6,785 research outputs found
Spectral distribution of solar radiation
Available quantitative data on solar total and spectral irradiance are examined in the context of utilization of solar irradiance for terrestrial applications of solar energy. The extraterrestrial solar total and spectral irradiance values are also reviewed. Computed values of solar spectral irradiance at ground level for different air mass values and various levels of atmospheric pollution or turbidity are presented. Wavelengths are given for computation of solar, absorptance, transmittance and reflectance by the 100 selected-ordinate method and by the 50 selected-ordinate method for air mass 1.5 and 2 solar spectral irradiance for the four levels of atmospheric pollution
Evaluation of an offshore wind farm computational fluid dynamics model against operational site data
Modelling wind turbine wake effects at a range of wind speeds and directions with actuator disk (AD) models can provide insight but also be challenging. With any model it is important to quantify the level of error, but this can also present a challenge when comparing a steady-state model to measurement data with scatter. This paper models wind flow in a wind farm at a range of wind speeds and directions using an AD implementation. The results from these models are compared to data collected from the actual farm being modelled. An extensive comparison is conducted, constituted from 35 cases where two turbulence models, the standard k-Īµ and k-Ļ SST are evaluated. The steps taken in building the models as well as processes for comparing the AD computational fluid dynamics (CFD) results to real-world data using the regression models of ensemble bagging and Gaussian process are outlined. Turbine performance data and boundary conditions are determined using the site data. Modifications to an existing opensource AD code are shown so that the predetermined turbine performance can be implemented into the CFD model. Steady state solutions are obtained with the OpenFOAM CFD solver. Results are compared in terms of velocity deficit at the measurement locations. Using the standard k-Īµ model, a mean absolute error for all cases together of roughly 8% can be achieved, but this error changes for different directions and methods of evaluating it
The Stock of External Sovereign Debt: Can We Take the Data At āFace Valueā?
The stock of sovereign debt is typically measured at face value. This is a misleading indicator when debts are issued with different contractual forms. In this paper we construct a new measure of the stock of external sovereign debt for 100 developing countries from 1979 to 2006 that is invariant to contractual form, and illustrate five problems with debt stocks measured at face value. First, we show that correcting for differences in the contractual form of debt paints a very different quantitative, and in some cases also qualitative, picture of the stock of developing country external sovereign debt. Second, rankings of indebtedness across countries, which were historically used to define eligibility for debt forgiveness, are sometimes inverted once we correct for differences in contractual form. Third, the empirical performance of the benchmark quantitative model of sovereign debt deteriorates by between 40 to 70 percent once model-consistent measures of debt are used. Fourth, we show how the spread of aggregation clauses in debt contracts which award creditors voting power in proportion to the contractual face value may introduce inefficiencies into the process of restructuring sovereign debts. Fifth, we show how the use of contractual face values gives issuing countries the ability to manipulate their debt stock data, and illustrate the use of these techniques in practice.
Metal-Insulator transition in the Generalized Hubbard model
We present the exact ground-state wave function and energy of the generalized
Hubbard model, subjected to the condition that the number of double occupied
sites is conserved, for a wide, physically relevant range of parameters. For
one hole and one double occupied site the existence of the ferromagnetic
ground-state is proved which allow one to determine the critical value of the
on-site repulsion corresponding to the point of metal-insulator transition. For
the one dimensional model the exact solution for special values of the
parameters is obtained.Comment: 20 pages, LaTex. Mod.Phys.Lett.B 7 (1993) 1397; Journal of Physics:
Condensed Matter (to appear
The influence of 6 weeks of maximal eccentric plantarflexor training on muscle-tendon mechanics
