[GTP] Two GTP seminars--Berengere Dubrulle

[Silvia Gentile]

Dear All,

Dr. Bérengère Dubrulle -- Centre national de la recherche scientifique-- 
will be visiting NCAR between August 10 and August 21.
She will be presenting the two talks below. For any questions, please 
contact Peter Sullivan or Annick Pouquet.

August 13, Foothills Laboratory 2, Room 1022
Lecture: 3:30pm

Statistical mechanics of axisymmetric turbulence: Theory and experiments

A turbulent flow is characterized by a very large number of degrees of
freedom which sets a number of practical challenges for simulations and
models. A long standing dream in turbulence theory is to devise a
"statistical mechanics" of turbulence, enabling the identification of a
few macroscopic global quantities (analog of temperature, pressure in
kinetic theory) describing the flow, thereby describing the effective
complexity of turbulence.
In my talk, I present such a theory, based on recent theoretical
breakthrough by Robert and Sommeria.  Specifically, I derive the
conservation laws of  an axisymmetric turbulent flow and a mixing
entropy characterizing the probability distribution of the turbulent
velocity. Then, using standard tools of statistical mechanics, I derive
the corresponding Gibbs distributions, the equilibrium states and
fluctuations around them. I show that the equilibrium states are
multi-stable and discuss the possible transitions between them.
The theoretical predictions are compared with experimental fields, and a
good agreement is found. I discuss the prespectives in terms of
turbulence models and effective numerical simulations.

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August 17, Center Green Laboratory 1, Room 3150
Lecture: 3:00pm

Dynamo in turbulent flows: Theory and experiments

The dynamo effect-the conversion of mechanical energy into magnetic
energy in a conducting fluid-is thought to be the source of many-perhaps
all cosmic magnetic fields (in the sun, in stars,  in black holes, in
galaxies, etc). In all these instances, the conducting medium is highly
turbulent. I present the challenges and the questions connecting
turbulence and dynamo. I discuss a stochastic model of turbulent dynamo,
and illustrate its predictions with numerical simulations.
In the second part of my talk, I discuss an experimental dynamo, the VKS
experiment, aiming at studying the dynamo in a turbulent fluid of liquid
sodium. Dynamical regimes are found, that mimics the regimes observed
in natural objects. The comparison with observations is discussed.

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Silvia Gentile
NCAR IMAGe