67 research outputs found
Turbulence-induced melting of a nonequilibrium vortex crystal in a forced thin fluid film
To develop an understanding of recent experiments on the turbulence-induced
melting of a periodic array of vortices in a thin fluid film, we perform a
direct numerical simulation of the two-dimensional Navier-Stokes equations
forced such that, at low Reynolds numbers, the steady state of the film is a
square lattice of vortices. We find that, as we increase the Reynolds number,
this lattice undergoes a series of nonequilibrium phase transitions, first to a
crystal with a different reciprocal lattice and then to a sequence of crystals
that oscillate in time. Initially the temporal oscillations are periodic; this
periodic behaviour becomes more and more complicated, with increasing Reynolds
number, until the film enters a spatially disordered nonequilibrium statistical
steady that is turbulent. We study this sequence of transitions by using
fluid-dynamics measures, such as the Okubo-Weiss parameter that distinguishes
between vortical and extensional regions in the flow, ideas from nonlinear
dynamics, e.g., \Poincare maps, and theoretical methods that have been
developed to study the melting of an equilibrium crystal or the freezing of a
liquid and which lead to a natural set of order parameters for the crystalline
phases and spatial autocorrelation functions that characterise short- and
long-range order in the turbulent and crystalline phases, respectively.Comment: 31 pages, 56 figures, movie files not include
Direct evidence of plastic events and dynamic heterogeneities in soft-glasses
By using fluid-kinetic simulations of confined and concentrated emulsion
droplets, we investigate the nature of space non-homogeneity in soft-glassy
dynamics and provide quantitative measurements of the statistical features of
plastic events in the proximity of the yield-stress threshold. Above the yield
stress, our results show the existence of a finite stress correlation scale,
which can be mapped directly onto the {\it cooperativity scale}, recently
introduced in the literature to capture non-local effects in the soft-glassy
dynamics. In this regime, the emergence of a separate boundary (wall) rheology
with higher fluidity than the bulk, is highlighted in terms of near-wall
spontaneous segregation of plastic events. Near the yield stress, where the
cooperative scale cannot be estimated with sufficient accuracy, the system
shows a clear increase of the stress correlation scale, whereas plastic events
exhibit intermittent clustering in time, with no preferential spatial location.
A quantitative measurement of the space-time correlation associated with the
motion of the interface of the droplets is key to spot the long-range amorphous
order at the yield stress threshold
Internal dynamics and activated processes in Soft-Glassy materials
Plastic rearrangements play a crucial role in the characterization of
soft-glassy materials, such as emulsions and foams. Based on numerical
simulations of soft-glassy systems, we study the dynamics of plastic
rearrangements at the hydrodynamic scales where thermal fluctuations can be
neglected. Plastic rearrangements require an energy input, which can be either
provided by external sources, or made available through time evolution in the
coarsening dynamics, in which the total interfacial area decreases as a
consequence of the slow evolution of the dispersed phase from smaller to large
droplets/bubbles. We first demonstrate that our hydrodynamic model can
quantitatively reproduce such coarsening dynamics. Then, considering
periodically oscillating strains, we characterize the number of plastic
rearrangements as a function of the external energy-supply, and show that they
can be regarded as activated processes induced by a suitable "noise" effect.
Here we use the word noise in a broad sense, referring to the internal
non-equilibrium dynamics triggered by spatial random heterogeneities and
coarsening. Finally, by exploring the interplay between the internal
characteristic time-scale of the coarsening dynamics and the external
time-scale associated with the imposed oscillating strain, we show that the
system exhibits the phenomenon of stochastic resonance, thereby providing
further credit to the mechanical activation scenario.Comment: 21 Pages, 9 figure
Growth, competition and cooperation in spatial population genetics
We study an individual based model describing competition in space between
two different alleles. Although the model is similar in spirit to classic
models of spatial population genetics such as the stepping stone model, here
however space is continuous and the total density of competing individuals
fluctuates due to demographic stochasticity. By means of analytics and
numerical simulations, we study the behavior of fixation probabilities,
fixation times, and heterozygosity, in a neutral setting and in cases where the
two species can compete or cooperate. By concluding with examples in which
individuals are transported by fluid flows, we argue that this model is a
natural choice to describe competition in marine environments.Comment: 29 pages, 14 figures; revised version including a section with
results in the presence of fluid flow
Particle algorithms for population dynamics in flows
We present and discuss particle based algorithms to numerically study the dynamics of population subjected to an advecting flow condition. We discuss few possible variants of the algorithms and compare them in a model compressible flow. A comparison against appropriate versions of the continuum stochastic Fisher equation (sFKPP) is also presented and discussed. The algorithms can be used to study populations genetics in fluid environments.Postprint (published version
Direct numerical simulations of statistically steady, homogeneous, isotropic fluid turbulence with polymer additives
We carry out a direct numerical simulation (DNS) study that reveals the
effects of polymers on statistically steady, forced, homogeneous, isotropic
fluid turbulence. We find clear manifestations of dissipation-reduction
phenomena: On the addition of polymers to the turbulent fluid, we obtain a
reduction in the energy dissipation rate, a significant modification of the
fluid energy spectrum, especially in the deep-dissipation range, a suppression
of small-scale intermittency, and a decrease in small-scale vorticity
filaments. We also compare our results with recent experiments and earlier DNS
studies of decaying fluid turbulence with polymer additives.Comment: consistent with the published versio
Convection in multiphase flows using Lattice Boltzmann methods
We present high resolution numerical simulations of convection in multiphase
flows (boiling) using a novel algorithm based on a Lattice Boltzmann method. We
first validate the thermodynamical and kinematical properties of the algorithm.
Then, we perform a series of 3d numerical simulations at changing the mean
properties in the phase diagram and compare convection with and without phase
coexistence at . We show that in presence of nucleating bubbles
non-Oberbeck Boussinesq effects develops, mean temperature profile becomes
asymmetric, heat-transfer and heat-transfer fluctuations are enhanced. We also
show that small-scale properties of velocity and temperature fields are
strongly affected by the presence of buoyant bubble leading to high
non-Gaussian profiles in the bulk.Comment: 5 pages, 3 figure
Stochastic resonance in soft-glassy materials
Flow in soft-glasses occurs via a sequence of reversible elastic deformations and local irreversible plastic rearrangements. Yield events in the material cause kicks adding up to an effectively thermal noise, an intuition that has inspired the development of phenomenological models aiming at explaining the main features of soft-glassy rheology. In this letter, we provide a specific scenario for such mechanical activation, based on a general paradigm of non-equilibrium statistical mechanics, namely {\it stochastic resonance}. By using mesoscopic simulations of emulsion droplets subject to an oscillatory strain, we characterize the response of the system and highlight a resonance-like behavior in the plastic rearrangements. This confirms that the synchronization of the system response to an external time-dependent load is triggered by the mechanical noise resulting from disordered configurations (polydispersity)
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