103 research outputs found
Micro-evaporators for kinetic exploration of phase diagrams
We use pervaporation-based microfluidic devices to concentrate species in
aqueous solutions with spatial and temporal control of the process. Using
experiments and modelling, we quantitatively describe the advection-diffusion
behavior of the concentration field of various solutions (electrolytes,
colloids, etc) and demonstrate the potential of these devices as universal
tools for the kinetic exploration of the phases and textures that form upon
concentration
Intermittency and the Slow Approach to Kolmogorov Scaling
From a simple path integral involving a variable volatility in the velocity
differences, we obtain velocity probability density functions with exponential
tails, resembling those observed in fully developed turbulence. The model
yields realistic scaling exponents and structure functions satisfying extended
self-similarity. But there is an additional small scale dependence for
quantities in the inertial range, which is linked to a slow approach to
Kolmogorov (1941) scaling occurring in the large distance limit.Comment: 10 pages, 5 figures, minor changes to mirror version to appear in PR
Mesoscopic two-phase model for describing apparent slip in micro-channel flows
The phenomenon of apparent slip in micro-channel flows is analyzed by means
of a two-phase mesoscopic lattice Boltzmann model including non-ideal
fluid-fluid and fluid-wall interactins. The weakly-inhomogeneous limit of this
model is solved analytically.
The present mesoscopic approach permits to access much larger scales than
molecular dynamics, and comparable with those attained by continuum methods.
However, at variance with the continuum approach, the existence of a gas layer
near the wall does not need to be postulated a priori, but emerges naturally
from the underlying non-ideal mesoscopic dynamics. It is therefore argued that
a mesoscopic Lattice Boltzmann approach with non-ideal fluid-fluid and
fluid-wall interactions might achieve an optimal compromise between physical
realism and computational efficiency for the study of channel micro-flows.Comment: 5 pages, 3 figure
Giant slip lengths of a simple fluid at vibrating solid interfaces
It has been shown recently [PRL 102, 254503 (2009)] that in the plane-plane
configuration a mechanical resonator vibrating close to a rigid wall in a
simple fluid can be overdamped to a frozen regime. Here, by solving
analytically the Navier Stokes equations with partial slip boundary conditions
at the solid fluid interface, we develop a theoretical approach justifying and
extending these earlier findings. We show in particular that in the perfect
slip regime the above mentioned results are, in the plane-plane configuration,
very general and robust with respect to lever geometry considerations. We
compare the results with those obtained previously for the sphere moving
perpendicularly and close to a plane in a simple fluid and discuss in more
details the differences concerning the dependence of the friction forces with
the gap distance separating the moving object (i.e., plane or sphere) from the
fixed plane. Finally, we show that the submicron fluidic effect reported in the
reference above, and discussed further in the present work, can have dramatic
implications in the design of nano-electromechanical systems (NEMS).Comment: submitted to PRE (see also PRL 102, 254503 (2009)
Geometrical statistics of the vorticity vector and the strain rate tensor in rotating turbulence
We report results on the geometrical statistics of the vorticity vector
obtained from experiments in electromagnetically forced rotating turbulence. A
range of rotation rates is considered, from non-rotating to rapidly
rotating turbulence with a maximum background rotation rate of rad/s
(with Rossby number much smaller than unity). Typically, in our experiments
. The measurement volume is located in the
centre of the fluid container above the bottom boundary layer, where the
turbulent flow can be considered locally statistically isotropic and
horizontally homogeneous for the non-rotating case, see van Bokhoven et al.,
Phys. Fluids 21, 096601 (2009). Based on the full set of velocity derivatives,
measured in a Lagrangian way by 3D Particle Tracking Velocimetry, we have been
able to quantify statistically the effect of system rotation on several flow
properties. The experimental results show how the turbulence evolves from
almost isotropic 3D turbulence ( rad/s) to quasi-2D
turbulence ( rad/s) and how this is reflected by several
statistical quantities. In particular, we have studied the orientation of the
vorticity vector with respect to the three eigenvectors of the local strain
rate tensor and with respect to the vortex stretching vector. Additionally, we
have quantified the role of system rotation on the self-amplification terms of
the enstrophy and strain rate equations and the direct contribution of the
background rotation on these evolution equations. The main effect is the strong
reduction of extreme events and related (strong) reduction of the skewness of
PDFs of several quantities such as, for example, the intermediate eigenvalue of
the strain rate tensor and the enstrophy self-amplification term.Comment: 17 pages, 6 figures, 3 table
Unified Multifractal Description of Velocity Increments Statistics in Turbulence: Intermittency and Skewness
The phenomenology of velocity statistics in turbulent flows, up to now,
relates to different models dealing with either signed or unsigned longitudinal
velocity increments, with either inertial or dissipative fluctuations. In this
paper, we are concerned with the complete probability density function (PDF) of
signed longitudinal increments at all scales. First, we focus on the symmetric
part of the PDFs, taking into account the observed departure from scale
invariance induced by dissipation effects. The analysis is then extended to the
