arXiv subjects

Ludovic Huguet

Publications and source records attributed to Ludovic Huguet.

6 recordsLinked to original sources

arXiv2026

Onset of dynamo action in planetesimals

Several meteorites have been found to carry primary remanent magnetizations imparted by fields generated within planetesimal cores during the early solar system. Thermal evolution models have shown that thermal convection likely drove the dynamo in the early evolution of these bodies. In such small cores, the magnetic Reynolds number is thought to be close to the threshold for dynamo action. However, the critical value of the magnetic Reynolds number, meaning its value at the onset, is also poorly constrained in dynamo simulations. We perform dynamo simulations to investigate the onset of the dynamo at different Ekman ($E$) and magnetic Prandtl ($Pm$) numbers, in a quasi-full sphere. Along two empirical paths in this $(E,Pm)$ space, the onset of dynamo action depends on whether the magnetic field is initially strong or weak. The onset of the dynamo occurs at larger supercriticality (ratio between the Rayleigh number ($Ra$) and its critical value ($Ra_c$)) when moving toward more realistic parameter values. Once extrapolated to planetesimal core conditions, the supercriticality for the onset of the dynamo is about $10^3$ for a strong initial magnetic field (meaning magnetic energy is of the same order of magnitude or larger than the kinetic energy). For a weak initial field, the required $Ra/Ra_c$ would be larger. Compared to previous estimates, this revised criterion facilitates dynamo activity, and so extends the lifetime of the magnetic field compared to previous models and allows the generation of a magnetic field in smaller planetesimals.

astro-ph.EP↗

arXiv2026

Geomagnetic signatures of the slurry F-layer inferred from dynamo simulations

Seismic observations indicate that the lowermost portion of Earth's liquid core is density stratified. The existence of this so-called F-layer challenges classical theories of core dynamics, where the geodynamo process that generates Earth's main magnetic field is assumed to be powered by heat and light element release at the inner core boundary. The seismically-inferred thickness, density, and velocity anomaly can be reproduced by a dynamical model that represents the F-layer as a two-phase two-component slurry on the liquidus, with a ``snow'' of solid iron particles falling through a quasi-static iron-oxygen liquid. Here, we present the first fluid dynamical simulations of thermochemically driven rotating convection and dynamo action that include a simple representation of the stratified slurry F-layer at the base of the spherical shell geometry. We show that the F-layer can create a barrier to columnar quasi-geostrophic flow, which is expressed near the core surface as a migration of peak radial and azimuthal flow speeds to lower latitudes as the thickness and stratification strength increase. In dynamo simulations, this effect induces polar minima in the radial magnetic field at the outer boundary ($B_r$) that strengthen and deepen with increasing stratification, and peaks in latitudinal profiles of $B_r$ moving to lower latitudes with reduced temporal variability. The geomagnetic signature of the F-layer is most prominent in time-averaged $B_r$, when resolved to at least spherical harmonic degree 5, and a trend of increasingly negative zonal degree 3 and 5 Gauss coefficients as the F-layer thickness and stratification strength increase. Our results suggest that an F-layer thickness of 600~km is incompatible with geomagnetic observations and favour weak stratification (normalised Brunt-Väisälä frequency $<1$) and a layer $<400$~km thick.

physics.geo-ph↗

arXiv2026

On the concentration distribution in turbulent thermals

Turbulent thermals emerge in a wide variety of geophysical and industrial flows, such as atmospheric cumulus convection and pollutant dispersal in oceans and lakes. When a buoyant fluid mass rises, or sinks, heat and mass transfers occur by the engulfment of the fresh surrounding fluid inside the thermal - a process that spans over multiple scales from macroscopic entrainment of ambient fluid to microscopic diffusive processes. Turbulent thermals are typically investigated through their integral properties (radius, depth, entrainment rate). However, mixing processes depend on the internal distribution of concentration or temperature inside a thermal, which remains poorly constrained. Here, we use laboratory fluid dynamics experiments and direct numerical simulations to investigate the mixing of a passive scalar in turbulent thermals with large Reynolds numbers. We track the evolution of the concentration field, computing its moments and the probability density function. The concentration distribution exhibits self-similarity over time, except at high concentrations, possibly because of the presence of undiluted cores. These distributions are well approximated by an exponential probability density function. Although diffusion has a strong effect on the spatial structure of the concentration field, we observe no significant effect of diffusivity on the concentration distributions in the investigated range of Peclet numbers.

