Basile Gallet


I am a physicist working at the "Service de Physique de l'Etat Condensé", CEA Saclay, France. I study the energetics of natural flows using the tools of nonlinear physics and experimental fluid mechanics. I often participate to the Geophysical Fluid Dynamics summer program of the Woods Hole Oceanographic Institution. I am a member of the Climate Modelling Alliance (CLIMA).


News:

02/2020: Our scaling theory for baroclinic turbulence has been published in PNAS.

09/2019: I am honored to have been awarded the Young Scientist Prize at the European Turbulence Conference in Torino.

08/2018: Our experimental work on radiatively driven convection and observation of the "ultimate" regime has been published in PNAS.

11/2017: Our work on the turbulent saturation of the dynamo effect soon to appear in Physical Review Letters.

09/2017: I received an ERC starting grant for the coming 5 years!

09/2016: I am very grateful to the Woods Hole Oceanographic Institution for awarding me the Distinguished Scholar Award.

Research interests

Baroclinic turbulence

Collaborators: R. Ferrari, CLIMA collaboration.

The mean state of the atmosphere and ocean is set through a balance between external forcing (radiation, winds, heat and freshwater fluxes) and the emergent turbulence, which transfers energy to dissipative structures. The forcing gives rise to jets in the atmosphere and currents in the ocean, which spontaneously develop turbulent eddies through the baroclinic instability. A critical step in the development of a theory of climate is to properly include the eddy-induced turbulent transport of properties like heat, moisture, and carbon. Through a combination of numerical and analytical work, we have developed a scaling theory that predicts the scaling behavior of the eddy diffusivity associated with baroclinic turbulence. The theory can be used as a quantitative parameterization in the case of meridionally dependent forcing, in the fully turbulent regime.

Radiatively driven convection

Collaborators: S. Aumaître, B. Miquel, V. Bouillaut, S. Lepot

Turbulent convection is ubiquitous in geophysical and astrophysical contexts: It drives winds in the atmosphere and currents in the ocean, it generates magnetic fields inside planets and stars, and it triggers supernova explosions inside collapsing stellar cores. In many such natural flows, convection is driven by the absorption of incoming radiation (light or neutrinos). We designed an experiment to reproduce such radiatively driven convection in the laboratory. Our radiative heating setup achieves the “ultimate” regime of turbulent convection, which is believed to be the one relevant to many natural flows. Such experiments can yield the constitutive laws of turbulent convection, to be implemented into geophysical and astrophysical models.


Wave mean-flow interactions

Collaborators: W.R. Young, S. Aumaître, T. Humbert, K. Seshasayanan.

In the Ocean: Swell are long surface waves propagating on the surface of oceans. Swell is generated in stormy regions of the globe and can travel for approximately one week before breaking on distant beaches. We have shown that this propagation is strongly affected by surface currents. This explains the inference of swell sources on land by Munk et al. (1959): because of refraction by surface currents, the swell did not propagate on great-circle routes, creating a mirage for the California-based observers.


In the laboratory: We reproduced such wave refraction by a vortex in the laboratory. We focused on the nonlinear regime of wave refraction: when strong enough surface waves are refracted by a vortex, the vortex gets distorted by the strong radiation pressure forces. This distortion in turn weakens the wave refraction: the laws of refraction then strongly depend on the wave intensity. When the vortex shown in the top-left panel interacts with the wave field in the top-right panel, we observe wave refraction (bottom-right) together with wave-induced vortex recoil (bottom-left).

Rotating and magnetohydrodynamic turbulence

Collaborators: C.R. Doering (theory); F. Moisy, P.-P. Cortet, A. Campagne, N. Machicoane (experiments).

Theory: When turbulent flows are subject to either rapid global rotation or a strong external magnetic field, they tend to become two-dimensional. How far does this two-dimensionalization proceed? Using rigorous analytical methods, I proved that such turbulence can become exactly two-dimensional for strong enough magnetic field or rotation. This has dramatic consequences for the energy dissipation rate of such flows: when the flow is 2D, it dissipates energy at a laminar rate, i.e., much less efficiently than 3D turbulence (by a ratio equal to the Reynolds number).

Experiments: how much energy does rotating turbulence dissipate?

We tested the above-mentioned theoretical predictions on the GYROFLOW rotating platform, focusing on the statistics of stationary rotating turbulence. We performed the first direct measurements of the power injected into rotating turbulence, showing that the two-dimensionalization of the velocity field is accompanied by a dramatic decrease in energy dissipation, by a factor of at least 10.

These studies were combined with additional Particle image velocimetry (PIV) experiments aimed at studying the cyclone-anticyclone asymmetry and the transition from a direct energy cascade to an inverse one in rapidly rotating flows.

The dynamo effect

Collaborators: K. Seshasayanan, S. Fauve, F. Pétrélis, the Von Karman Sodium collaboration

I am a member of the Von Karman Sodium collaboration. This experiment generates magnetic field through dynamo action in a turbulent flow of liquid sodium. My main contribution was to establish a connexion between randomly reversing magnetic fields, such as the geodynamo, and hemispherically localized magnetic fields, such as the remanent magnetic field of Mars. We then confirmed this prediction experimentally, with the first observation of localized dynamo magnetic fields.

Other works

My research in MHD also includes several smaller scale liquid metal experiments, together with numerical simulations aimed at studying random reversals of large-scale fields generated over a turbulent background. Other works include granular matter experiments, an original instability destabilizing rotating and accreting flows, the influence of ferromagnetic boundaries on the threshold for dynamo action, the stability of stratified flows at low-Péclet number and wave turbulence. I am also part of the SHREK collaboration for the experimental study of superfluid turbulence.