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Jérémie Vidal (Ph.D.)
CNRS Researcher, LGL-TPE (ENS Lyon - Univ. Lyon 1)
Email: jeremie [dot] vidal [at] ens-lyon [dot] fr
Please find below a short overview of my (interdisciplinary) research interests in Geophysics, Fluid Mechanics and Astrophysics.
Waves in geophysical flows
A large part of my research work is devoted to the study of waves in rotating fluids. Since these waves often play a fundamental role in the dynamics of geophysical flows, I study their fundamental properties using fluid-dynamics models.
A thermally stably stratified layer could exist below the Earth's core-mantle boundary (CMB), which may affect our ability to detect core flows from geomagnetic data. I developed some years ago the numerical code SINGE to investigate waves in spherical rotating cores in the presence of an outermost density stratification (which inhibits the radial motions below the CMB). I used it to explore the properties of the quasi-geostrophic modes (QG), which are almost invariant along the rotation axis and are sustained by core convection.
I have also developed a novel spectral Galerkin method to study waves in rotating fluid-filled ellipsoids (e.g. to model planetary deformations due to centrifugal and tidal effects). This code can account for buoyancy effects, core compressibility and ambient magnetic fields.
Penetration of the azimuthal velocity of a QG wave in an outermost stratified layer below the CMB.
Convection in geophysics (and astrophysics)
Convection is believed to sustain dynamo magnetic fields in planetary interiors (e.g. in the Earth's liquid core). Numercal models are widely used to simulate planetary convective flows but, so far, with parameters that are still far removed from the geophysical range. Thus, fundamental studies are still required to get physical insight into planetary conditions.
Vorticity field in rotating convection.
Many geophysical models of convection assume that the thermal and kinematic diffusivities are equal, but this is not in agreement with the molecular properties of liquid metals (e.g. iron in the Earth's core). Since the nature of the convection may be different, we investigated thermal convection of liquid metals in rotating full spheres. We found that rotating thermal convection exhibits (nonlinear) hysteresis cycles and subcriticality.
In geophysical interiors, the density usually depends on both temperature and chemical composition in light elements (which diffuse at very different rates). This leads to various convective instabilities, bearing the name of double-diffusive convection (DDC). It has been suggested that the Early Earth was prone to rotating DDC to explain the past magnetic field of the Earth before the nucleation of the inner core (as recorded at leat -3.6 Ga ago in Jack Hills zircons). In the early Earth the temperature profile was likely stably stratified and the compositional one unstably stratified, leading to fingering convection. We investigated the linear & nonlinear dynamics of rotating fingering convection in full spheres. Recently, we found that such flows are dynamo capable (AGU 2022).
Dynamo magnetic field (radial component) of rotating fingering convection at the CMB (AGU 2022).
Motivated by the dynamics of the top magma ocean, we investigated the effects of the radiation boundary condition on (rotating) thermal convection in thin spherical shells.
Tidal flows in planetary interiors
Planets (and stars) are usually rotating and subject to tidal forcing due to gravitational interactions in the presence of orbital partners. Tidal forcings could be important for the flow dynamics in some rapidly rotating bodies (e.g. the Early Earth, Mars or some Moons in the Solar system), because tidal effects can sustain turbulent flows and (possibly) dynamo magnetic fields. Yet, several physical ingredients are usually neglected in the models. Thus, I strive to develop more realistic physical models for a better planetary extrapolation.
I have studied in details the (tidal) elliptical instability, which is a flow instability excited by nonlinear interactions between fluid waves and the large-scale flow driven by tidal forcing in planetary cores. In particular, I have investigated the transition towards turbulence of tidally driven flows, showing that tidal forcing can sustain turbulent flows even in the presence of density stratification (as in the liquid core of the Early Earth or some intermediate-mass stars).
Existence (colors) of the tidal instability
Simulation of a tidally driven dynamo.
I showed that tidal forcing can sustain turbulence and large-scale dynamo magnetic fields in rapidly rotating fluids, even in the presence of a density stratification. This mechanism may explain the magnetism of some massive stars in astrophysics, or that of the Early Earth (before -3 Ga, when tidal effects were sufficiently strong to overcome diffusive effects in the liquid core).
Fluid dynamics of the lunar core
Measurements of the Earth-Moon distance (through Lunar Laser Ranging) are accurate enough to estimate the the dissipation in the lunar core. However, explaining this dissipation is still disputed as it requires some turbulence in the core. Motivated by the lunar application, we studied the mean (steady) flows in ellipsoids subject to precession or libration forcings. These mean flows exhibit strong geostrophic shears, which could sustain secondary turbulent flows partly accounting for the observed dissipation in the lunar core.
The origin of the Moon's magnetic field (generated billions years ago) remains enigmatic, as turbulent convection in the lunar core seems unable to generate magnetic fields with a sufficiently large amplitude. Thus, I explore alternative mechanisms that could have sustained dynamo action in the lunar core (e.g. precession in the non-spherical lunar core).
I have developed a novel mathematical/numerical method to study the onset of dynamo magnetic fields in triaxial ellipsoids.
I studied the transition towards turbulence of precession-driven flows in ellipsoids.
Moon's magnetic field as a function of time, as recorded in rocks brought back by Apollo missions. Its amplitude is larger than that of the magnetic field currently measured at the Earth's surface.
Experimental collaborations
Laboratory experiments can be used to reproduce some features of geophysical flows but, sometimes, experimental models are over-simplified. Hence, it is often worth combining experimental studies with theoretical (and numerical) models for extrapolating the results to geophysical (and astrophysical) conditions.
Laboratory and numerical studies of mechanically driven flows, performed in the achievable range of parameters (i.e. large deformations and overestimated diffusive effects), are often not in agreement with theoretical predictions representative of celestial fluid bodies (for extremely small deformation and vanishing diffusive effects). I have developed an asymptotic theory that reconciles theoretical predictions with laboratory (and numerical) results.
Values of the Ekman number and of the tidal equatorial deformation of the fluid region in models and planetary bodies.
Spheroidal experiment ZoRo (Zonal flows in Rotating Fluids).
As a member of the experimental collaboration ZoRo (developped in Grenoble by H.-C. Nataf), I study the theoretical properties of the acoustic modes in rapidly rotating gas-filled ellipsoids. Indeed, we aim to use the acoustic modes to image the large-scale components of the velocity in rotating fluid experiments.
Tidal flows & interactions with turbulent convection in stellar interiors
The magnetism of intermediate-mass stars has sparked the interest of astronomers for a long time. Recent spectropolarimetric surveys have detected surface magnetic fields in 5 to 10% of pre-main sequence and main-sequence massive stars. The origin of these fields remain unclear. I proposed that nonlinear tides could (partially?) explain the dearth of strongly magnetic stars in some short-period binaries, made of hot massive stars.
Frequency-reduction of the turbulent viscosity acting on the large-scale tidal flows tide in solar-type binaries. Red area: No frequency-reduction. Grey area: Linear reduction. Green area: Quadratic frequency reduction. Symbols show astrophysical measurements (where e is the orbital eccentricity).
Tidal dissipation is thought to play a crucial role in shaping the properties of short-period extrasolar planetary systems and close binary stars. Several mechanisms have been invoked, such as the interaction of large-scale tidal flows with turbulent convection (which could act as a turbulent viscosity in damping the tidal flows). Yet, the underlying physics is not fully understood, and the astrophysical applications are highly disputed. I studied the nonlinear interactions between turbulent convection and the large-scale tidal flow in stellar interiors.
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