8,093 research outputs found
On the generation and the nonlinear dynamics of X-waves of the Schroedinger equation
The generation of finite energy packets of X-waves is analysed in normally
dispersive cubic media by using an X-wave expansion. The 3D nonlinear
Schroedinger model is reduced to a 1D equation with anomalous dispersion. Pulse
splitting and beam replenishment as observed in experiments with water and Kerr
media are explained in terms of a higher order breathing soliton. The results
presented also hold in periodic media and Bose-condensed gases.Comment: 18 pages, 6 figures, corrected version to be published in Physical
Review
Steam storage systems for flexible biomass CHP plants - Evaluation and initial model based calculation
The file attached to this record is the author's final peer reviewed version. The Publisher's final version can be found by following the DOI link.Within the present study a novel concept for the demand-oriented power generation of a solid-biomass fueled combined heat and power (CHP) plant is investigated. The integration of a steam storage system into the plants process enables a decoupling of the steam (boiler) and the power generation (steam turbine). By buffering the steam, the power output of the turbine can be adjusted without changing the rated thermal capacity of the plant. Various available storage systems are selected and comparatively evaluated applying the adapted analytic hierarchy process (AHP). The technology assessment revealed that the combination of a steam accumulator and solid concrete storage represents the best suiting option. An initial model based simulation study is performed to identify the fundamental behaviour of this system, integrated in a biomass CHP plant. The operation principle is has proved their technical feasibility and seems to be applicable at a commercial scale. According to the modelling results flexible short term power generation in a time range of up to fifteen minutes is applicable. A load-range of almost the plants rated capacity can be achieved
Continuum elasticity theory of edge excitations in a two-dimensional electron liquid with finite range interactions
We make use of continuum elasticity theory to investigate the collective
modes that propagate along the edge of a two-dimensional electron liquid or
crystal in a magnetic field. An exact solution of the equations of motion is
obtained with the following simplifying assumptions: (i) The system is {\it
macroscopically} homogeneous and isotropic in the half-plane delimited by the
edge (ii) The electron-electron interaction is of finite range due to screening
by external electrodes (iii) The system is nearly incompressible. At
sufficiently small wave vector we find a universal dispersion curve independent of the shear modulus. At larger wave vectors the dispersion
can change its form in a manner dependent on the comparison of various length
scales. We obtain analytical formulas for the dispersion and damping of the
modes in various physical regimes.Comment: 3 figure
Quantal charge redistributions accompanying the structural transitions of sodium channels.
Asymmetric displacement currents, Ig, associated with the gating of nerve sodium channels have been recorded in cell-attached macropatches of Xenopus laevis oocytes injected with exogenous mRNA coding for rat-brain-II sodium channels. The Ig properties were found to be similar to those of gating currents previously observed in native nerve preparations. Ig fluctuations were measured in order to ascertain the discreteness of the conformational changes which precede the channel opening. The autocorrelation of the fluctuations is consistent with a shot-like character of the elementary Ig contributions. The variance of the fluctuations indicates that most of the gating-charge movement that accompanies the activation of a single sodium channel occurs in 2 to 3 brief packets, each carrying an equivalent of about 2.3 electron charges
Homogenization in magnetic-shape-memory polymer composites
Magnetic-shape-memory materials (e.g. specific NiMnGa alloys) react with a
large change of shape to the presence of an external magnetic field. As an
alternative for the difficult to manifacture single crystal of these alloys we
study composite materials in which small magnetic-shape-memory particles are
embedded in a polymer matrix. The macroscopic properties of the composite
depend strongly on the geometry of the microstructure and on the
characteristics of the particles and the polymer.
