Patrice Le Gal Home Page
Institut de Recherche sur les Phénomènes Hors Equilibre (IRPHE)
UMR 6594, CNRS - Universités d'Aix-Marseille I et II.
Abstract
This paper is devoted to the experimental study and theoretical
modeling of the coupled wakes of a pair of cylinders placed side by
side in a constant flow. We propose here a dynamical system which
models the behavior of these coupled wakes. This model is constituted
by two coupled Landau equations whose coefficients are estimated from
experimental observations. A stability diagram is analytically and
numerically computed. Finally, we present some new experiments which
complete previous observations of locked and unlocked wakes.
In phase wakes
Out of phase wakes
Asymmetric wakes
Asymmetric wakes
Abstract
This experimental study is devoted to visualisation and ultrasonic
velocity measurement of the wakes formed behind a row of parallel
cylinders placed side by side, perpendicular to an incoming flow at low
Reynolds numbers. When the distance separating the cylinders is small
compared to their diameter, two instability mechanisms, associated with
different patterns and dynamics compete. A first spatial symmetry
breaking appears when the stationary wakes behind each cylinder are
deviated towards one side or the other and form large clusters
containing from 2 to sometimes more than 10 wakes.(Figure 1). These clusters are separated by
intense recirculating zones. When the Reynolds number is increased, the
wakes belonging to the widest clusters experience a secondary temporal
oscillatory bifurcation. Classical Benard-Von Karman vortex streets are
thus shed in phase by these cylinders (acoustic mode), by contrast with
the wakes outside these cells which stay stationary(Figure 2) (Figure
3). Finally, the flow around far appart cylinders is also
investigated. The primary instability does not occur in this case and a
perfect optical mode of vortex shedding, with neighbours in phase
opposition, takes place in the flow.(Figure 4).
Abstract
We study the flow behind an array of equally spaced parallel cylinders.
snapshot, space-time
diagram. A system of Stuart-Landau equations with complex
parameters is used to model the oscillating wakes. Our purpose is to
identify the 6 scalar parameters which most accurately reproduce the
experimental data of Chauve and Le Gal [{Physica D {\bf 58}}, pp
407--413, (1992)]. To do so, we perform a computational search for the
minimum of a distance $\calj$. We define $\calj$ as the sum-square
difference of the data and amplitudes reconstructed using coupled
equations. The search algorithm is made more efficient through the use
of a partially analytical expression for the gradient $\nabla \cal J$.
Indeed $\nabla \cal J$ can be obtained by the integration of a
dynamical system propagating backwards in time (a backpropagation
equation for the Lagrange multipliers). Using the parameters computed
via the backpropagation method, the coupled Stuart-Landau equations
accurately predicted the experimental data from Chauve and Le Gal over
a correlation time of the system. Our method turns out to be quite
robust as evidenced by using noisy synthetic data obtained from
integrations of the coupled Stuart-Landau equations. However, a
difficulty remains with experimental data: in that case the several
sets of identified parameters are shown to yield equivalent
predictions. This is due to a strong discretisation or ``round-off"
error arising from the digitalization of the video images in the
experiment.This ambiguity in parameter identification has been
reproduced with synthetic data subjected to the same kind of
discretisation.
Abstract
The wake of a short aspect ratio cylinder placed in a uniform flow is
experimentally investigated. After having characterized the temporal
behavior of the Benard-Von Karman vortex shedding by the use of a
classical hot-wire anemometer, an ultrasound anemometry technique is
applied to study the spatial critical behavior of the envelope of the
transverse velocity of the wake (tranverse
velocity profiles). It is shown that this envelope which represents
the spatial form of the global mode of the wake, follows universal
scaling laws which are in agreement with second order phase transition.
In a second set of experiments, the behavior of the longitudinal
velocity fluctuations is also investigated (longitudinal
velocity profiles). It is discovered that there is a special point
several diameters behind the cylinder, which plays a role of a wave
maker. Finally for very small aspect ratio cylinders, symmetric vortex
shedding (symmetric shedding)is
reported and modeled using system of coupled oscillators.
