63,745 research outputs found
Negative Linear Compressibility
While all materials reduce their intrinsic volume under hydrostatic (uniform)
compression, a select few actually \emph{expand} along one or more directions
during this process of densification. As rare as it is counterintuitive, such
"negative compressibility" behaviour has application in the design of pressure
sensors, artificial muscles and actuators. The recent discovery of surprisingly
strong and persistent negative compressibility effects in a variety of new
families of materials has ignited the field. Here we review the phenomenology
of negative compressibility in this context of materials diversity, placing
particular emphasis on the common structural motifs that recur amongst known
examples. Our goal is to present a mechanistic understanding of negative
compressibility that will help inform a clear strategy for future materials
design.Comment: Submitted to PCC
Efficient Dynamic Compressor Optimization in Natural Gas Transmission Systems
The growing reliance of electric power systems on gas-fired generation to
balance intermittent sources of renewable energy has increased the variation
and volume of flows through natural gas transmission pipelines. Adapting
pipeline operations to maintain efficiency and security under these new
conditions requires optimization methods that account for transients and that
can quickly compute solutions in reaction to generator re-dispatch. This paper
presents an efficient scheme to minimize compression costs under dynamic
conditions where deliveries to customers are described by time-dependent mass
flow. The optimization scheme relies on a compact representation of gas flow
physics, a trapezoidal discretization in time and space, and a two-stage
approach to minimize energy costs and maximize smoothness. The resulting
large-scale nonlinear programs are solved using a modern interior-point method.
The proposed optimization scheme is validated against an integration of dynamic
equations with adaptive time-stepping, as well as a recently proposed
state-of-the-art optimal control method. The comparison shows that the
solutions are feasible for the continuous problem and also practical from an
operational standpoint. The results also indicate that our scheme provides at
least an order of magnitude reduction in computation time relative to the
state-of-the-art and scales to large gas transmission networks with more than
6000 kilometers of total pipeline
Redshift distribution of {\bf Ly-} lines and metal systems
The observed redshift distribution of Ly- lines and metal systems is
examined in order to discriminate and to trace the evolution of structure
elements observed in the galaxy distribution, at small redshifts, and to test
the theoretical description of structure evolution. We show that the expected
evolution of filamentary component of structure describes quite well the
redshift distribution of metal systems and stronger Ly- lines with
14, at 3. The redshift distribution of weaker
Ly- lines can be attributed to the population of poorer structure
elements (Zel'dovich pancakes), which were formed at high redshifts from the
invisible DM and non luminous baryonic matter, and at lower redshifts they
mainly merged and dispersed.Comment: 13 pages, 5 figures, accepted in MNRA
SANS study of hybrid silica aerogels under "in situ" uniaxial compression
We have modified the inorganic silica network of aerogels with polydimethylsiloxane (PDMS), a hydroxyl-terminated polymer, to obtain an organic modified silicate (ORMOSIL). Reactions were assisted by high-power ultrasounds. The resulting gels were dried under supercritical conditions of the solvent to obtain a monolithic sono-aerogel. The mechanical behaviour of these aerogels can be tuned from brittle to rubbery as a function of the organic polymer content. In order to determine the links between the mechanical behaviour and modifications made to the microstructure, SANS (small-angle neutron scattering) experiments were carried out. To measure the intensities under "in situ" uniaxial compression of the aerogel, a specific sample-holder was built. Under uniaxial compression the 2D-diagrams were significantly anisotropic (butterfly pattern), indicating the rearrangement of the polymer. The form factor of these aerogels is described well by two correlation lengths, small microporous silica clusters surrounded by entangled polymer chains of 6 nm average size (blobs), which form a larger secondary level of agglomerates governed by the "frozen-in" elastic constraints.Comisión Interministerial de Ciencia y Tecnología MAT2005-1583European Commission CT-2003-50592
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