4,817 research outputs found
Water footprint of cotton, wheat and rice production in Central Asia
The hydrology of the Aral Sea Basin during the past few decades has been largely determined by the decision to\ud
develop irrigated agriculture on a large scale to produce cotton for export in the 1960s. The irrigated area has\ud
grown to 8 million hectares, using practically the entire available flow of the two main rivers, the Amu Darya\ud
and Syr Darya. Almost two decades after the disintegration of the Soviet Union, the five states of the Aral Sea\ud
Basin face the challenge of restoring a sustainable equilibrium while offering development opportunities for an\ud
increasing population. Sustainable water management is thus an imperative that will require coordinated\ud
political action of all the states involved.\ud
The Soviet Union established its cotton-producing areas in Uzbekistan, Turkmenistan, Tajikistan, and\ud
Kyrgyzstan. Today, while cotton remains relatively important, cereal production to reduce imports has become a\ud
priority in all four nations. The cotton crop area has decreased over the past ten years, while that of winter wheat\ud
â the main grain crop â has doubled. At 39 per cent of the total (blue and green) water consumption in\ud
agriculture, wheat is the largest water-consuming crop in the five basin states, followed by cotton at 33 per cent.\ud
The present study analyses the water footprint of Central Asian cotton (Gossypium hirsutum L.), wheat\ud
(Triticum aestivum L.) and rice (Oryza sativa L.) production, differentiating between the green and blue\ud
components, in order to know how the scarce water resources in the region are apparently allocated
Shear Thickening of Dense Suspensions: The Role of Friction
Shear thickening of particle suspensions is characterized by a transition
between lubricated and frictional contacts between the particles. Using 3D
numerical simulations, we study how the inter-particle friction coefficient
influences the effective macroscopic friction coefficient and hence the
microstructure and rheology of dense shear thickening suspensions. We propose
expressions for effective friction coefficient in terms of distance to jamming
for varying shear stresses and particle friction coefficient values. We find
effective friction coefficient to be rather insensitive to interparticle
friction, which is perhaps surprising but agrees with recent theory and
experiments
Light Turning Mirrors in SiON Optical Waveguides for Hybrid Integration with CMOS Photo-detectors
A new method is proposed for hybrid integration of SiON optical waveguides and standard CMOS photo-detectors based on anisotropic etching of 45° facets in a Si substrate. After removal of anisotropically etched Si structures in cladding SiO2, the fabricated total-internal-reflection mirrors can direct the output of the waveguides to photo-detectors placed on top of the chip. The metal-free fabrication process, designed to create these mirrors, is convenient for batch production. Fourier optics based simulations predict that the reflection efficiency of the mirrors is 68.5 %. The far field pattern obtained from the fabricated device is similar to the simulated one
Direct measurement of the on-chip insertion loss of high finesse microring resonators in Si3N4-SiO2 technology.
Microring resonators show the possibility for designing Very Large Scale Integrated (VLSI) photonic circuits by cascading them. In order to realize the devices, the on-chip insertion loss becomes an important parameter. The direct measurement of the on-chip insertion loss of a high finesse microring resonator will be presented. Its value (0.1 ± 0.1) dB is low, in agreement with calculations
45° light turning mirrors for hybrid integration of silica optical waveguides and photo-detectors
For hybrid integration of an optical chip with an electronic chip with photo diodes and electronic processing, light must be coupled from the optical chip to the electronic chip. This paper presents a method to fabricate metal-free 45° quasi-total internal reflecting mirrors in optical chips that enable 90° out-of-plane light coupling between flip-chip bonded chips. This method is fully compatible with fabrication of conventional optical chips. The mirrors are created using anisotropic etching of 45° facets in a Si substrate followed by fabrication of optical structures. After removal of the mirror-defining Si structures by isotropic etching, the obtained air-optical structure interface directs the output of the waveguides to out-of-plane photo detectors that are mounted flip-chip on the optical chip. Simulations show a reflection efficiency of 72.3 %, while experimentally 47% was measured on a not fully optimized first batch
The mass-L_x relation for moderate luminosity X-ray clusters
We present measurements of the masses of a sample of 25 moderate X-ray
luminosity clusters of galaxies from the 160 square degree ROSAT survey. The
masses were obtained from a weak lensing analysis of deep F814W images obtained
using the Advanced Camera for Surveys (ACS). We present an accurate empirical
correction for the effect of charge transfer (in)efficiency on the shapes of
faint galaxies. A significant lensing signal is detected around most of the
clusters. The lensing mass correlates tightly with the cluster richness. We
measured the intrinsic scatter in the scaling relation between M_2500 and L_X
and find the best fit power law slope and normalisation to be alpha=0.68+-0.07
and M_X=(1.2+-0.12)10^14M_sun (for L_X=2x10^44 erg/s). These results agree well
with a number of recent studies, but the normalisation is lower compared to the
study of Rykoff et al. (2008b). One explanation for this difference may be the
fact that (sub)structures projected along the line-of-sight boost both the
galaxy counts and the lensing mass. Such superpositions lead to an increased
mass at a given L_X when clusters are binned by richness.Comment: accepted for publication in the Astrophysical Journal; 15 pages, 11
figure
PKS 1004+13: A High-Inclination, Highly-Absorbed Radio-Loud QSO -- The First Radio-Loud BAL QSO at Low Redshift?