Abstract
Very often in hydrodynamics, the description of the complexity of flows
can only be achieved by the use of simple models. These models,
obtained usually by phenomenological arguments, need in general the
knowledge of some parameters. The challenge is then to determine the
values of these parameters from experiments. Here, our concern is the
description of a coupled wakes experiment using a complex
Ginzburg-Landau Equation (GLE). Our analysis is based on a proper
decomposition of experimental spatio-temporal chaotic flow fields,
followed by a projection of the GLE onto the proper directions. We show
that our method is able to recover the parameters of the model which
permit to reconstruct the spatio-temporal chaos observed in the
experiment. As it is based on a general projection principle, this
method is general and could be applied to other systems.
Abstract
The discrete Ginzburg-Landau model for a family of oscillators linearly
coupled with their first neighbors is studied. The full linear
stability analysis of the nonlinear plane wave solutions is performed
by considering both the wavenumber ($k$) of the basic states and the
wavenumber ($q$) of the perturbations as free parameters. In
particular, it is shown that nonlinear plane waves can be destabilized
not only by long ($q\rightarrow 0$) or short ($q=\pi$) wave
perturbations, but also by intermediate wavenumbers
($0<q<\pi$). Finite size effects are also considered and
discussed in connection with experiments on coupled oscillating wakes.
Abstract
The well-known Benard-Von Karman cylinder wake is one of the most
challenging phenomena of fluid mechanics. As the Reynolds number of the
flow around a cylinder passes through a critical value, alternating
vortex shedding appears via a Hopf bifurcation. Theoretical studies of
the wake have described the appearance of this self-sustained
oscillation as the result of a convective to absolute transition
resulting in the formation of a global mode. We illustrate here the
convective global regime of the sub-critical wake by analyzing
visualizations of its impulse response.
Abstract
The complex Stuart-Landau equation models a prototypical Hopf
bifurcation in which, when the control parameter exceeds a critical
value, the null solution bifurcates into a finite amplitude
time-periodic solution. We study the response of this equation to
time-harmonic forcing in the subcritical regime (i.e., before the
bifurcation). We show that when a second parameter in the Stuart-Landau
equation passes a critical value, a portion of the solution surface as
a function of forcing frequency and amplitude becomes multivalued. For
instance, at a fixed forcing amplitude, one finds a well-defined range
of frequencies over which two stable periodic responses may coexist,
having different amplitudes. We apply this result to predict the
behaviour of the wake downstream of an oscillating cylinder, and
compare the predictions with experimental (Re-Rec =-10) and
computational observations of such a wake.
Abstract
There are many examples where the description of the complexity of
flows can
only be achieved by the use of simple models. These models, obtained
usually from phenomenological arguments, need in general the knowledge
of
some parameters. The challenge is then to determine the values of these
parameters from experiments. We will give two examples where we have
been able to evaluate the coefficients of the complex Ginzburg-Landau
equation from space-time chaotic data applied to first a row of coupled
cylinder wakes and then to wave propagation in the Ekman layer of a
rotating
disk. In the first case, our analysis is based on a proper
decomposition of
experimental chaotic flow fields, followed by a projection of the CGLE
onto
the proper directions. We show that our method is able to recover the
parameters of the model which permits to reconstruct the
spatio-temporal chaos
observed in the experiment. The second physical system under
consideration is
the flow above a rotating disk and its cross-flow instability. Our aim
is to
study the properties of the wavefield through a Volterra series
equation. The kernels of the Volterra expansion, which contain relevant
physical information about the system, are estimated by fitting
two-point
measurements via a nonlinear parametric model. We then consider
describing the
wavefield with the complex Ginzburg-Landau equation, and derive
analytical
relations which express the coefficients of the Ginzburg-Landau
equation in
terms of the kernels of the Volterra expansion.
Abstract
Following previous experimental and computational studies, this article
further investigates the applicability of the Stuart-Landau
equation to describe the Hopf bifurcation occurring for flow past a
circular cylinder. It is shown that when the amplitude variable is
taken as the transverse velocity component at a point in the wake, the
so-called Landau constant varies considerably with position and
importantly is generally far from constant during the saturation phase
of wake development. The variation with downstream distance is
quantified. However, it is found that the Landau constant at saturation
is indeed a position-independent constant and this value is close to
that generally measured previously both experimentally and numerically.
