37 research outputs found
Fluctuations in 2D reversibly-damped turbulence
Gallavotti proposed an equivalence principle in hydrodynamics, which states
that forced-damped fluids can be equally well represented by means of the
Navier-Stokes equations and by means of time reversible dynamical systems
called GNS. In the GNS systems, the usual viscosity is replaced by a
state-dependent dissipation term which fixes one global quantity. The principle
states that the mean values of properly chosen observables are the same for
both representations of the fluid. In the same paper, the chaotic hypothesis of
Gallavotti and Cohen is applied to hydrodynamics, leading to the conjecture
that entropy fluctuations in the GNS system verify a relation first observed in
nonequilibrium molecular dynamics. We tested these ideas in the case of
two-dimensional fluids. We examined the fluctuations of global quadratic
quantities in the statistically stationary state of a) the Navier-Stokes
equations; b) the GNS equations. Our results are consistent with the validity
of the fluctuation relation, and of the equivalence principle, indicating
possible extensions thereof. Moreover, in these results the difference between
the Gallavotti-Cohen fluctuation theorem and the Evans-Searles identity is
evident.Comment: latex-2e, 14 pages, 6 figures, submitted to Nonlinearity. Revised
version following the referees' comments: text polished, a few algebraic
mistakes corrected, figures improved, reference to the Evans-Searles identity
adde
Vesicle dynamics in elongation flow: Wrinkling instability and bud formation
We present experimental results on the relaxation dynamics of vesicles
subjected to a time-dependent elongation flow. We observed and characterized a
new instability, which results in the formation of higher order modes of the
vesicle shape (wrinkles), after a switch in the direction of the gradient of
the velocity. This surprising generation of membrane wrinkles can be explained
by the appearance of a negative surface tension during the vesicle deflation,
due to compression in a sign-switching transient. Moreover, the formation of
buds in the vesicle membrane has been observed in the vicinity of the dynamical
transition point.Comment: 4 pages, 4 figure
Wettability Stabilizes Fluid Invasion into Porous Media via Nonlocal, Cooperative Pore Filling
We study the impact of the wetting properties on the immiscible displacement
of a viscous fluid in disordered porous media. We present a novel pore-scale
model that captures wettability and dynamic effects, including the
spatiotemporal nonlocality associated with interface readjustments. Our
simulations show that increasing the wettability of the invading fluid (the
contact angle) promotes cooperative pore filling that stabilizes the invasion,
and that this effect is suppressed as the flow rate increases, due to viscous
instabilities. We use scaling analysis to derive two dimensionless numbers that
predict the mode of displacement. By elucidating the underlying mechanisms, we
explain classical yet intriguing experimental observations. These insights
could be used to improve technologies such as hydraulic fracturing, CO
geo-sequestration, and microfluidics.Comment: In review, Physics Review Letter
Dynamics of Vesicles in shear and rotational flows: Modal Dynamics and Phase Diagram
Despite the recent upsurge of theoretical reduced models for vesicle shape
dynamics, comparisons with experiments have not been accomplished. We review
the implications of some of the recently proposed models for vesicle dynamics,
especially the Tumbling-Trembling domain regions of the phase plane and show
that they all fail to capture the essential behavior of real vesicles for
excess areas, \Delta, greater than 0.4. We emphasize new observations of shape
harmonics and the role of thermal fluctuations.Comment: (removed forgotten leftover figure files
Cellular Automaton for Realistic Modelling of Landslides
A numerical model is developed for the simulation of debris flow in
landslides over a complex three dimensional topography. The model is based on a
lattice, in which debris can be transferred among nearest neighbors according
to established empirical relationships for granular flows. The model is then
validated by comparing a simulation with reported field data. Our model is in
fact a realistic elaboration of simpler ``sandpile automata'', which have in
recent years been studied as supposedly paradigmatic of ``self-organized
criticality''.
Statistics and scaling properties of the simulation are examined, and show
that the model has an intermittent behavior.Comment: Revised version (gramatical and writing style cleanup mainly).