asymmetric part of the PDFs, with the specific goal to predict the skewness of
the velocity derivatives. It opens the route to the complete description of all
measurable quantities, for any Reynolds number, and various experimental
conditions. This description is based on a single universal parameter function
D(h) and a universal constant R*.Comment: 13 pages, 3 figures, Extended version, Publishe
Transitions and Probes in Turbulent Helium
Previous analysis of a Paris turbulence experiment \cite{zoc94,tab95} shows a
transition at the Taylor Reynolds number \rel \approx 700. Here correlation
function data is analyzed which gives further evidence for this transition. It
is seen in both the power spectrum and in structure function measurements. Two
possible explanations may be offered for this observed transition: that it is
intrinsic to the turbulence flow in this closed box experiment or that it is an
effect of a change in the flow around the anemometer. We particularly examine a
pair of ``probe effects''. The first is a thermal boundary layer which does
exist about the probe and does limit the probe response, particularly at high
frequencies. Arguments based on simulations of the response and upon
observations of dissipation suggests that this effect is only crucial beyond
\rel\approx 2000. The second effect is produced by vortex shedding behind the
probe. This has been seen to produce a large modification in some of the power
spectra for large \rel. It might also complicate the interpretation of the
experimental results. However, there seems to be a remaining range of data for
\rel < 1300 uncomplicated by these effects, and which are thus suggestive of
an intrinsic transition.Comment: uuencoded .ps files. submitted to PRE. 12 figures are sent upon
request to jane wang ([email protected]
Studying Flow Close to an Interface by Total Internal Reflection Fluorescence Cross Correlation Spectroscopy: Quantitative Data Analysis
Total Internal Reflection Fluorescence Cross Correlation Spectroscopy
(TIR-FCCS) has recently (S. Yordanov et al., Optics Express 17, 21149 (2009))
been established as an experimental method to probe hydrodynamic flows near
surfaces, on length scales of tens of nanometers. Its main advantage is that
fluorescence only occurs for tracer particles close to the surface, thus
resulting in high sensitivity. However, the measured correlation functions only
provide rather indirect information about the flow parameters of interest, such
as the shear rate and the slip length. In the present paper, we show how to
combine detailed and fairly realistic theoretical modeling of the phenomena by
Brownian Dynamics simulations with accurate measurements of the correlation
functions, in order to establish a quantitative method to retrieve the flow
properties from the experiments. Firstly, Brownian Dynamics is used to sample
highly accurate correlation functions for a fixed set of model parameters.
Secondly, these parameters are varied systematically by means of an
importance-sampling Monte Carlo procedure in order to fit the experiments. This
provides the optimum parameter values together with their statistical error
bars. The approach is well suited for massively parallel computers, which
allows us to do the data analysis within moderate computing times. The method
is applied to flow near a hydrophilic surface, where the slip length is
observed to be smaller than 10nm, and, within the limitations of the
experiments and the model, indistinguishable from zero.Comment: 18 pages, 12 figure
Mean- Field Approximation and a Small Parameter in Turbulence Theory
Numerical and physical experiments on two-dimensional (2d) turbulence show
that the differences of transverse components of velocity field are well
described by a gaussian statistics and Kolmogorov scaling exponents. In this
case the dissipation fluctuations are irrelevant in the limit of small
viscosity. In general, one can assume existence of critical
space-dimensionality , at which the energy flux and all odd-order
moments of velocity difference change sign and the dissipation fluctuations
become dynamically unimportant. At the flow can be described by the
``mean-field theory'', leading to the observed gaussian statistics and
Kolmogorov scaling of transverse velocity differences. It is shown that in the
vicinity of the ratio of the relaxation and translation
characteristic times decreases to zero, thus giving rise to a small parameter
of the theory. The expressions for pressure and dissipation contributions to
the exact equation for the generating function of transverse velocity
differences are derived in the vicinity of . The resulting equation
describes experimental data on two-dimensional turbulence and demonstrate onset
of intermittency as and in three-dimensional flows in
close agreement with experimental data. In addition, some new exact relations
between correlation functions of velocity differences are derived. It is also
predicted that the single-point pdf of transverse velocity difference in
developing as well as in the large-scale stabilized two-dimensional turbulence
is a gaussian.Comment: 25 pages, 1 figur
Vorticity statistics in the two-dimensional enstrophy cascade
We report the first extensive experimental observation of the two-dimensional
enstrophy cascade, along with the determination of the high order vorticity
statistics. The energy spectra we obtain are remarkably close to the Kraichnan
Batchelor expectation. The distributions of the vorticity increments, in the
inertial range, deviate only little from gaussianity and the corresponding
structure functions exponents are indistinguishable from zero. It is thus shown
that there is no sizeable small scale intermittency in the enstrophy cascade,
in agreement with recent theoretical analyses.Comment: 5 pages, 7 Figure
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