It is shown that if the amplitude variable is taken as the lift
coefficient of the cylinder (a global variable) then the same Landau
constant is measured at saturation and the zero amplitude Landau
constant corresponds to that for the transverse velocity at the back of
the cylinder. These findings are used to interpret the wake behaviour
of a transversely oscillating circular at subcritical Reynolds numbers.
Abstract
Abstract
Abstract
This article is devoted to the study of rotating disk flow instability.
This inflectional-type instability, called cross-flow instability (image), is exemplary of transition to
turbulence in the three-dimensional boundary layers. In a first part,
we present the experimental marginal stability curve of unstable waves
obtained by hot-film probe measurements and compare it with the
theoretical results available in the literature. The experiment is in
accordance with the different theoretical determinations of the linear
threshold but we note a difference between experimental and theoretical
critical wave number values. The unstable waves dynamics is then
investigated by means of experimental dispersion curves (linking
frequencies to the wave number vector components) determined by
two-probe measurements. The results show in particular the existence of
travelling dispersive waves, in the boundary layer of the rotating
disk. Finally, we show that the emergence of non-linear effects occurs
very early in the system, far from the transition point.
Abstract
The destabilization of the rotating disk flow subject to a forcing is
experimentally investigated. An isolated roughness element of order
$\delta$ (the constant boundary layer thickness) is placed under the
linear threshold of the cross-flow instability in order to create a
hydrodynamic pattern of finite amplitude and localized in space. The
experimental neutral stability curve is first established. The
resulting double parabolic curve exhibits two minima: one of which is
due to the amplification of fundamental modes, the other one is linked
to the emergence of superharmonic modes. Dispersion curves determined
by means of two-point measurements appear to be shifted away from those
measures in the natural case (without forcing). We show that this
discrepancy is due to the presence of weak nonlinear effects which can
be described by a Ginzburg-Landau amplitude equation. Lastly, we
present an original method that enables to determine both components of
the group velocity vector using the measured dispersion relations and
wave packet propagation angle.
Abstract
Circular and spiral waves are observed in the flow between a rotating
and a stationary disk. These waves are generated by instabilities of
the stationary disk boundary layer. This experimental work is devoted
to their study by means of flow visualization (circular waves) (travelling spiral waves) (stationary spiral waves)and measurements
of the associated velocity fields. In particular, instantaneous
velocity profiles (axial profiles of
circular waves) (axial profiles of
spiral waves) (radial profiles of
circular waves and spiral waves)are measured by ultrasonic Doppler
anemometry (ultrasound pulse). The
spatio-temporal characteristics of the waves are studied with the help
of Fourier transforms of these velocity signals.
Abstract
The stability of a traveling roll system, which results from the
development of the flow between a stationary and a rotating disk, is
experimentally studied. The characteristics of this traveling pattern
and of the bifurcation from which it results are obtained. We show in
particular that the band of the stable roll modes is limited by the
Eckhaus secondary instability.
Phys. Fluids, volume 10, number 11, p.2695, 1998.
Abstract
The physical system under consideration is the flow above a rotating
disk and its cross-flow instability, which is a typical route to
turbulence in three-dimensionnal boundary layers. Out aim is to study
the nonlinear properties of the wavefield through a Volterra series
equation. The kernels of the volterra expansion, which contain relevant
physical information about the system, are estimated by fitting
two-point measurements via a nonlinear parametric model. We then
consider describing the wavefield wit hthe complex Ginzburg-Landau
equation, and derive analytical relations which express the
coefficients of the Ginzburg-Landau equation in terms of the kernels of
the volterra expansion. These relations must hold for a large class of
weakly nonlinear systems, in fluid as well as in plasma physics.
Abstract
This experimental study is devoted to the description of the different
patterns resulting from instabilities which appear in the flow between
a rotating and a stationary disk enclosed by a stationary sidewall.
With the help of visualisations we describe the different flow regimes
as functions of two control parameters: the Reynolds number and the
aspect ratio of the gap separating the disks, which are varied on large
continuous ranges. Moreover visualizsations and ultrasonic anemometry
lead to the description of the different instabilities and to the
construction of a transition diagram that summarizes the domains of
existence of the various patterns. Two different scenarios of
transition are maimly followed by the flow. When the gap between the
two disks is more than the thickness of the two disk boundary layers,
circular and spiral waves destabilize the stationary disk boundary
layer. transition occurs in this case by the mixing of these waves. On
the contrary, when the two boundary layers are merged, finite size
turbulent structuress can appear. They consist of turbulent spots or
turbulent spirals which invade the laminar domains as the Reynolds
number of the flow
is increased.
