Direct Numerical Simulation of structural vacillation in the transition to geostrophic turbulence

The onset of small-scale fluctuations around a steady convection pattern in a rotating baroclinic annulus filled with air is investigated using Direct Numerical Simulation. In previous laboratory experiments of baroclinic waves, such fluctuations have been associated with a flow regime termed Structural Vacillation which is regarded as the first step in the transition to fully-developed geostrophic turbulence.Comment: 6 page

DNS of bifurcations in an air-filled rotating baroclinic annulus

Three-dimensional Direct Numerical Simulation (DNS) on the nonlinear dynamics and a route to chaos in a rotating fluid subjected to lateral heating is presented here and discussed in the context of laboratory experiments in the baroclinic annulus. Following two previous preliminary studies by Maubert and Randriamampianina, the fluid used is air rather than a liquid as used in all other previous work. This study investigated a bifurcation sequence from the axisymmetric flow to a number of complex flows. The transition sequence, on increase of the rotation rate, from the axisymmetric solution via a steady, fully-developed baroclinic wave to chaotic flow followed a variant of the classical quasi-periodic bifurcation route, starting with a subcritical Hopf and associated saddle-node bifurcation. This was followed by a sequence of two supercritical Hopf-type bifurcations, first to an amplitude vacillation, then to a three-frequency quasi-periodic modulated amplitude vacillation (MAV), and finally to a chaotic MAV\@. In the context of the baroclinic annulus this sequence is unusual as the vacillation is usually found on decrease of the rotation rate from the steady wave flow. Further transitions of a steady wave with a higher wave number pointed to the possibility that a barotropic instability of the side wall boundary layers and the subsequent breakdown of these barotropic vortices may play a role in the transition to structural vacillation and, ultimately, geostrophic turbulence.Comment: 31 page

New World Hantavirus in Humans, French Guiana

International audienceNew World hantavirus in humans, French Guiana

Instabilité elliptique sous champ magnétique

Nous présentons une approche théorique, expérimentale et numérique de la dynamique de l'instabilité elliptique dans un fluide tournant conducteur lorsque qu'un champ magnétique est imposé le long de l'axe de rotation. Nous démontrons que le taux de croissance de l'instabilité et le carré de l'amplitude du champ induit diminuent linéairement avec le carré du champ appliqué. Une dynamique non-linéaire particulièrement riche est mise en évidence. Enfin, l'éventualité d'une dynamo par instabilité elliptique est discutée

DNS and experimental study of the transition to chaos in the rotating baroclinic annulus

We discuss the nonlinear dynamics and routes to chaos of baroclinic waves as found in a laboratory experiment, the baroclinic annulus, and in Direct Numerical Simulation (DNS). Thermal convection in a rapidly rotating fluid subject to differential heating occurs in many systems, ranging from the atmosphere to roating machinery. Our focus is the transition sequence from a steady wave to chaotic mixed-mode vacillations via amplitude vacillation. The main techniques used are phase space reconstruction and a phase coherence measure of the different waves to estimate the strength of the nonlinear wave-wave interactions. While experimental evidence tends to show the onset of amplitude vacillation and further bifurcations towards a mode transition to next-lower mode, recent numerical solutions have shown vacillation and modulated vacillation before a mode transition to the next-higher mode. These apparently inconsistent results will be discussed in detail