arXiv subjects

Leonardo Primavera

Publications and source records attributed to Leonardo Primavera.

10 recordsLinked to original sources

arXiv2026

Scaling laws and local enhancements of buoyancy flux in stratified turbulent flows

In the presence of stratification, turbulent flows exhibit intermittency not only at small scales but also at large scales, comparable to the mean flow, as observed in the atmosphere and oceans. We study such flows through a large parametric exploration using direct numerical simulations of the Boussinesq equations with different forcing types. We examine two Prandtl numbers (1 and 6) and vary the Froude number ($Fr$) over a range of geophysical interest values, $0.01\le Fr \le 1$, corresponding to a variation in terms of the buoyancy Reynolds number ($R_{IB}$) of $0.06\le R_{IB} \le 2300$. We analyze the dependence on $R_{IB}$ of the buoyancy flux ($B_f$), the mixing efficiency, the shear parameters, and the vertical momentum flux. Strongly non-Gaussian tails in the spatio-temporal distribution of the $B_f$ are observed, with kurtosis reaching $\approx 10^2$, indicating the potential for stratified geophysical flows to be characterized by highly variable transport properties along the direction of gravity even under stable stratification. This is associated with long-time intermittent behavior of vertical velocity and temperature at large scale, which produces local turbulence and enhances dissipation and transport. We present evidence that the skewness of $B_f$ increases with $R_{IB}$ as a power-law and saturates in the passive-scalar limit. We also show that the domain-averaged $B_f$ exhibits two distinct trends: logarithmic growth with $R_{IB}$ and approach to a small offset as stratification strengthens. A simple model for the temporal evolution of energy and $B_f$ indicates that the defect between vertical and potential energy drives strong $B_f$ events. This trend directly leads to convective instabilities, the formation of two-dimensional and three-dimensional eddies, and rapid dissipation on a turnover timescale, allowing the energetic cycle to restart-also occurring in bursts.

physics.flu-dyn↗

arXiv2025

Measuring the properties of homogeneous turbulence in curved spacetimes

Turbulence in curved spacetimes in general, and in the vicinity of black holes (BHs) in particular, represents a poorly understood phenomenon that is often analysed employing techniques developed for flat spacetimes. We here propose a novel approach to study turbulence in strong gravitational fields that is based on the computation of structure functions on generic manifolds and is thus applicable to arbitrary curved spacetimes. In particular, we introduce, for the first time, a formalism to compute the characteristic properties of turbulence, such as the second-order structure function or the power spectral density, in terms of proper lengths and volumes and not in terms of coordinate lengths and volumes, as customarily done. By applying the new approach to the turbulent rest-mass density field from simulations of magnetised disc accretion onto a Kerr BH, we inspect in a rigorous way turbulence in regions close to the event horizon, but also in the disc, the wind, and in the jet. We demonstrate that the new approach can capture the typical behavior of an inertial-range cascade and that differences up to $40-80\%$ emerge in the vicinity of the event horizon with respect to the standard flat-spacetime approach. While these differences become smaller at larger distances, our study highlights that special care needs to be paid when analysing turbulence in strongly curved spacetimes.

gr-qc↗

arXiv2025

The Interaction of a Supernova Remnant with background interstellar turbulence

Supernova explosions (SNe) are among the most energetic events in the Universe. After the explosion, the material ejected by the Supernova expands throughout the interstellar medium (ISM) forming what is called Supernova Remnant (SNR). Shocks associated with the expanding SNR are sources of galactic cosmic rays, that can reach energy of the PeV order. In these processes, a key role is played by the magnetic field. It is known that the ISM is turbulent with an observed magnetic field of about a few $\mu$G, made by the superposition of a uniform and a fluctuating component. During the SNR expansion, the shock interacts with a turbulent environment, leading to a distortion of the shock front and a compression of the medium. In this work, we use the MagnetoHydroDynamics (MHD) PLUTO code to mimic the evolution of the blast wave associated with the SNR. We make a parametric study varying the level of density and magnetic field fluctuations in the interstellar medium, with the aim of understanding the best parameter values able to reproduce real observations. We introduce a novel analysis technique based on two-dimensional autocorrelation function $C({\ell})$ and the second order structure function $S_2(\ell)$, quantifying the level of anisotropy and the turbulence correlation lengths. By interpolating the autocorrelation function on a polar grid, we extract the power spectra of turbulence at the SNR. Finally, a preliminary comparison with Chandra observations of SN 1006 is also presented.

astro-ph.HE↗

arXiv2023

Multipoint Turbulence Analysis with Helioswarm

Exploration of plasma dynamics in space, including turbulence, is entering a new era of multi-satellite constellation measurements that will determine fundamental properties with unprecedented precision. Familiar but imprecise approximations will need to be abandoned and replaced with more advanced approaches. We present a preparatory study of the evaluation of second- and third-order statistics, using simultaneous measurements at many points. Here, for specificity, the orbital configuration of the NASA Helioswarm mission is employed in conjunction with three-dimensional magnetohydrodynamics numerical simulations of turbulence. The Helioswarm 9-spacecraft constellation flies virtually through the turbulence to compare results with the exact numerical statistics. We demonstrate novel increment-based techniques for the computation of (1) the multidimensional spectra and (2) the turbulent energy flux. This latter increment-space estimate of the cascade rate, based on the third-order Yaglom-Politano-Pouquet theory, uses numerous increment-space tetrahedra. Our investigation reveals that Helioswarm will provide crucial information on the nature of astrophysical turbulence.