Abstract
This work is devoted to the experimental study of the transition to
turbulence of a flow confined in a
narrow gap between a rotating and a stationary disk. When the fluid
layer thickness is of the same
order of magnitude as the boundary layer depths, the azimuthal velocity
axial gradient is nearly
constant and this rotating disk flow tends to be a torsional Couette
flow. As in the plane Couette
flow or the Taylor-Couette flow, transition to turbulence occurs via
the appearance of turbulent
domains inside a laminar background. In the rotating disk case, the
nucleation of turbulent spirals,
previously called "solitary waves" in the rotating disk flow
literature, is connected to the birth of
structural defects in a periodic underlying roll pattern. As the
rotation rate is increased, the lifetime
of these turbulent structures increases until a threshold is reached
where they then form permanent
turbulent spirals arranged nearly periodically all around a
circumference. However, since the
number of these turbulent spirals decreases with the rotational
frequency, the transition to a fully
turbulent regime is not achieved. Thus the turbulent fraction of the
pattern saturates to a value
lower than 0.5. After a geometrical description of the structures, we
present a statistical analysis of
sizes and lifetimes of the turbulent and laminar domains in order to
compare this transition to
already observed spatiotemporal intermittent behavior.
(Spatio-temporal intermittency in the rotating frame)
Abstract
Torsional Couette flow between a rotating disk and a stationary wall is
studied experimentally. The surface of the disk is either rigid or,
alternatively, covered with a compliant coating. The influence of wall
compliance on characteristic flow instabilities and on the
laminar-turbulent flow transition is investigated. Data obtained from
analysing flow visualisations are discussed. It is found that wall
compliance favours two of the three characteristic wave patterns
associated with the transition process and broadens the parameter
regime in which these patterns are observed. The results for the
effects of wall compliance on the third pattern are inconclusive.
However, the experiments indicate that the third pattern is not a
primary constituent of the laminar-turbulent transition process of
torsional Couette flow.
Abstract
Our experimental study is devoted to the transition to defect
turbulence of a periodic spiral wave pattern occurring in the flow
between a rotating and a stationary disk. As the rotation rate
$\Omega$ of the disk is increased, the radial phase velocity of
the waves changes its sign: the waves that propagate first outward
on average, then become stationary and finally propagate inward.
As they become stationary, the nucleation of topological defects
breaks the periodicity of the pattern. For higher $\Omega$, more
and more defects are generated in the flow pattern. This article
presents the statistical study of this defect mediated
turbulence.
Abstract
The evolution of the entrainment coefficient K of the
rotating fluid in a rotor-stator cavity with an imposed
centripetal flux and pre-rotation is studied according to the flow
parameters. Measurements are realized in water for a turbulent
Batchelor type of flow by the means of a two component laser Doppler
anemometer (LDA) and the results are compared to those performed
by pressure transducers. We show that the entrainment coefficient $K$
depends on a local flow rate coefficient $Cq_r$ according
to a 5/7 power law, whose coefficients
depend on the boundary
conditions. A theoretical analysis confirms the asymptotic behavior of
K.
Abstract
This experimental study is devoted to the transition to
turbulence of the flow confined between a stationary and a rotating
disk.
Using visualization and video image
analysis, we describe the different transitions occurring in the flow
as the rotating velocity
of the disk is varied. The space–time behavior of the wave patterns is
analyzed using the Bi-Orthogonal Decomposition (BOD)
technique. This decomposition of the experimental signals on proper
modes permits to project the dynamics of the waves in a
reduced embedding phase space. By this means, a torus doubling
bifurcation is revealed before its complete destruction during
the transition to a weak turbulence. Finally, a more classical 2D-Fourier analysis completes our description
of the transition and
shows for higher rotation rates, the appearance of a more developed
turbulence issued from the former chaotic waves.
