12,411 research outputs found
On large deviation regimes for random media models
The focus of this article is on the different behavior of large deviations of
random subadditive functionals above the mean versus large deviations below the
mean in two random media models. We consider the point-to-point first passage
percolation time on and a last passage percolation time
. For these functionals, we have and
. Typically, the large deviations for such
functionals exhibits a strong asymmetry, large deviations above the limiting
value are radically different from large deviations below this quantity. We
develop robust techniques to quantify and explain the differences.Comment: Published in at http://dx.doi.org/10.1214/08-AAP535 the Annals of
Applied Probability (http://www.imstat.org/aap/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Suspension flow: do particles act as mixers?
Recently, Roht et al. [J. Contam. Hydrol. 145, 10-16 (2013)] observed that
the presence of suspended non-Brownian macroscopic particles decreased the
dispersivity of a passive solute, for a pressure-driven flow in a narrow
parallel-plates channel at low Reynolds number. This result contradicts the
idea that the streamline distortion caused by the random diffusive motion of
the particles increases the dispersion and mixing of the solute. Therefore, to
estimate the influence of this motion on the dispersivity of the solute, and
investigate the origin of the reported decrease, we experimentally studied the
probability density functions (pdf) of the particle velocities, and
spatio-temporal correlations, in the same experimental configuration. We
observed that, as the mean suspension velocity exceeds a critical value, the
pdf of the streamwise velocities of the particles markedly changes from a
symmetric distribution to an asymmetric one strongly skewed to high velocities
and with a peak of most probable velocity close to the maximum velocity. The
latter observations and the analysis of suspension microstructure indicate that
the observed decrease in the dispersivity of the solute is due to particle
migration to the mid-plane of the channel, and consequent flattening of the
velocity profile. Moreover, we estimated the contribution of particle diffusive
motion to the solute dispersivity to be three orders of magnitude smaller than
the reported decrease, and thus negligible. Solute dispersion is then much more
affected by how particles modify the flow velocity profile across the channel,
than by their diffusive random motion
- …