arXiv subjects

Nicolás Pablo Müller

Publications and source records attributed to Nicolás Pablo Müller.

5 recordsLinked to original sources

arXiv2026

Dynamics of fast magnetosonic wave turbulence

Fast magnetosonic waves are among the fundamental oscillation modes of astrophysical plasmas. To study their dynamics, we carry out numerical simulations of the wave turbulence kinetic equation, which describes the evolution of the energy spectrum of a set of weakly nonlinear fast magnetosonic waves. This kinetic equation, which involves three-wave interactions, has recently been derived from compressible magnetohydrodynamics in the low-$β$ limit (Galtier 2023). It has an exact stationary solution, the Kolmogorov-Zakharov spectrum, corresponding to a direct energy cascade. Here we perform free decay simulations of the kinetic equation for which we propose a Kolmogorov-type phenomenology to explain the temporal decay laws of energy and integral length scale. In the forced simulations, we show that the cascade is in fact composed of a mixture of a forward cascade for counter-propagating waves, and a backward cascade for co-propagating waves, with the former being stronger than the latter. The Kolmogorov-Zakharov energy spectrum in $k^{-3/2}$ is found in the radial direction with an anisotropy due to the amplitude that depends on the angle relative to the strong mean magnetic field. We give the analytical expression of the Kolmogorov-Zakharov constant, which is numerically verified in the high Reynolds number limit. Our study provides a theoretical explanation for certain observations in the solar wind plasma (Zhao et al. 2022), where a regime of weak turbulence has been identified for fast magnetosonic waves, alongside a critical balance regime for strong Alfvén wave turbulence.

physics.plasm-ph↗

arXiv2026

Pressure and Size Dependence of Roton Emission and Vortex Creation by Moving Objects in He~II in $T \to 0$ Limit: Generalized Nonlocal Gross-Pitaevskii Model

In the framework of generalized, nonlocal Gross-Pitaevskii (GP) model, we study numerically the pressure- and size-dependent mechanisms of roton emission and vortex nucleation by objects moving in superfluid $^4$He. As far as the authors are aware, this is the first attempt to analyze the pressure dependence of these mechanisms and the associated critical velocities within a single theoretical framework. For each of several pressures in the range from 0 to the solidification pressure of $\approx25$~bar, we chose the parameters of the interatomic interaction potential such that the resulting excitation spectrum for the generalized, nonlocal GP equation approximates fairly accurately the pressure-dependent dispersion curve determined experimentally by Godfrin \textit{et al.}, Phys. Rev. B \textbf{103}, 104516 (2021). In the two-dimensional approximation, for circular obstacles (disks) moving in quiescent $^4$He, we calculated two critical velocities -- one corresponding to the roton emission and the other to the nucleation of quantized vortices -- as functions of pressure and the obstacle's size. We also comment briefly on three-dimensional simulations of the roton emission and vortex nucleation by moving spherical obstacles.

cond-mat.quant-gas↗

arXiv2026

Magnetic reversals in a geodynamo model with a stably-stratified layer

We study the process of magnetic reversals in the presence of a stably-stratified layer below the core-mantle boundary using direct numerical simulations of the incompressible magnetohydrodynamics equations under the Boussinesq approximation in a spherical shell. We show that the dipolar-multipolar transition shifts to larger Rayleigh numbers in the presence of a stably-stratified layer, and that the dipolar strength of the magnetic field at the core-mantle boundary increases due to the skin effect. By imposing an heterogeneous heat flux at the outer boundary, we break the equatorial symmetry of the flow, and show that different heat flux patterns can trigger different dynamo solutions, such as hemispheric dynamos and polarity reversals. Using kinematic dynamo simulations, we show that the stably-stratified layer leads to similar growth rates of the dipole and quadrupole components of the magnetic field, playing the role of a conducting boundary layer, favouring magnetic reversals, and a dynamics predicted by low-dimensional models.

physics.flu-dyn↗

arXiv2024

Lack of self-similarity in transverse velocity increments and circulation statistics in two-dimensional turbulence

We study the statistics of longitudinal and transverse structure functions, as well as velocity circulation in the inverse energy cascade of two-dimensional turbulence. By means of direct numerical simulations of the incompressible Navier-Stokes equations, we show that transverse structure functions exhibit an anomalous scaling, in contrast to the self-similar behavior of longitudinal ones. We derive an analytical relation that shows that the scaling exponents of transverse structure functions and velocity circulation are related in two-dimensional turbulence.

physics.flu-dyn↗

arXiv2020

Kolmogorov and Kelvin wave cascades in a generalized model for quantum turbulence

We performed numerical simulations of decaying quantum turbulence by using a generalized Gross-Pitaevskii equation, that includes a beyond mean field correction and a nonlocal interaction potential. The nonlocal potential is chosen in order to mimic He II by introducing a roton minimum in the excitation spectrum. We observe that at large scales the statistical behavior of the flow is independent of the interaction potential, but at scales smaller than the intervortex distance a Kelvin wave cascade is enhanced in the generalized model. In this range, the incompressible kinetic energy spectrum obeys the weak wave turbulence prediction for Kelvin wave cascade not only for the scaling with wave numbers but also for the energy fluxes and the intervortex distance.

cond-mat.other↗