Abstract
This article presents some
results on the statistical behavior of localized structures—called
“spots”—that propagate in the flow between a rotating and a stationary
disk when those are very close
one to the other. Under these conditions the rotating-disk flow belongs
to the Couette-flow family and is
called the torsional Couette flow. Some visualizations of its
transition to turbulence have already revealed
the propagation of these spots (Schouveiler et al., J Fluid Mech
443:329–350, 2001) from the rim of the disk
towards its center.Using flow visualizations and
an original image analysis, the present study aims to better
describe the characteristics of the spots whose number continuously
increases with the Reynolds number
until they invade the whole flow. Moreover, we propose a statistical
model that predicts an error-function
shape for the probability to observe a spot at a given radial position.
This prediction is confirmed by an image
analysis of the flow and the stability curve of torsional Couette flow
is deduced from these observations.
Abstract
This paper reports an experimental data analysis which clearly
emphasizes the complex nature of the mechanisms governing the mixing of
passive scalars such as temperature in fully developed turbulence. for
that purpose, we compare our measurements of temperature increments and
their probability
density functions (pdfs) to theoretical predictions available in the
literature for a scalar field evolving within a fully developed
turbulent field. The observed disagreements lesd us to propose some
improvements of the existing models. This in fact underlines the
tremendous evolution through the scales of turbulence of the statistics
of temperature increments, whose coupling with the velocity field
appears to be an essential feature of the mixing process.
Abstract
This letter presents some quantitative predictions for the small-scale
statistics for a passive scalar mixed in a fully turbulent flow. Our
method uses an equation (Vaienti et al. Physica D 73D, 99, 1994) which
controls the evolution through the scales of the probability density
function of temperature increments. This equation, which needs some
closure as well as initial data, uses results from direct numerical
simulations or from exeriments. for the dissipative and small inertial
range scales, our predictions are in excellent agreement with the
measured or numerically calculated probability density functions. The
associated Yaglom equation is also well verified for the same range of
scales.
Abstract
We analyze experimentally the statistical properties of velocity and
temperature fluctuations, considered as a passive scalar, in a
turbulent flow generated in the gap between coaxial disks. One of the
disk is heated at a temperature higher than ambient, while the other
one is cooled. With this arrangement one obtains a flow with either
positive, negative or null temperature-velocity correlations. The
velocity, temperature and heat flux fields are measured locally and
studied via the moments of their increments. we find that the velocity
field characteristics are independent of the large scale anisotropy,
with scaling laws in agreement with the ones generally observed in
fully developed turbulence. In contrast, the scalar field is strongly
affected by the injection conditions at large scale; only in the
central well-mixed region does one observe a behavior consistent with
the existence of scaling. In the same region, the heat transport dudt^2
follows the same scaling laws as the kinetic energy flux du^3.
Abstract
We analyze experimentally the statistical properties of a turbulent
mixing created in the gap between two counter-rotating disks at a
Taylor Reynolds number R_l = 400. Local isotropy is investigated for
the inertial and dissipative scales r, using two tests, one applied on
C(r), the correlation coefficient between temperature increments and
velocity increments, and the other one on S(r), the temperature
increment skewness factor. When heating one of the disks and cooling
the other one, either positive, negative or almost null values of C and
S can be obtained at small scales as a direct result of the presence of
several temperature sources. In particular, we emphasize the fact that
null or small values for these quantities in the inertial range are an
evidence of local isotropy of the temperature field. In these cases, we
use the Vaienti et al. equation [Physica D 73, 99 (1994)] for the
evolution of the temperature increments probability density functions
(PDFs) to predict the inertial and dissipative range PDFs, using an
initial PDF, and two measurable closure functions. The intermittent
behaviour quantified through these statistics is well reproduced by the
numerical integration of this evolution equation.
Abstract
The instability of a vortex subject to a stationary dipolar or tripolar
constraint is studied experimentally using a rotating deformable
cylinder on which two or three rollers are applied. As the Reynolds
number and the aspect ratio of the cylinder are varied, different modes
of instability are observed and their wavelength and frequency are
compared to theoretical predictions. Secondary instability and cyclic
break up are also evidenced in the elliptic geometry.