physics.flu-dyn↗

arXiv2024

Solid-liquid phase change in planetary cores

The ubiquitous phenomena of crystallization and melting occur in various geophysical contexts across many spatial and temporal scales. In particular, they take place in the iron core of terrestrial planets and moons, profoundly influencing their dynamics and magnetic field generation. Crystallization and melting entail intricate multiphase flows, buoyancy effects, and out-of-equilibrium thermodynamics, posing challenges for theoretical modeling and numerical simulations. Besides, due to the inaccessible nature of the planetary deep interior, our understanding relies on indirect data from seismology, mineral physics, geochemistry, and magnetism. Consequently, phase-change-driven flows in planetary cores constitute a compelling yet challenging area of research. This paper provides an overview of the role of laboratory fluid dynamics experiments in elucidating the solid-liquid phase change phenomena occurring thousands of kilometers beneath our feet and within other planetary depths, along with their dynamic repercussions. Drawing parallel with metallurgy, it navigates through all scales of phase change dynamics, from microscopic processes (nucleation and crystal growth) to macroscopic consequences (solid-liquid segregation and large-scale flows). The review delves into the two primary planetary solidification regimes, top-down and bottom-up, and elucidates the formation of mushy and/or slurry layers in the various relevant configurations. Additionally, it outlines remaining challenges, including insights from ongoing space missions poised to unveil the diverse planetary regimes.

astro-ph.EP↗

arXiv2020

Dynamics of a reactive spherical particle falling in a linearly stratified fluid

Motivated by numerous geophysical applications, we have carried out laboratory experiments of a reactive (i.e. melting) solid sphere freely falling by gravity in a stratified environment, in the regime of large Reynolds ($Re$) and Froude numbers. We compare our results to non-reactive spheres in the same regime. First, we confirm for larger values of $Re$, the stratification drag enhancement previously observed for low and moderate Re (Magnaudet and Mercier, 2020). We also show an even more significant drag enhancement due to melting, much larger than the stratification-induced one. We argue that the mechanism for both enhancements is similar, due to the specific structure of the vorticity field sets by buoyancy effects and associated baroclinic torques, as deciphered for stratification by Zhang et al. (2019). Using particle image velocimetry, we then characterize the long-term evolution (at time $t \gg 1/N$ with $N$ the Brünt-Väisälä frequency) of the internal wave field generated by the wake of the spheres. Measured wave field is similar for both reactive and inert spheres: indeed, each sphere fall might be considered as a quasi impulsive source of energy in time and the horizontal direction, as the falling time (resp. the sphere radius) is much smaller than $N$ (resp. than the tank width). Internal gravity waves are generated by wake turbulence over a broad spectrum, with the least damped component being at the Brünt-Väisälä frequency and the largest admissible horizontal wavelength. About 1% of the initial potential energy of each sphere is converted in to kinetic energy in the internal waves, with no significant dependence on the Froude number over the explored range.

physics.flu-dyn↗

arXiv2019

Experimental study of convection in the compressible regime

An experiment of thermal convection with significant compressible effects is presented. The high-gravity environment of a centrifuge and the choice of xenon gas enable us to observe an average adiabatic temperature gradient up to 3.5 K cm$^{-1}$ over a 4 cm high cavity. At the highest rotation rate investigated, 9990 rpm, the superadiabatic temperature difference applied to the gas layer is less than the adiabatic temperature difference. The convective regime is characterized by a large Rayleigh number, about 10$^{12}$, and dominant Coriolis forces (Ekman number of order 10$^{-6}$). The analysis of temperature and pressure fluctuations in our experiments shows that the dynamics of the flow is in a quasi-geostrophic regime. Still, a classical power law (exponent 0.3 $\pm$ 0.04) is observed between the Nusselt number (dimensionless heat flux) and the superadiabatic Rayleigh number (dimensionless superadiabatic temperature difference). However, a potential hysteresis is seen between this classical high flux regime and a lower heat flux regime. It is unclear whether this is due to compressible or Coriolis effects. In the transient regime of convection from an isothermal state, we observe a local decrease of temperature which can only be explained by adiabatic decompression.

physics.flu-dyn↗