3,744 research outputs found
Weakly nonlinear investigation of the Saffman-Taylor problem in a rectangular Hele-Shaw cell
We analyze the Saffman-Taylor viscous fingering problem in rectangular
geometry. We investigate the onset of nonlinear effects and the basic
symmetries of the mode coupling equations, highlighting the link between
interface asymmetry and viscosity contrast. Symmetry breaking occurs through
enhanced growth of sub-harmonic perturbations. Our results explain the absence
of finger tip-splitting in the early flow stages, and saturation of growth
rates compared with the predictions of linear stability.Comment: 42 pages, 5 figures, added references, minor changes, to appear in
Int. J. Mod. Phys. B (1998
Viscous fingering patterns in ferrofluids
Viscous fingering occurs in the flow of two immiscible, viscous fluids
between the plates of a Hele-Shaw cell. Due to pressure gradients or gravity,
the initially planar interface separating the two fluids undergoes a
Saffman-Taylor instability and develops finger-like structures. When one of the
fluids is a ferrofluid and a perpendicular magnetic field is applied, the
labyrinthine instability supplements the usual viscous fingering instability,
resulting in visually striking, complex patterns. We consider this problem in a
rectangular flow geometry using a perturbative mode-coupling analysis. We
deduce two general results: viscosity contrast between the fluids drives
interface asymmetry, with no contribution from magnetic forces; magnetic
repulsion within the ferrofluid generates finger tip-splitting, which is absent
in the rectangular geometry for ordinary fluids.Comment: 29 pages, 5 figures, Late
Suppression of viscous fluid fingering: a piecewise constant-injection process
The injection of a fluid into another of larger viscosity in a Hele-Shaw cell
usually results in the formation of highly branched patterns. Despite the
richness of these structures, in many practical situations such convoluted
shapes are quite undesirable. In this letter we propose an efficient and easily
reproducible way to restrain these instabilities based on a simple piecewise
constant pumping protocol. It results in a reduction in the size of the viscous
fingers by one order of magnitude.Comment: Published in Phys. Rev.
Environmental context of endophyte symbioses: Interacting effects of water stress and insect herbivory
Symbiotic associations between grasses and fungal endophytes are generally regarded as mutualistic, yet benefits to host plants may vary with environmental context. Previous studies have emphasized how endophytes influence plant responses to single stressors. In contrast, the outcome of endophyte-grass interactions under simultaneous biotic and abiotic stresses remains poorly explored. We hypothesized that benefits from endophyte symbiosis become most apparent in "complex" environments where hosts experience multiple stresses. We evaluated the performance of endophyte-infected (E+) vs. endophyte-uninfected (E-) Lolium multiflorum plants in a factorial experiment with water supply (control vs. drought) and insect herbivory (with aphids vs. without aphids). Endophyte infection delayed tiller production in well-watered plants, while water stress reduced tillering in E- plants. Endophyte mediation of herbivory tolerance was contingent on water supply. Whereas aphid herbivory was detrimental to E+ plants in well-watered soils, aphids interacted with drought stress in decreasing the reproductive output of E- but not E+ plants. Moreover, endophyte presence decreased aphid densities on drought-stressed plants only. Thus, endophyte symbiosis enhanced host tolerance to overlapping biotic and abiotic stresses, although infected plants failed to outgrow their uninfected counterparts. These results support the view that mutualistic endophyte effects may not arise in low-stress environments. © 2011 by The University of Chicago. All rights reserved.Fil: Miranda, M. Isabel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; ArgentinaFil: Omacini, Marina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; ArgentinaFil: Chaneton, Enrique Jose. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentin
Quantum thermal transistor
We demonstrate that a thermal transistor can be made up with a quantum system
of 3 interacting subsystems , coupled to a thermal reservoir each. This thermal
transistor is analogous to an electronic bipolar one with the ability to
control the thermal currents at the collector and at the emitter with the
imposed thermal current at the base. This is achieved determining the heat
fluxes by means of the strong-coupling formalism. For the case of 3 interacting
spins, in which one of them is coupled to the other 2, that are not directly
coupled, it is shown that high amplification can be obtained in a wide range of
energy parameters and temperatures. The proposed quantum transistor could, in
principle, be used to develop devices such as a thermal modulator and a thermal
amplifier in nano systems.Comment: Physical Review Letters, American Physical Society, 2016, 116,
pp.20060
Gravity-driven instability in a spherical Hele-Shaw cell
A pair of concentric spheres separated by a small gap form a spherical
Hele-Shaw cell. In this cell an interfacial instability arises when two
immiscible fluids flow. We derive the equation of motion for the interface
perturbation amplitudes, including both pressure and gravity drivings, using a
mode coupling approach. Linear stability analysis shows that mode growth rates
depend upon interface perimeter and gravitational force. Mode coupling analysis
reveals the formation of fingering structures presenting a tendency toward
finger tip-sharpening.Comment: 13 pages, 4 ps figures, RevTex, to appear in Physical Review
PROTECTING COPYRIGHT INFORMATION THROUGH INFORMATION HIDING TECHNIQUES
poster abstractIn the age of technology, copyright information is a vital component to organizations or individuals that utilizes the Internet as means of productivity and communication. Unfortunately, this type of information is public and any individual can alter or falsify the information to commit negative actions. This research project analyzes different information hiding techniques that involves the use of steganography and digital watermarking to embed messages in a digital image without a visual difference and without harming the image. The procedure is to encode the message utilizing a hashing function that will return a location (i, j, k) based on a generated key that will be utilized to provide a collision-free distribution among the image. This technique will result on an image I’ that will be visible publicly, but the embedded message will be retrieved only and only if the other party contains the key
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