Abstract
In this article, the multipolar vortex instability of the flow in a
finite cylinder is addressed. The experimental study uses a rotating
elastic
deformable tube filled with water which is elliptically or triangularly
deformed by two or three rollers. The experimental control
parameters are the cylinder aspect ratio and the Reynolds number based
on the angular frequency. For Reynolds numbers close to threshold,
different instability modes are visualized using anisotropic particles,
according to the value of
the aspect ratio. These modes are compared with those predicted by an
asymptotic stability theory in the limit of small deformations and
large Reynolds numbers. A very good agreement is obtained which
confirms the instability mechanism: for both elliptic and triangular
configurations, the instability is due to the resonance of two normal
modes (Kelvin modes) of the underlying rotating flow with the
deformation field. At least four different elliptic instability modes,
including combinations of Kelvin modes with azimuthal wavenumbers
m=0 and m=2 and Kelvin modes m=1 and m=3 are visualised. Two different
triangular instability modes which are combination of Kelvin modes m=-1
and
m=2 and combination of Kelvin modes m=0 and m=3 are also evidenced. The
nonlinear dynamics of a particular elliptic instability mode, which
corresponds to the combination of two stationary Kelvin modes m=-1 and
m=1, is examined in more detail using Particle Image Velocimetry (PIV).
The
dynamics of the phase and amplitude of the instability mode is shown to
be
well-predicted by the weakly nonlinear analysis for moderate Reynolds
numbers.
For larger Reynolds number, a secondary instability is observed. Below
a
Reynolds number threshold, the amplitude of this instability mode
saturates
and its frequency is shown to agree with the predictions of Kerswell
(1999). Above this threshold, a more complex dynamics develops which is
only
sustained during a finite time. The flow then relaminarises and the
destabilisation process starts again. Experimental visualizations:
Abstract
This paper concerns the elliptical instability of a flow in a rotating
deformed sphere. The aim of our work is to observe and measure the
characterics of this
instability in experiments and to compare them with theorical
predictions. For
this purpose, an elastic and transparent hollow sphere has been
moulded. The
flow is visualised using Kalliroscope flakes as the sphere is set into
rotation and compressed by two rollers. The elliptical instability
occurs by
the appearance of the so-called 'spin-over' mode whose growth rates and
saturations are measured for different Eckman numbers by video image
analysis.
These growth rates compare avantageously to theorical calculations
which are
performed using classical asymptotic expansions. The linear analysis is
then
completed by a non linear model which predicts correctly the asymptotic
regimes for high Eckman numbers. Experimental visualization:
Abstract
A
theoretical and experimental study of the spin-over mode induced by the
elliptical instability of a flow contained in a slightly deformed
rotating spherical shell is presented. This geometrical configuration
mimics the liquid rotating cores of planets when deformed by tides
coming from neighboring gravitational bodies. Theoretical estimations
for the growth rates and for the non linear amplitude saturations of
the unstable mode are obtained and compared to experimental data
obtained from Laser Doppler anemometry measurements. Visualizations and
descriptions of the various characteristics of the instability are
given as functions of the flow parameters.
Résumé
Le phénomène de bioluminescence est bien connu des marins
ou des nageurs nocturnes. Il provient de l'excitation de certains
organismes planctoniques, notamment des dinoflagellés,
provoquée
par les mouvements de l'eau occasionnés par le
passage de bateaux ou de nageurs. Ces organismes uni-cellulaires
possèdent une taille de quelques dizaines à quelques
centaines de microns, et quand leur densité est
suffisamment importante, cette bioluminescence naturelle pourrait
devenir un moyen de visualisation et d'études originales des
écoulements
hydrodynamiques. Il semble donc interressant
de réaliser des expériences de stimulation de la
bioluminescence en
laboratoire afin de comprendre et calibrer la réponse du
plancton à des
stimuli contrôlés. Plusieurs espèces de
dinoflagellés
sont actuellement cultivées:
Pyrocystis lunula, P. noctiluca et Gonyaulax
polyedra. Une fois les densités des cultures de
dinoflagellés suffisantes,
ces solutions sont transférées sur des installations
expérimentales, afin de
procéder aux essais de bioluminescence stimulée par
différents écoulements.
Le stimulus naturel de la bioluminescence des dinoflagellés
est mécanique; les cellules répondent à une
déformation de la membrane cellulaire produite par des forces
"brusques", induites par une forte agitation
forte agitation de l'eau (içi dans un flacon
de culture), comme dans le cas du brisement des vagues ou la nage
rapide des mammifères, des poissons ou des
invertébrés.
