390 research outputs found
Transition from viscous to inertial regime in dense suspensions
Non-Brownian suspensions present a transition from Newtonian behavior in the
zero-shear limit to a shear thickening behaviour at a large shear rate, none of
which is clearly understood so far. Here, we carry out numerical simulations of
such an athermal dense suspension under shear, at an imposed confining
pressure. This set-up is conceptually identical to the recent experiments of
Boyer and co-workers [Phys. Rev. Lett. 107,188301 (2011)]. Varying the
interstitial fluid viscosities, we recover the Newtonian and Bagnoldian regimes
and show that they correspond to a dissipation dominated by viscous and contact
forces respectively. We show that the two rheological regimes can be unified as
a function of a single dimensionless number, by adding the contributions to the
dissipation at a given volume fraction.Comment: 4 pages, 3 figure
Adaptabilidade de linhagens e cultivares de feijao (Phaseolus vulgaris L.) em Rondonia e resistencia a "mela" (Thanatephorus cucumeris (Frank.) Donk).
bitstream/item/60390/1/PA-1588-0001.pd
Rheology of mobile sediment beds sheared by viscous, pressure-driven flows
We present a detailed comparison of the rheological behaviour of sheared
sediment beds in a pressure-driven, straight channel configuration based on
data that was generated by means of fully coupled, grain-resolved direct
numerical simulations and experimental measurements reviously published by
Aussillous {\it et al.} (J. Fluid Mech., vol. 736, 2013, pp. 594-615). The
highly-resolved simulation data allows to compute the stress balance of the
suspension in the streamwise and vertical directions and the stress exchange
between the fluid and particle phase, which is information needed to infer the
rheology, but has so far been unreachable in experiments. Applying this
knowledge to the experimental and numerical data, we obtain the
statistically-stationary, depth-resolved profiles of the relevant rheological
quantities. The scaling behavior of rheological quantities such as the shear
and normal viscosities and the effective friction coefficient are examined and
compared to data coming from rheometry experiments and from widely-used
rheological correlations. We show that rheological properties that have
previously been inferred for annular Couette-type shear flows with neutrally
buoyant particles still hold for our setup of sediment transport in a
Poiseuille flow and in the dense regime we found good agreement with empirical
relationships derived therefrom. Subdividing the total stress into parts from
particle contact and hydrodynamics suggests a critical particle volume fraction
of 0.3 to separate the dense from the dilute regime. In the dilute regime,
i.e., the sediment transport layer, long-range hydrodynamic interactions are
screened by the porous media and the effective viscosity obeys the Einstein
relation
Spreading fronts and fluctuations in sedimentation
International audienceA diffuse interface or ''front'' at the top of the suspension is investigated experimentally and numerically. The width of the front is found to grow linearly in time, mainly due to a polydispersity of particle size in the very dilute experiments, and due only to fluctuations in particle density in the simulations. Away from the front, the fluctuations in the particle velocities are found not to decay
On stratification control of the velocity fluctuations in sedimentation
International audienceWe have tested whether stratification can govern local velocity fluctuations in suspensions of sedimenting spheres. Comparison of the proposed scaling for local control of fluctuations by stratification to experimental data demonstrates that this mechanism cannot account for the reduction of the observed velocity fluctuations
Fluctuations and stratification in sedimentation of dilute suspensions of spheres
International audienceWe have tested in experiments and simulations whether stratification can control velocity fluctuations in suspensions of sedimenting spheres. The initial value and early decay of the velocity fluctuations are not affected by stratification. On the other hand, in the descending front where the stratification is strong and well defined, the velocity fluctuations are inhibited according to a previously proposed scaling. In between, after the initial decay and before the arrival of the front, the local value of the stratification does not always play a role
Spreading fronts in sedimentation of dilute suspension of spheres
International audienceThe thickness of the diffuse front between a sedimenting dilute suspension and the clear fluid above grows linearly in time due to polydispersity in the size of the particles and due to a hydrodynamic effect in which randomly heavy clusters fall out of the front leaving it depleted. Experiments and simplified point-particle numerical simulations agree that these two effects are not simply linearly additive
Frozen water waves over rough topographical bottoms
The propagation of surface water waves over rough topographical bottoms is
investigated by the multiple scattering theory. It is shown that the waves can
be localized spatially through the process of multiple scattering and wave
interference, a peculiar wave phenomenon which has been previously discussed
for frozen light in optical systems (S. John, Nature {\bf 390}, 661, (1997)).
We demonstrate that when frozen, the transmission of the waves falls off
exponentially, and a cooperative behavior appears, fully supporting previous
predictions. A phase diagram method is used to illustrate this distinct phase
states in the wave propagation.Comment: 4 pages and 5 figure
Gravity waves over topographical bottoms: Comparison with the experiment
In this paper, the propagation of water surface waves over one-dimensional
periodic and random bottoms is investigated by the transfer matrix method. For
the periodic bottoms, the band structure is calculated, and the results are
compared to the transmission results. When the bottoms are randomized, the
Anderson localization phenomenon is observed. The theory has been applied to an
existing experiment (Belzons, et al., J. Fluid Mech. {\bf 186}, 530 (1988)). In
general, the results are compared favorably with the experimental observation.Comment: 15 pages, 7 figure
A NARRATIVA HISTÓRICA: REVELAÇÃO E ENGANO
Nas últimas quatro décadas, a historiografia tem privilegiado o estabelecimento de diálogos com a antropologia, a teoria literária, a hermenêutica e alguns campos da semiologia ao discutir o estatuto da história como campo específico do saber e a natureza das verdades que ela produz. Esses diálogos trouxeram grandes avanços, mas limitaram o debate a um espaço específico e tautológico, que busca dar conta das diferentes interações entre história e Narrativa, ora assumindo o ofício do historiador como produtor de ficções controladas que fazem as vezes do pretérito, ora relativizando tal assunção ao distinguir a narrativa literária - comprometida com o verossímil - e a narrativa histórica - comprometida com o verdadeiro. O presente trabalho busca, a partir da análise de obras canônicas no campo da história e narrativa, utilizar os conceitos de ceticismo, dogmatismo e criticismo derivados da teoria do conhecimento de Johannes Hessen para delimitar o lugar epistemológico em que se inserem as abordagens de Leopold Von Ranke, Hayden White, Roland Barthes, Paul Ricoeur, Michel de Certeau e Michel Foucault.
- …