9,131 research outputs found
The mass and dynamical state of Abell 2218
Abell 2218 is one of a handful of clusters in which X-ray and lensing
analyses of the cluster mass are in strong disagreement. It is also a system
for which X-ray data and radio measurements of the Sunyaev-Zel'dovich decrement
have been combined in an attempt to constrain the Hubble constant. However, in
the absence of reliable information on the temperature structure of the
intracluster gas, most analyses have been carried out under the assumption of
isothermality. We combine X-ray data from the ROSAT PSPC and the ASCA GIS
instruments, enabling us to fit non-isothermal models, and investigate the
impact that this has on the X-ray derived mass and the predicted
Sunyaev-Zel'dovich effect.
We find that a strongly non-isothermal model for the intracluster gas, which
implies a central cusp in the cluster mass distribution, is consistent with the
available X-ray data and compatible with the lensing results. At r<1 arcmin,
there is strong evidence to suggest that the cluster departs from a simple
relaxed model. We analyse the dynamics of the galaxies and find that the
central galaxy velocity dispersion is too high to allow a physical solution for
the galaxy orbits. The quality of the radio and X-ray data do not at present
allow very restrictive constraints to be placed on H_0. It is apparent that
earlier analyses have under-estimated the uncertainties involved. However,
values greater than 50 km/s/Mpc are preferred when lensing constraints are
taken into account.Comment: 16 pages, 9 postscript figures, accepted for publication in MNRA
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Dynamic Long-Term Modelling of Generation Capacity Investment and Capacity Margins: a GB Market Case Study
Many governments who preside over liberalised energy markets are developing policies aimed at promoting investment in renewable generation whilst maintaining the level of security of supply customers have come to expect. Of particular interest is the mix and amount of generation investment over time in response to policies promoting high penetrations of variable output renewable power such as wind. Modelling the dynamics of merchant generation investment in market environments can inform the debate. Such models need improved methods to calculate expected output, costs and revenue of thermal generation subject to varying load and random independent thermal outages in a power system with high penetrations of wind. This paper presents a dynamic simulation model of the aggregated Great Britain (GB) generation investment market. The short-term energy market is simulated using probabilistic production costing based on the Mix of Normals distribution technique with a residual load calculation (load net of wind output). Price mark-ups due to market power are accounted for. These models are embedded in a dynamic model in which generation companies use a Value at Risk (VaR) criterion for investment decisions. An `energy-only' market setting is used to estimate the economic profitability of investments and forecast the evolution of security of supply. Simulated results for the GB market case study show a pattern of increased relative security of supply risk during the 2020s. In addition, fixed cost recovery for many new investments can only occur during years in which more frequent supply shortages push energy prices higher. A sensitivity analyses on a number of key model assumptions provides insight into factors affecting the simulated timing and level of generation investment. This is achieved by considering the relative change in simulated levels of security of supply risk metric such as de-rated capacity margins and expected energy unserved. The model can be used as a decision support tool in policy design, in particular how to address the increased `energy-only market revenue risk facing thermal generation, particularly peaking units, that rely on a small number of high price periods to recover fixed costs and make adequate returns on investment
Rotor redesign for a highly loaded 1800 ft/sec tip speed fan. 3: Laser Doppler velocimeter report
Laser Doppler velocimeter (LDV) techniques were employed for testing a highly loaded, 550 m/sec (1800 ft/sec) tip speed, test fan stage, the objective to provide detailed mapping of the upstream, intrablade, and downstream flowfields of the rotor. Intrablade LDV measurements of velocity and flow angle were obtained along four streamlines passing through the leading edge at 45%, 69%, 85%, and 95% span measured from hub to tip, at 100% of design speed, peak efficiency; 100% speed, near surge; and 95% speed, peak efficiency. At the design point, most passages appeared to have a strong leading edge shock, which moved forward with increasing strength near surge and at part speeds. The flow behind the shock was of a complex mixed subsonic and supersonic form. The intrablade flowfields were found to be significantly nonperiodic at 100% design speed, peak efficiency
Ab-initio spin dynamics applied to nanoparticles: canted magnetism of a finite Co chain along a Pt(111) surface step edge
In order to search for the magnetic ground state of surface nanostructures we
extended first principles adiabatic spin dynamics to the case of fully
relativistic electron scattering. Our method relies on a constrained density
functional theory whereby the evolution of the orientations of the spin-moments
results from a semi-classical Landau-Lifshitz equation. This approach is
applied to a study of the ground state of a finite Co chain placed along a step
edge of a Pt(111) surface. As far as the ground state spin orientation is
concerned we obtain excellent agreement with the experiment. Furthermore we
observe noncollinearity of the atom-resolved spin and orbital moments. In terms
of magnetic force theorem calculations we also demonstrate how a reduction of
symmetry leads to the existence of canted magnetic states.Comment: 4 pages, ReVTeX + 3 figures (Encapsulated Postscript), submitted to
PR
A conjugate gradient algorithm for the astrometric core solution of Gaia
The ESA space astrometry mission Gaia, planned to be launched in 2013, has
been designed to make angular measurements on a global scale with
micro-arcsecond accuracy. A key component of the data processing for Gaia is
the astrometric core solution, which must implement an efficient and accurate
numerical algorithm to solve the resulting, extremely large least-squares
problem. The Astrometric Global Iterative Solution (AGIS) is a framework that
allows to implement a range of different iterative solution schemes suitable
for a scanning astrometric satellite. In order to find a computationally
efficient and numerically accurate iteration scheme for the astrometric
solution, compatible with the AGIS framework, we study an adaptation of the
classical conjugate gradient (CG) algorithm, and compare it to the so-called
simple iteration (SI) scheme that was previously known to converge for this
problem, although very slowly. The different schemes are implemented within a
software test bed for AGIS known as AGISLab, which allows to define, simulate
and study scaled astrometric core solutions. After successful testing in
AGISLab, the CG scheme has been implemented also in AGIS. The two algorithms CG
and SI eventually converge to identical solutions, to within the numerical
noise (of the order of 0.00001 micro-arcsec). These solutions are independent
of the starting values (initial star catalogue), and we conclude that they are
equivalent to a rigorous least-squares estimation of the astrometric
parameters. The CG scheme converges up to a factor four faster than SI in the
tested cases, and in particular spatially correlated truncation errors are much
more efficiently damped out with the CG scheme.Comment: 24 pages, 16 figures. Accepted for publication in Astronomy &
Astrophysic
Time-Correlated Structure in Spin Fluctuations in Pulsars
We study statistical properties of stochastic variations in pulse arrival
times, timing noise, in radio pulsars using a new analysis method applied in
the time domain. The method proceeds in two steps. First, we subtract
low-frequency wander using a high-pass filter. Second, we calculate the
discrete correlation function of the filtered data. As a complementary method
for measuring correlations, we introduce a statistic that measures the
dispersion of the data with respect to the data translated in time. The
analysis methods presented here are robust and of general usefulness for
studying arrival time variations over timescales approaching the average
sampling interval. We apply these methods to timing data for 32 pulsars. In two
radio pulsars, PSRs B1133+16 and B1933+16, we find that fluctuations in arrival
times are correlated over timescales of 10 - 20 d with the distinct signature
of a relaxation process. Though this relaxation response could be
magnetospheric in origin, we argue that damping between the neutron star crust
and interior liquid is a more likely explanation. Under this interpretation,
our results provide the first evidence independent from pulsar spin glitches of
differential rotation in neutron stars. PSR B0950+08, shows evidence for
quasi-periodic oscillations that could be related to mode switching.Comment: 25 pages, Final journal version (MNRAS
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