408 research outputs found
Theory of Concentration Dependence in Drag Reduction by Polymers and of the MDR asymptote
A simple model of the effect of polymer concentration on the amount of drag
reduction in turbulence is presented, simulated and analyzed. The qualitative
phase diagram of drag coefficient vs. Reynolds number (Re) is recaptured in
this model, including the theoretically elusive onset of drag reduction and the
Maximum Drag Reduction (MDR) asymptote. The Re-dependent drag and the MDR are
analytically explained, and the dependence of the amount of drag on material
parameters is rationalized
Drag Reduction by Polymers in Wall Bounded Turbulence
We address the mechanism of drag reduction by polymers in turbulent wall
bounded flows. On the basis of the equations of fluid mechanics we present a
quantitative derivation of the "maximum drag reduction (MDR) asymptote" which
is the maximum drag reduction attained by polymers. Based on Newtonian
information only we prove the existence of drag reduction, and with one
experimental parameter we reach a quantitative agreement with the experimental
measurements.Comment: 4 pages, 1 fig., included, PRL, submitte
Comparison of Theory and Direct Numerical Simulations of Drag Reduction by Rodlike Polymers in Turbulent Channel Flows
Numerical simulations of turbulent channel flows, with or without additives,
are limited in the extent of the Reynolds number \Re and Deborah number \De.
The comparison of such simulations to theories of drag reduction, which are
usually derived for asymptotically high \Re and \De, calls for some care. In
this paper we present a study of drag reduction by rodlike polymers in a
turbulent channel flow using direct numerical simulation and illustrate how
these numerical results should be related to the recently developed theory
Additive Equivalence in Turbulent Drag Reduction by Flexible and Rodlike Polymers
We address the "Additive Equivalence" discovered by Virk and coworkers: drag
reduction affected by flexible and rigid rodlike polymers added to turbulent
wall-bounded flows is limited from above by a very similar Maximum Drag
Reduction (MDR) asymptote. Considering the equations of motion of rodlike
polymers in wall-bounded turbulent ensembles, we show that although the
microscopic mechanism of attaining the MDR is very different, the macroscopic
theory is isomorphic, rationalizing the interesting experimental observations.Comment: 8 pages, PRE, submitte
Toward a structural understanding of turbulent drag reduction: nonlinear coherent states in viscoelastic shear flows
Nontrivial steady flows have recently been found that capture the main
structures of the turbulent buffer layer. We study the effects of polymer
addition on these "exact coherent states" (ECS) in plane Couette flow. Despite
the simplicity of the ECS flows, these effects closely mirror those observed
experimentally: Structures shift to larger length scales, wall-normal
fluctuations are suppressed while streamwise ones are enhanced, and drag is
reduced. The mechanism underlying these effects is elucidated. These results
suggest that the ECS are closely related to buffer layer turbulence.Comment: 5 pages, 3 figures, published version, Phys. Rev. Lett. 89, 208301
(2002
Hydrogen effusion from tritiated amorphous silicon
Results for the effusion and outgassing of tritium from tritiated hydrogenated amorphous silicon (a-Si:H:T) films are presented. The samples were grown by dc-saddle field glow discharge at various substrate temperatures between 150 and 300 °C. The tracer property of radioactive tritium is used to detect tritium release. Tritium effusion measurements are performed in a nonvacuum ion chamber and are found to yield similar results as reported for standard high vacuum technique. The results suggest for decreasing substrate temperature the growth of material with an increasing concentration of voids. These data are corroborated by analysis of infrared absorption data in terms of microstructure parameters. For material of low substrate temperature (and high void concentration) tritium outgassing in air at room temperature was studied, and it was found that after 600 h about 0.2% of the total hydrogen (hydrogen+tritium) content is released. Two rate limiting processes are identified. The first process, fast tritium outgassing with a time constant of 15 h, seems to be related to surface desorption of tritiated water (HTO) with a free energy of desorption of 1.04 eV. The second process, slow tritium outgassing with a time constant of 200-300 h, appears to be limited by oxygen diffusivity in a growing oxide layer. This material of lowest H stability would lose half of the hydrogen after 60 years. © 2008 American Institute of Physics
Transition From Targeted Breeding to Mainstreaming of Biofortification Traits in Crop Improvement Programs
Biofortification breeding for three important micronutrients for human health, namely,
iron (Fe), zinc (Zn), and provitamin A (PVA), has gained momentum in recent years.
HarvestPlus, along with its global consortium partners, enhances Fe, Zn, and PVA
in staple crops. The strategic and applied research by HarvestPlus is driven by
product-based impact pathway that integrates crop breeding, nutrition research, impact
assessment, advocacy, and communication to implement country-specific crop delivery
plans. Targeted breeding has resulted in 393 biofortified crop varieties by the end of
2020, which have been released or are in testing in 63 countries, potentially benefitting
more than 48 million people. Nevertheless, to reach more than a billion people by 2030,
future breeding lines that are being distributed by Consultative Group on International
Agricultural Research (CGIAR) centers and submitted by National Agricultural Research
System (NARS) to varietal release committees should be biofortified. It is envisaged that
the mainstreaming of biofortification traits will be driven by high-throughput micronutrient
phenotyping, genomic selection coupled with speed breeding for accelerating genetic
gains. It is noteworthy that targeted breeding gradually leads to mainstreaming, as the
latter capitalizes on the progress made in the former. Efficacy studies have revealed the
nutritional significance of Fe, Zn, and PVA biofortified varieties over non-biofortified ones.
Mainstreaming will ensure the integration of biofortified traits into competitive varieties
and hybrids developed by private and public sectors. The mainstreaming strategy has
just been initiated in select CGIAR centers, namely, International Maize and Wheat
Improvement Center (CIMMYT), International Rice Research Institute (IRRI), International
Crops Research Institute for the Semi-Arid Tropics (ICRISAT), International Institute of
Tropical Agriculture (IITA), and International Center for Tropical Agriculture (CIAT). This
review will present the key successes of targeted breeding and its relevance to the
mainstreaming approaches to achieve scaling of biofortification to billions sustainably
Statistical Properties of Turbulence: An Overview
We present an introductory overview of several challenging problems in the
statistical characterisation of turbulence. We provide examples from fluid
turbulence in three and two dimensions, from the turbulent advection of passive
scalars, turbulence in the one-dimensional Burgers equation, and fluid
turbulence in the presence of polymer additives.Comment: 34 pages, 31 figure
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