Resistance training can influence muscle-tendon properties including strength, flexibility, stretch tolerance and muscle-tendon stiffness; however the specific influence of eccentric-only training is unknown. Therefore, the aims of the present study were to examine the effects of a 6-week maximal eccentric resistance training programme on isometric plantarflexor moment (MVC), dorsiflexion range of motion (ROM), stretch tolerance (peak passive moment), muscle and tendon stiffness and running economy. Thirteen recreationally active men (age = 20.0 Ā± 0.9 yr, mass = 75.9 Ā± 8.5 kg, height = 1.8 Ā± 0.1 m) volunteered for the study after giving written informed consent; ethical approval was granted from the University of Northampton. Training was performed twice weekly for six weeks and consisted of 5 sets of 12 repetitions of 3-s maximal eccentric contractions at 10Ā°ā¢s-1 from 20Ā° plantarflexion to 10Ā° dorsiflexion. Maximal isometric plantarflexor moment, dorsiflexion ROM, stretch tolerance, and muscle, tendon and muscle-tendon unit (MTU) stiffness were measured using isokinetic dynamometry, real-time ultrasound and 3D motion analyses before and after the training. Running economy (VO2) was determined at a running speed equating to 70%VO2max using online gas analysis. Repeated measures t-tests were used to determine significant differences between pre- and post-training data, significance accepted at p0.05). Analysis of ultrasound data revealed a significant decrease in muscle stiffness (20.6%; p0.05). While the training-induced increase in plantarflexor strength was expected, the substantial increases in ROM, stretch tolerance and tendon stiffness, and the reduction in passive muscle stiffness, were important and novel findings. Interestingly, when measured during passive stretch, MTU stiffness remained unchanged while tendon stiffness increased and muscle stiffness decreased. These disparate findings have clear implications for testing methodologies, and indicate that imaging techniques must be utilised in order to examine the effects of interventions on specific tissues. As the training clearly enhanced the capacity of the muscle to tolerate both tissue loading and deformation, which are commonly associated with muscle strain injury, these data have clear implications for both muscular performance and injury risk
Towards a Continuous Record of the Sky
It is currently feasible to start a continuous digital record of the entire
sky sensitive to any visual magnitude brighter than 15 each night. Such a
record could be created with a modest array of small telescopes, which
collectively generate no more than a few Gigabytes of data daily.
Alternatively, a few small telescopes could continually re-point to scan and
reco rd the entire sky down to any visual magnitude brighter than 15 with a
recurrence epoch of at most a few weeks, again always generating less than one
Gigabyte of data each night. These estimates derive from CCD ability and
budgets typical of university research projects. As a prototype, we have
developed and are utilizing an inexpensive single-telescope system that obtains
optical data from about 1500 square degrees. We discuss the general case of
creating and storing data from a both an epochal survey, where a small number
of telescopes continually scan the sky, and a continuous survey, composed of a
constellation of telescopes dedicated each continually inspect a designated
section of the sky. We compute specific limitations of canonical surveys in
visible light, and estimate that all-sky continuous visual light surveys could
be sensitive to magnitude 20 in a single night by about 2010. Possible
scientific returns of continuous and epochal sky surveys include continued
monitoring of most known variable stars, establishing case histories for
variables of future interest, uncovering new forms of stellar variability,
discovering the brightest cases of microlensing, discovering new novae and
supernovae, discovering new counterparts to gamma-ray bursts, monitoring known
Solar System objects, discovering new Solar System objects, and discovering
objects that might strike the Earth.Comment: 38 pages, 9 postscript figures, 2 gif images. Revised and new section
added. Accepted to PASP. Source code submitted to ASCL.ne
Uniform infinite planar triangulation and related time-reversed critical branching process
We establish a connection between the uniform infinite planar triangulation
and some critical time-reversed branching process. This allows to find a
scaling limit for the principal boundary component of a ball of radius R for
large R (i.e. for a boundary component separating the ball from infinity). We
show also that outside of R-ball a contour exists that has length linear in R.Comment: 27 pages, 5 figures, LaTe
Natural Exact Covering Systems and the Reversion of the Mƶbius Series
We prove that the number of natural exact covering systems of cardinality k is equal to the coefficient of xkin the reversion of the power series ākā„1Ī¼(k) xk, whereĀ Ī¼(k) is the usual number-theoretic Mƶbius function. Using this result, we deduce an asymptotic expression for the number of such systems
Distinguishing cancerous from non-cancerous cells through analysis of electrical noise
Since 1984, electric cell-substrate impedance sensing (ECIS) has been used to
monitor cell behavior in tissue culture and has proven sensitive to cell
morphological changes and cell motility. We have taken ECIS measurements on
several cultures of non-cancerous (HOSE) and cancerous (SKOV) human ovarian
surface epithelial cells. By analyzing the noise in real and imaginary
electrical impedance, we demonstrate that it is possible to distinguish the two
cell types purely from signatures of their electrical noise. Our measures
include power-spectral exponents, Hurst and detrended fluctuation analysis, and
estimates of correlation time; principal-component analysis combines all the
measures. The noise from both cancerous and non-cancerous cultures shows
correlations on many time scales, but these correlations are stronger for the
non-cancerous cells.Comment: 8 pages, 4 figures; submitted to PR
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