Accepted for publication by Nonlinear Processes in Geophysics. 16 pages, 98Kb
uuencoded compressed dvi file (that's the way life is easiest). Big (6Mb)
postscript figures available upon request from [email protected] /
[email protected]
The Interplay Between PoreâScale Heterogeneity, Surface Roughness, and Wettability Controls Trapping in TwoâPhase Fluid Displacement in Porous Media
Predicting the compactness of the invasion front and the amount of trapped fluid left behind is of crucial importance to applications ranging from microfluidics and fuel cells to subsurface storage of carbon and hydrogen. We examine the interplay of wettability, macroâ and pore scale heterogeneity (pore angularity and pore wall roughness), and its influence on flow patterns formation and trapping efficiency in porous media by a combination of 3D microâCT imaging with 2D direct visualization of micromodels. We observe various phase transitions between the following capillary flow regimes (phases): (a) compact advance, (b) wetting and drainage Invasion percolation, (c) Ordinary percolation
A New Phase Diagram for Fluid Invasion Patterns as a Function of PoreâScale Heterogeneity, Surface Roughness, and Wettability
Understanding how different flow patterns emerge at various macroâ and pore scale heterogeneity,pore wettability and surface roughness is remains a long standing scientific challenge. Such understandingallows to predict the amount of trapped fluid left behind, of crucial importance to applications ranging frommicrofluidics and fuel cells to subsurface storage of carbon and hydrogen. We examine the interplay ofwettability and poreâscale heterogeneity including both pore angularity and roughness, by a combination ofmicroâCT imaging of 3D grain packs with direct visualization of 2D micromodels. The micromodels aredesigned to retain the key morphological and topological properties derived from the microâCT images.Different manufacturing techniques allow us to control pore surface roughness. We study the competitionbetween flow through the pore centers and flow along rough pore walls and corners in media of increasingcomplexity in the capillary flow regime. The resulting flow patterns and their trapping efficiency are in excellentagreement with previous ÎŒâCT results. We observe different phase transitions between the following flowregimes (phases): (a) Frontal/compact advance, (b) wetting and drainage Invasion percolation, and (c) Ordinarypercolation. We present a heterogeneityâwettabilityâroughness phase diagram that predicts these regimes
Elastic turbulence in von Karman swirling flow between two disks
We discuss the role of elastic stress in the statistical properties of
elastic turbulence, realized by the flow of a polymer solution between two
disks. The dynamics of the elastic stress are analogous to those of a small
scale fast dynamo in magnetohydrodynamics, and to those of the turbulent
advection of a passive scalar in the Batchelor regime. Both systems are
theoretically studied in literature, and this analogy is exploited to explain
the statistical properties, the flow structure, and the scaling observed
experimentally. Several features of elastic turbulence are confirmed
experimentally and presented in this paper: (i) saturation of the rms of the
vorticity and of velocity gradients in the bulk, leading to the saturation of
the elastic stress; (ii) large rms of the velocity gradients in the boundary
layer, linearly growth with Wi; (iii) skewed PDFs of the injected power, with
exponential tails, which indicate intermittency; PDF of the acceleration
exhibit well-pronounced exponential tails too; (iv) a new length scale, i.e the
thickness of the boundary layer, as measured from the profile of the rms of the
velocity gradient, is found to be relevant and much smaller than the vessel
size; (v) the scaling of the structure functions of the vorticity, velocity
gradients, and injected power is found to be the same as that of a passive
scalar advected by an elastic turbulent velocity field.Comment: submitted to Physics of Fluids; 31 pages, 29 figures (resolution
reduced to screen quality
Chaotic flow and efficient mixing in a micro-channel with a polymer solution
Microscopic flows are almost universally linear, laminar and stationary
because Reynolds number, , is usually very small. That impedes mixing in
micro-fluidic devices, which sometimes limits their performance. Here we show
that truly chaotic flow can be generated in a smooth micro-channel of a uniform
width at arbitrarily low , if a small amount of flexible polymers is added
to the working liquid. The chaotic flow regime is characterized by randomly
fluctuating three-dimensional velocity field and significant growth of the flow
resistance. Although the size of the polymer molecules extended in the flow may
become comparable with the micro-channel width, the flow behavior is fully
compatible with that in a table-top channel in the regime of elastic
turbulence. The chaotic flow leads to quite efficient mixing, which is almost
diffusion independent. For macromolecules, mixing time in this microscopic flow
can be three to four orders of magnitude shorter than due to molecular
diffusion.Comment: 8 pages,7 figure