that have been proposed to describe transitions between
low and high confinement states (L-H transitions) in confined plasmas are
analysed using singularity theory and stability theory. It is shown that the
stationary-state bifurcation sets have qualitative properties identical to
standard normal forms for the pitchfork and transcritical bifurcations. The
analysis yields the codimension of the highest-order singularities, from which
we find that the unperturbed systems are overdetermined bifurcation problems
and derive appropriate universal unfoldings. Questions of mutual equivalence
and the character of the state transitions are addressed.Comment: Latex (Revtex) source + 13 small postscript figures. Revised versio
Generation of scalar-tensor gravity effects in equilibrium state boson stars
Boson stars in zero-, one-, and two-node equilibrium states are modeled
numerically within the framework of Scalar-Tensor Gravity. The complex scalar
field is taken to be both massive and self-interacting. Configurations are
formed in the case of a linear gravitational scalar coupling (the Brans-Dicke
case) and a quadratic coupling which has been used previously in a cosmological
context. The coupling parameters and asymptotic value for the gravitational
scalar field are chosen so that the known observational constraints on
Scalar-Tensor Gravity are satisfied. It is found that the constraints are so
restrictive that the field equations of General Relativity and Scalar-Tensor
gravity yield virtually identical solutions. We then use catastrophe theory to
determine the dynamically stable configurations. It is found that the maximum
mass allowed for a stable state in Scalar-Tensor gravity in the present
cosmological era is essentially unchanged from that of General Relativity. We
also construct boson star configurations appropriate to earlier cosmological
eras and find that the maximum mass for stable states is smaller than that
predicted by General Relativity, and the more so for earlier eras. However, our
results also show that if the cosmological era is early enough then only states
with positive binding energy can be constructed.Comment: 20 pages, RevTeX, 11 figures, to appear in Class. Quantum Grav.,
comments added, refs update
Catastrophic regime shifts in model ecological communities are true phase transitions
Ecosystems often undergo abrupt regime shifts in response to gradual external
changes. These shifts are theoretically understood as a regime switch between
alternative stable states of the ecosystem dynamical response to smooth changes
in external conditions. Usual models introduce nonlinearities in the
macroscopic dynamics of the ecosystem that lead to different stable attractors
among which the shift takes place. Here we propose an alternative explanation
of catastrophic regime shifts based on a recent model that pictures ecological
communities as systems in continuous fluctuation, according to certain
transition probabilities, between different micro-states in the phase space of
viable communities. We introduce a spontaneous extinction rate that accounts
for gradual changes in external conditions, and upon variations on this control
parameter the system undergoes a regime shift with similar features to those
previously reported. Under our microscopic viewpoint we recover the main
results obtained in previous theoretical and empirical work (anomalous
variance, hysteresis cycles, trophic cascades). The model predicts a gradual
loss of species in trophic levels from bottom to top near the transition. But
more importantly, the spectral analysis of the transition probability matrix
allows us to rigorously establish that we are observing the fingerprints, in a
finite size system, of a true phase transition driven by background
extinctions.Comment: 19 pages, 11 figures, revised versio
Green Infrastructure in Coastal Landscapes: Ecological Design, Hydrological Function, and Sustainable Land Use Goals
2012 S.C. Water Resources Conference - Exploring Opportunities for Collaborative Water Research, Policy and Managemen
The Clumping Transition in Niche Competition: a Robust Critical Phenomenon
We show analytically and numerically that the appearance of lumps and gaps in
the distribution of n competing species along a niche axis is a robust
phenomenon whenever the finiteness of the niche space is taken into account. In
this case depending if the niche width of the species is above or
below a threshold , which for large n coincides with 2/n, there are
two different regimes. For the lumpy pattern emerges
directly from the dominant eigenvector of the competition matrix because its
corresponding eigenvalue becomes negative. For the lumpy
pattern disappears. Furthermore, this clumping transition exhibits critical
slowing down as is approached from above. We also find that the number
of lumps of species vs. displays a stair-step structure. The positions
of these steps are distributed according to a power-law. It is thus
straightforward to predict the number of groups that can be packed along a
niche axis and it coincides with field measurements for a wide range of the
model parameters.Comment: 16 pages, 7 figures;
http://iopscience.iop.org/1742-5468/2010/05/P0500
Canonical Melnikov theory for diffeomorphisms
We study perturbations of diffeomorphisms that have a saddle connection
between a pair of normally hyperbolic invariant manifolds. We develop a
first-order deformation calculus for invariant manifolds and show that a
generalized Melnikov function or Melnikov displacement can be written in a
canonical way. This function is defined to be a section of the normal bundle of
the saddle connection.
We show how our definition reproduces the classical methods of Poincar\'{e}
and Melnikov and specializes to methods previously used for exact symplectic
and volume-preserving maps. We use the method to detect the transverse
intersection of stable and unstable manifolds and relate this intersection to
the set of zeros of the Melnikov displacement.Comment: laTeX, 31 pages, 3 figure
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