991 research outputs found
Three dimensional scattering center imaging techniques
Two methods to image scattering centers in 3-D are presented. The first method uses 2-D images generated from Inverse Synthetic Aperture Radar (ISAR) measurements taken by two vertically offset antennas. This technique is shown to provide accurate 3-D imaging capability which can be added to an existing ISAR measurement system, requiring only the addition of a second antenna. The second technique uses target impulse responses generated from wideband radar measurements from three slightly different offset antennas. This technique is shown to identify the dominant scattering centers on a target in nearly real time. The number of measurements required to image a target using this technique is very small relative to traditional imaging techniques
Integral Glass Encapsulation for Solar Arrays
Work reported was performed during the period from August 1977 to December 1978. The program objective was to continue the development of electrostatic bonding (ESB) as an encapsulation technique for terrestrial cells. Economic analyses shows that this process can be a cost-effective method of producing reliable, long lifetime solar modules. When considered in sufficient volume, both material and equipment costs are competitive with conventional encapsulation systems. In addition, the possibility of integrating cell fabrication into the encapsulation process, as in the case of the preformed cell contacts discussed in this report, offers the potential of significant overall systems cost reduction
Vortex-induced vibration of catenary riser: reduced-order modeling and lock-in analysis using wake oscillator
A new reduced-order model capable of analyzing the vortex-induced vibration of catenary riser in the ocean current has been developed. This semi analytical-numerical approach is versatile and allows for a significant reduction in computational effort for the analysis of fluid-riser interactions. The incoming current flow is assumed to be steady, uniform, unidirectional and perpendicular to the riser plane of initial equilibrium curvatures
Breaking Cosmological Degeneracies in Galaxy Cluster Surveys with a Physical Model of Cluster Structure
Forthcoming large galaxy cluster surveys will yield tight constraints on
cosmological models. It has been shown that in an idealized survey, containing
> 10,000 clusters, statistical errors on dark energy and other cosmological
parameters will be at the percent level. It has also been shown that through
"self-calibration", parameters describing the mass-observable relation and
cosmology can be simultaneously determined, though at a loss in accuracy by
about an order of magnitude. Here we examine the utility of an alternative
approach of self-calibration, in which a parametrized ab-initio physical model
is used to compute cluster structure and the resulting mass-observable
relations. As an example, we use a modified-entropy ("pre-heating") model of
the intracluster medium, with the history and magnitude of entropy injection as
unknown input parameters. Using a Fisher matrix approach, we evaluate the
expected simultaneous statistical errors on cosmological and cluster model
parameters. We study two types of surveys, in which a comparable number of
clusters are identified either through their X-ray emission or through their
integrated Sunyaev-Zel'dovich (SZ) effect. We find that compared to a
phenomenological parametrization of the mass-observable relation, using our
physical model yields significantly tighter constraints in both surveys, and
offers substantially improved synergy when the two surveys are combined. These
results suggest that parametrized physical models of cluster structure will be
useful when extracting cosmological constraints from SZ and X-ray cluster
surveys. (abridged)Comment: 22 pages, 8 figures, accepted to Ap
What are the key issues regarding the role of geothermal energy in meeting energy needs in the global south?
Cosmological Simulations of the Preheating Scenario for Galaxy Cluster Formation: Comparison to Analytic Models and Observations
We perform a set of non--radiative cosmological simulations of a preheated
intracluster medium in which the entropy of the gas was uniformly boosted at
high redshift. The results of these simulations are used first to test the
current analytic techniques of preheating via entropy input in the smooth
accretion limit. When the unmodified profile is taken directly from
simulations, we find that this model is in excellent agreement with the results
of our simulations. This suggests that preheated efficiently smoothes the
accreted gas, and therefore a shift in the unmodified profile is a good
approximation even with a realistic accretion history. When we examine the
simulation results in detail, we do not find strong evidence for entropy
amplification, at least for the high-redshift preheating model adopted here. In
the second section of the paper, we compare the results of the preheating
simulations to recent observations. We show -- in agreement with previous work
-- that for a reasonable amount of preheating, a satisfactory match can be
found to the mass-temperature and luminosity-temperature relations. However --
as noted by previous authors -- we find that the entropy profiles of the
simulated groups are much too flat compared to observations. In particular,
while rich clusters converge on the adiabatic self--similar scaling at large
radius, no single value of the entropy input during preheating can
simultaneously reproduce both the core and outer entropy levels. As a result,
we confirm that the simple preheating scenario for galaxy cluster formation, in
which entropy is injected universally at high redshift, is inconsistent with
observations.Comment: 11 pages, 13 figures, accepted for publication in Ap
What are the key issues regarding the role of geothermal energy in meeting energy needs in the global south?
Globally, the potential of geothermal far exceeds that of all other renewable sources
together, although investment in the other sources to date has far exceeded investment in
geothermal. World Energy Assessment estimates in 2000 for the global potential of all
renewables (EJ/yr) were Geothermal 5000, Solar 1575, Wind 640, Biomass 276, Hydro 50,
giving a total of 7541 (UNDP, 2000). When installed, geothermal plants have a far higher
capacity factor than other sources (solar depends on the level of direct insolation, wind
power on wind, etc.); estimates (REN21, 2009) give wind-power 21%, solar PV 14% but
geothermal is at least as high as 75% and often more than 95%, given that once a plant is
established it operates continuously except for routine down-time for maintenance and rare
break-downs
Evolution of the Cluster Mass and Correlation Functions in LCDM Cosmology
The evolution of the cluster mass function and the cluster correlation
function from z = 0 to z = 3 are determined using 10^6 clusters obtained from
high-resolution simulations of the current best-fit LCDM cosmology (\Omega_m =
0.27, \sigma_8 = 0.84, h = 0.7). The results provide predictions for
comparisons with future observations of high redshift clusters. A comparison of
the predicted mass function of low redshift clusters with observations from
early Sloan Digital Sky Survey data, and the predicted abundance of massive
distant clusters with observational results, favor a slightly larger amplitude
of mass fluctuations (\sigma_8 = 0.9) and lower density parameter (\Omega_m =
0.2); these values are consistent within 1-\sigma with the current
observational and model uncertainties. The cluster correlation function
strength increases with redshift for a given mass limit; the clusters were more
strongly correlated in the past, due to their increasing bias with redshift -
the bias reaches b = 100 at z = 2 for M > 5 x 10^13 h^-1 M_sun. The
richness-dependent cluster correlation function, represented by the correlation
scale versus cluster mean separation relation, R0-d, is generally consistent
with observations. This relation can be approximated as R_0 = 1.7 d^0.6 h^-1
Mpc for d = 20 - 60 h^-1 Mpc. The R0-d relation exhibits surprisingly little
evolution with redshift for z < 2; this can provide a new test of the current
LCDM model when compared with future observations of high redshift clusters.Comment: 20 pages, 9 figures, accepted for publication in Ap
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