The existence of BAL outflows in only radio-quiet QSOs was thought to be an
important clue to mass ejection and the radio-loud - radio-quiet dichotomy.
Recently a few radio-loud BAL QSOs have been discovered at high redshift. We
present evidence that PKS 1004+13 is a radio-loud BAL QSO. It would be the
first known at low-redshift (z = 0.24), and one of the most radio luminous. For
PKS 1004+13, there appear to be broad absorption troughs of O VI, N V, Si IV,
and C IV, indicating high-ionization outflows up to about 10,000 km/s. There
are also two strong, broad (~500 km/s), high-ionization, associated absorption
systems that show partial covering of the continuum source. The strong UV
absorption we have detected suggests that the extreme soft-X-ray weakness of
PKS 1004+13 is primarily the result of absorption. The large radio-lobe
dominance indicates BAL and associated gas at high inclinations to the central
engine axis, perhaps in a line-of-sight that passes through an accretion disk
wind.Comment: To appear in Ap.J. Letters, 1999 (June or July); 4 pages, 5 figure
Multiple Weak Deflections in Galaxy-Galaxy Lensing
The frequency and effects of multiple weak deflections in galaxy-galaxy
lensing are investigated via Monte Carlo simulations. The lenses in the
simulations are galaxies with known redshifts and known rest-frame blue
luminosities. The frequency of multiple deflections above a given threshold
shear value is quantified for discrete source redshifts, as well as for a set
of sources that are broadly distributed in redshift space. In general, the
closest lens in projection on the sky is not the only lens for a given source.
In addition, ~50% of the time the closest lens is not the most important lens
for a given source. Compared to a naive single-deflection calculation in which
only the lensing due to the closest weak lens is considered, a full
multiple-deflection calculation yields a higher net shear for individual
sources, as well as a higher mean tangential shear around the lens centers. The
full multiple-deflection calculation also shows that galaxy-galaxy lensing may
contribute a substantial amount to cosmic shear on small angular scales. The
degree to which galaxy-galaxy lensing contributes to the small-scale cosmic
shear is, however, quite sensitive to the mass adopted for the halos of L_B*
galaxies. Changing the halo mass by a factor of ~2.5 changes the contribution
of galaxy-galaxy lensing to the cosmic shear by a factor of ~3 on scales of
order 1 arcmin. The contribution of galaxy-galaxy lensing to cosmic shear
decreases rapidly with angular scale and extrapolates to zero at scales of
order 5 arcmin. This last result is roughly independent of the halo mass and
suggests that for scales greater than about 5 arcmin, cosmic shear is
insensitive to the details of the gravitational potentials of large galaxies.Comment: accepted for publication in ApJ; 35 pages, 15 figures; full text with
high-resolution Figure 1 available at
http://firedrake.bu.edu/preprints/preprints.htm
The partial cavity on a 2D foil revisited
The partial cavity on a 2D NACA0015 foil at 6 degrees angle-of-attack is studied numerically. Assuming the fluid to be a continuum of variable density, we solve the RANS equations, complemented with turbulence and cavitation models. Some important details of the mathematical model are pointed out first. We study then carefully what occurs in the numerical simulations in and near the cavity from the inception phase to the stage well before serious unsteadiness (cavity shedding) starts. By making the computations on grids of different densities we get an impression of numerical uncertainties. This is important for the interpretation and the subsequent comparison with what experimental investigations have learned us about the physics of these almost steady partial cavities on foils. The results show that close to inception a cavity exists while the boundary layer is nonseparating. The liquid-vapour interface turns out not to be a material surface, neither at the front end nor at the tail of the cavity. It also appears that the widely accepted re-entrant jet model as conceived from free-streamline theory is not a good description of the flow at the tail. The confrontation of the numerical results with information from experiments indicates that there is agreement and corroboration in several respects, but also intriguing discrepancies are found which require further elucidation.http://deepblue.lib.umich.edu/bitstream/2027.42/84243/1/CAV2009-final43.pd
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