We present a variational model based on micromagnetism and elasticity, and
derive via homogenization an effective macroscopic model under the assumption
that the microstructure is periodic. We then study numerically the resulting
cell problem, and discuss the effect of the microstructure on the macroscopic
material behavior. Our results may be used to optimize the shape of the
particles and the microstructure.Comment: 17 pages, 4 figure
Methodological Issues in Multistage Genome-Wide Association Studies
Because of the high cost of commercial genotyping chip technologies, many
investigations have used a two-stage design for genome-wide association
studies, using part of the sample for an initial discovery of ``promising''
SNPs at a less stringent significance level and the remainder in a joint
analysis of just these SNPs using custom genotyping. Typical cost savings of
about 50% are possible with this design to obtain comparable levels of overall
type I error and power by using about half the sample for stage I and carrying
about 0.1% of SNPs forward to the second stage, the optimal design depending
primarily upon the ratio of costs per genotype for stages I and II. However,
with the rapidly declining costs of the commercial panels, the generally low
observed ORs of current studies, and many studies aiming to test multiple
hypotheses and multiple endpoints, many investigators are abandoning the
two-stage design in favor of simply genotyping all available subjects using a
standard high-density panel. Concern is sometimes raised about the absence of a
``replication'' panel in this approach, as required by some high-profile
journals, but it must be appreciated that the two-stage design is not a
discovery/replication design but simply a more efficient design for discovery
using a joint analysis of the data from both stages. Once a subset of
highly-significant associations has been discovered, a truly independent
``exact replication'' study is needed in a similar population of the same
promising SNPs using similar methods.Comment: Published in at http://dx.doi.org/10.1214/09-STS288 the Statistical
Science (http://www.imstat.org/sts/) by the Institute of Mathematical
Statistics (http://www.imstat.org
Experimental measurement of photothermal effect in Fabry-Perot cavities
We report the experimental observation of the photothermal effect. The
measurements are performed by modulating the laser power absorbed by the
mirrors of two high-finesse Fabry-Perot cavities. The results are very well
described by a recently proposed theoretical model [M. Cerdonio, L. Conti, A.
Heidmann and M. Pinard, Phys. Rev. D 63 (2001) 082003], confirming the
correctness of such calculations. Our observations and quantitative
characterization of the photothermal effect demonstrate its critical importance
for high sensitivity interferometric displacement measurements, as those
necessary for gravitational wave detection.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
Properties of Hot Stars in the Wolf-Rayet galaxy NGC5253 from ISO Spectroscopy
ISO-SWS spectroscopy of the WR galaxy NGC5253 is presented, and analysed to
provide estimates of its hot young star population. Our approach differs from
previous investigations in that we are able to distinguish between the regions
in which different infrared fine-structure lines form, using complementary
ground-based observations. The high excitation nebular [SIV] emission is formed
in a very compact region, which we attribute to the central super-star-nucleus,
and lower excitation [NeII] nebular emission originates in the galactic core.
We use photo-ionization modelling coupled with the latest theoretical O-star
flux distributions to derive effective stellar temperatures and ionization
parameters of Teff>38kK, logQ=8.25 for the compact nucleus, with Teff=35kK,
logQ<8 for the larger core. Results are supported by more sophisticated
calculations using evolutionary synthesis models. We assess the contribution
that Wolf-Rayet stars may make to highly ionized nebular lines (e.g. [OIV]).
From our Br(alpha) flux, the 2" nucleus contains the equivalent of
approximately 1000 O7V star equivalents and the starburst there is 2-3Myr old;
the 20" core contains about 2500 O7V star equivalents, with a representative
age of 5Myr. The Lyman ionizing flux of the nucleus is equivalent to the 30
Doradus region. These quantities are in good agreement with the observed mid-IR
dust luminosity of 7.8x10^8 L(sun) Since this structure of hot clusters
embedded in cooler emission may be common in dwarf starbursts, observing a
galaxy solely with a large aperture may result in confusion. Neglecting the
spatial distribution of nebular emission in NGC5253, implies `global' stellar
temperatures (or ages) of 36kK (4.8Myr) and 39kK (2.9 or 4.4Myr) from the
observed [NeIII/II] and [SIV/III] line ratios, assuming logQ=8.Comment: 16 pages, 7 figures, uses mn.sty, to appear in MNRA
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