107 research outputs found

### THE CUSP: FROM FINITE TIME SINGULARITIES TO BREAKING WAVES

International audienceWe all have in mind the sound of the ping-pong ball bouncing repeatedly faster and faster off the ground before it comes to a halt. This phenomenon of divergence in a finite duration of a physical quantity-here the frequency of rebounds-carries in physics the name of "singularity in finite time". Another example of a singularity in finite time comes from the wave breaking phenomenon. In the vicinity of this subtle instant preceding breaking, the temporal derivative of the free surface of water diverges as the inverse of the square root of time. A geometric representation of this singular behavior is the cusp, the simplest catastrophe depending on two parameters as classified in his Catastrophes Theory by RenÃ© Thom. The drawing of the cusp shows that for certain time and space positions, the surface is a single valued function, whereas outside this domain, it takes three possible values. Therefore, the cusp gives a good representation of the tilting of the wave before breaking. The deliminating curves between these two behavior are called caustics and merge at the singularity point. These lines share in fact the exact mathematical description of the pattern drawn by focusing light at the bottom of a cup after reflecting off the cup wall. My wish during my SCIENTIFIC DELIRIUM MADNESS residency was to create an art piece with the intention to share the contemplation of this subtle and fragile instant where the singularity arises, where the curvature of forms changes sign, where waves tilt over. To extend this infinitely small duration, the singularity will be unfolded-as mathematicians say. The result of my creative process is an array of wooden rods whose arrangement makes straight rays to cooperate and interfere in an apparent continuous and smooth motion, giving rise to a regular but cusped surface that sifts and transforms our own perception of the world around it. Like a filter, the CUSP will let wind, sound, light and thoughts to go through its beams. Fig. 1. The CUSP under the full moon in the Djerassi land. (Â© Patrice Le Gal. Photo: Kristen Stipanov.

### The Universal Aspect Ratio of Vortices in Rotating Stratified Flows: Theory and Simulation

We derive a relationship for the vortex aspect ratio $\alpha$ (vertical
half-thickness over horizontal length scale) for steady and slowly evolving
vortices in rotating stratified fluids, as a function of the Brunt-Vaisala
frequencies within the vortex $N_c$ and in the background fluid outside the
vortex $\bar{N}$, the Coriolis parameter $f$, and the Rossby number $Ro$ of the
vortex: $\alpha^2 = Ro(1+Ro) f^2/(N_c^2-\bar{N}^2)$. This relation is valid for
cyclones and anticyclones in either the cyclostrophic or geostrophic regimes;
it works with vortices in Boussinesq fluids or ideal gases, and the background
density gradient need not be uniform. Our relation for $\alpha$ has many
consequences for equilibrium vortices in rotating stratified flows. For
example, cyclones must have $N_c^2 > \bar{N}^2$; weak anticyclones (with $|Ro|
\bar{N}^2$. We verify our relation for $\alpha$ with numerical simulations of
the three-dimensional Boussinesq equations for a wide variety of vortices,
including: vortices that are initially in (dissipationless) equilibrium and
then evolve due to an imposed weak viscous dissipation or density radiation;
anticyclones created by the geostrophic adjustment of a patch of locally mixed
density; cyclones created by fluid suction from a small localised region;
vortices created from the remnants of the violent breakups of columnar
vortices; and weakly non-axisymmetric vortices. The values of the aspect ratios
of our numerically-computed vortices validate our relationship for $\alpha$,
and generally they differ significantly from the values obtained from the
much-cited conjecture that $\alpha = f/\bar{N}$ in quasi-geostrophic vortices.Comment: Submitted to the Journal of Fluid Mechanics. Also see the companion
paper by Aubert et al. "The Universal Aspect Ratio of Vortices in Rotating
Stratified Flows: Experiments and Observations" 201

### The Universal Aspect Ratio of Vortices in Rotating Stratifi?ed Flows: Experiments and Observations

We validate a new law for the aspect ratio $\alpha = H/L$ of vortices in a
rotating, stratified flow, where $H$ and $L$ are the vertical half-height and
horizontal length scale of the vortices. The aspect ratio depends not only on
the Coriolis parameter f and buoyancy (or Brunt-Vaisala) frequency $\bar{N}$ of
the background flow, but also on the buoyancy frequency $N_c$ within the vortex
and on the Rossby number $Ro$ of the vortex such that $\alpha = f \sqrt{[Ro (1
+ Ro)/(N_c^2- \bar{N}^2)]}$. This law for $\alpha$ is obeyed precisely by the
exact equilibrium solution of the inviscid Boussinesq equations that we show to
be a useful model of our laboratory vortices. The law is valid for both
cyclones and anticyclones. Our anticyclones are generated by injecting fluid
into a rotating tank filled with linearly-stratified salt water. The vortices
are far from the top and bottom boundaries of the tank, so there is no Ekman
circulation. In one set of experiments, the vortices viscously decay, but as
they do, they continue to obey our law for $\alpha$, which decreases over time.
In a second set of experiments, the vortices are sustained by a slow continuous
injection after they form, so they evolve more slowly and have larger |Ro|, but
they also obey our law for $\alpha$. The law for $\alpha$ is not only validated
by our experiments, but is also shown to be consistent with observations of the
aspect ratios of Atlantic meddies and Jupiter's Great Red Spot and Oval BA. The
relationship for $\alpha$ is derived and examined numerically in a companion
paper by Hassanzadeh et al. (2012).Comment: Submitted to the Journal of Fluid Mechanics. Also see the companion
paper by Hassanzadeh et al. "The Universal Aspect Ratio of Vortices in
Rotating Stratifi?ed Flows: Theory and Simulation" 201