Abstract
The excitation of bioluminescence by different flow regimes
generated within a Couette chamber was examined using the
dinoflagellates Pyrocystis noctiluca. Cultured cells of Pyrocystis
noctiluca were gently transferred into a cylindrical Couette chamber in
a dark room. In initial experiments, the velocity of the outer Couette
cylinder was then gradually increased. The bioluminescence emissions in
response to stationary-laminar and turbulent flows were quantified
using a photomultiplier tube. Video images were also recorded in order
to identify the location of bioluminescence emissions within the
Couette chamber. Reflective flake flow visualizations were used to
correlate these locations to the flow regimes in those parts of the
chamber. These experiments clearly demonstrated that the strongest
bioluminescence emissions were only triggered by the onset of
turbulence at high rotation speeds. Below the turbulence threshold,
much lower bioluminescence emissions were detected, and appeared to be
in response to a non-homogeneity in the stationary-laminar flow (end
cap effects and Ekman cells). In a second set of experiments, the
excitation of bioluminescence in response to acceleration was studied
by abrupt starts of the rotating Couette cylinder. These experiments
also triggered massive bioluminescence emissions. We conclude that pure
laminar-stationary, homogenous shear flow excites very little
bioluminescence in Pyrocystis noctiluca. The bulk of bioluminescence
emissions primarily occurred under non homogenous or non stationary
flow conditions, where the cells experience velocity changes as they
move through the flow. These findings are discussed in relation to the
theory that bioluminescence
in dinoflagellates is an anti-predation mechanism. (bioluminescence
in the turbulent Couette flow)
Résumé
Contrairement aux molécules d'un fluide classique tel que l'eau,
les longues molécules d'un cristal liquide peuvent être
orientées lorsqu'elles sont soumises à une contrainte
extérieure. Dans notre expérience, le champ
appliqué à la couche de nématiques
(orientée préalablement perpendiculairement au rotor et
au stators) est dû à un cisaillement créé
par la rotation d'une des deux plaques (le rotor) entre lesquelles est
contenue la couche de nématiques d'une épaisseur de 100
microns. Le cristal liquide utilisé lors de ces
expériences est le MLC-6610 de la société Merck.
Ses caractéristiques physico-chimiques le rendent proche d'un
nématique phase 9A. L'effet d'un cisaillement sur une couche de
nématiques est un sujet qui a reçu beaucoup d'attention
il y a quelques dizaines d'années car l'étude de la
réponse de telles couches fluides cisaillées permet de
mesurer les coefficients visco-élastiques du cristal liquide.
Ainsi plusieurs expériences se sont attachées à
l'étude de l'orientation des molécules (observée
entre polariseurs croisés) dans des configurations
d'écoulement de Couette plan ou de type rotor-stator. Nous avons
alors noté que dans ces dernières expériences, des
réseaux de lignes de disinclinaisons avaient été
observées, mais qu'elles avaient été
considérées comme néfastes compte tenu des
objectifs de la recherche menée. Notre étude
expérimentale montre en effet que le cisaillement provoque le
basculement des molécules appartenant à certaines
régions de l'espace séparées du fond
homogène par des lignes de disinclinaison et dont l'organisation
spatio-temporelle est fonction du taux de rotation appliqué. En
particulier, nous avons mis en évidence à fort taux de
cisaillement une forme de turbulence que nous appelons Turbulence d'orientation qui fond
à l'arrêt du rotor. Les propriétés des
textures observées ont été alors
étudiées: taille des structures, vitesse de propagation
des fronts séparant les domaines lors des régimes de fonte ou de croissance... Mais, si
le rotor est mis à nouveau en mouvement avant la totale
disparition des derniers domaines, les quelques gouttes qui n'ont pas
encore fondu, vont constituer des germes où la nouvelle phase va
pouvoir s'établir. Étonnement, comme le montrent les
clichés successifs de la figure suivante, seuls les domaines
ayant une double structure
peuvent survivre; les
``mono-domaines'' sont tout d'abord étirés par le
cisaillement puis ils
disparaissent.
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Last modified: February 28, 2008.