### L'instabilitÃ© elliptique en gÃ©ophysique

L'Ã©tude de l'instabilitÃ© elliptique a Ã©tÃ© motivÃ©e par des problÃ¨mes rencontrÃ©s en aÃ©rodynamique (instabiltÃ©s secondaires diverses, vortex de bout d'ailes, turbulence), mais un autre intÃ©rÃªt suscitÃ© par cette instabilitÃ© elliptique relÃ¨ve de la gÃ©ophysique. En effet, lorsqu'une planÃ¨te tourne autour d'un soleil, et (ou) qu'une lune tourne autour de la planÃ¨te, le noyau liquide de celle-ci subit une dÃ©formation elliptique (une marÃ©e) causÃ©e par le champ de gravitation. La rotation de la planÃ¨te sur son axe pourrait alors faire rÃ©sonner des ondes de Kelvin dans le noyau liquide ce qui rÃ©sulterait en l'apparition de l'instabilitÃ© elliptique. Nos rÃ©sultats expÃ©rimentaux et thÃ©oriques montrent qu'en effet, le mode dit de "spin-over" apparaÃ®t au seuil de l' instabilitÃ© dans la gÃ©omÃ©trie sphÃ©rique et qu'une dynamique intermittente existe Ã plus haut nombre de Reynolds. Finalement, une expÃ©rience sous champ magnÃ©tique et utilisant un mÃ©tal liquide, met en Ã©vidence la gÃ©nÃ©ration d'un champ magnÃ©tique directement induit par l'instabilitÃ©

### Elliptical instability in hot Jupiter systems

Several studies have already considered the influence of tides on the
evolution of systems composed of a star and a close-in companion to tentatively
explain different observations such as the spin-up of some stars with hot
Jupiters, the radius anomaly of short orbital period planets and the
synchronization or quasi-synchronization of the stellar spin in some extreme
cases. However, the nature of the mechanism responsible for the tidal
dissipation in such systems remains uncertain. In this paper, we claim that the
so-called elliptical instability may play a major role in these systems,
explaining some systematic features present in the observations. This
hydrodynamic instability, arising in rotating flows with elliptical
streamlines, is suspected to be present in both planet and star of such
systems, which are elliptically deformed by tides. The presence and the
influence of the elliptical instability in gaseous bodies, such as stars or hot
Jupiters, are most of the time neglected. In this paper, using numerical
simulations and theoretical arguments, we consider several features associated
to the elliptical instability in hot-Jupiter systems. In particular, the use of
ad hoc boundary conditions makes it possible to estimate the amplitude of the
elliptical instability in gaseous bodies. We also consider the influence of
compressibility on the elliptical instability, and compare the results to the
incompressible case. We demonstrate the ability for the elliptical instability
to grow in the presence of differential rotation, with a possible synchronized
latitude, provided that the tidal deformation and/or the rotation rate of the
fluid are large enough. Moreover, the amplitude of the instability for a
centrally-condensed mass of fluid is of the same order of magnitude as for an
incompressible fluid for a given distance to the threshold of the instability.
Finally, we show that the assumption of the elliptical instability being the
main tidal dissipation process in eccentric inflated hot Jupiters and
misaligned stars is consistent with current data.Comment: Icarus (2013) http://dx.doi.org/10.1016/j.icarus.2012.12.01

### Flow-induced vibrations of high mass ratio flexible filaments freely hanging in a flow

8 p.The behavior of high mass ratio flexible filaments freely hanging in steady horizontal uniform flows is experimentally and theoretically investigated. When the flow velocity is small, static equilibrium states, where the filaments are inclined to the flow, are observed. Then, above a critical value of the wind velocity, the filaments exhibit periodic oscillations in the vertical plane. The problem is theoretically addressed considering the beam theory equations for the filament dynamics where the action of the flowing fluid is modeled using semi-empirical expressions. These equations are first solved for the stationary equilibrium states. Then, the stability of these steady solutions relatively to small perturbations is analyzed. A good agreement between experimental and theoretical results is found

### Experimental study of internal wave generation by convection in water

We experimentally investigate the dynamics of water cooled from below at 0^oC
and heated from above. Taking advantage of the unusual property that water's
density maximum is at about 4^oC, this set-up allows us to simulate in the
laboratory a turbulent convective layer adjacent to a stably stratified layer,
which is representative of atmospheric and stellar conditions. High precision
temperature and velocity measurements are described, with a special focus on
the convectively excited internal waves propagating in the stratified zone.
Most of the convective energy is at low frequency, and corresponding waves are
localized to the vicinity of the interface. However, we show that some energy
radiates far from the interface, carried by shorter horizontal wavelength,
higher frequency waves. Our data suggest that the internal wave field is
passively excited by the convective fluctuations, and the wave propagation is
correctly described by the dissipative linear wave theory

### Transition to turbulence of the Batchelor flow in a rotor/stator device

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 spaceâ€“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

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