physics.plasm-ph↗

arXiv2022

Turbulence generation by large-scale extreme vertical drafts and the modulation of local energy dissipation in stably stratified geophysical flows

We observe the emergence of strong vertical drafts in direct numerical simulations of the Boussinesq equations in a range of parameters of geophysical interest. These structures, which appear intermittently in space and time, generate turbulence and enhance kinetic and potential energy dissipation, providing a possible explanation for the observed variability of the local energy dissipation in the bulk of oceanic flows, and the modulation of its probability distribution function. We show how, due to the extreme drafts, in runs with Froude numbers observable in geophysical scenarios, roughly 10% of the domain flow can account for up to 50% of the global volume dissipation, reminiscent of estimates based on oceanic models.

physics.flu-dyn↗

arXiv2022

Magnetic Switchback Occurrence Rates in the Inner Heliosphere: Parker Solar Probe and 1 au

The subject of switchbacks, defined either as large angular deflections or polarity reversals of the magnetic field, has generated substantial interest in the space physics community since the launch of Parker Solar Probe (PSP) in 2018. Previous studies have characterized switchbacks in several different ways, and have been restricted to data available from the first few orbits. Here, we analyze the frequency of occurrence of switchbacks per unit distance for the first full eight orbits of PSP. In this work, are considered switchback only the events that reverse the sign of magnetic field relative to a regional average. A significant finding is that the rate of occurrence falls off sharply approaching the sun near 0.2 au (40 $R_\odot$), and rises gently from 0.2 au outward. The analysis is varied for different magnetic field cadences and for different local averages of the ambient field, confirming the robustness of the results. We discuss implications for the mechanisms of switchback generation. A publicly available database has been created with the identified reversals.

astro-ph.SR↗

arXiv2021

Connecting Large-Scale Velocity and Temperature Bursts with Small-Scale Intermittency in Stratified Turbulence

Non-Gaussian statistics of large-scale fields are routinely observed in data from atmospheric and oceanic campaigns and global models. Recent direct numerical simulations (DNSs) showed that large-scale intermittency in stably stratified flows is due to the emergence of sporadic, extreme events in the form of bursts in the vertical velocity and the temperature. This phenomenon results from the interplay between waves and turbulent motions, affecting mixing. We provide evidence of the enhancement of the classical small-scale (or internal) intermittency due to the emergence of large-scale drafts, connecting large- and small-scale bursts. To this aim we analyze a large set of DNSs of the stably stratified Boussinesq equations over a wide range of values of the Froude number ($Fr\approx 0.01-1$). The variation of the buoyancy field kurtosis with $Fr$ is similar to (though with smaller values than) the kurtosis of the vertical velocity, both showing a non-monotonic trend. We present a mechanism for the generation of extreme vertical drafts and vorticity enhancements which follows from the exact equations for field gradients.

physics.flu-dyn↗

arXiv2020

Single-particle Lagrangian statistics from direct numerical simulations of rotating-stratified turbulence

Geophysical fluid flows are predominantly turbulent and often strongly affected by the Earth's rotation, as well as by stable density stratification. Using direct numerical simulations of forced Boussinesq equations, we study the influence of these effects on the motion of fluid particles, focusing on cases where the frequencies associated with rotation and stratification (RaS), $N$ and $f$ respectively, are held at a fixed ratio $N/f=5$. As the intensity of RaS increases, a sharp transition is observed between a regime dominated by eddies to a regime dominated by waves, which can also be seemingly described by simply comparing the time scale $1/N$ and $τ_η$ (the Kolmogorov time scale). We perform a detailed study of Lagrangian statistics of acceleration, velocity and related quantities in the two regimes. The flow anisotropy induces a clear difference between particle motion in the horizontal and vertical directions. In the regime $Nτ_η<1$, acceleration statistics in both horizontal and vertical directions, exhibit well known characteristics of isotropic turbulence. In contrast for $Nτ_η>1$, they are directly influenced by imposed RaS. The Lagrangian velocity statistics exhibit visible anisotropy for all runs; nevertheless the degree of anisotropy becomes very strong in the regime $Nτ_η>1$. We find that in the regime $Nτ_η<1$, rotation enhances the mean displacement of particles in horizontal planes at short times, but inhibits them at longer times. This inhibition of horizontal displacement becomes stronger for $Nτ_η>1$, with no clear diffusive behavior. Displacements in the vertical direction are always inhibited. The inhibition becomes extremely strong when $Nτ_η>1$, with the particles almost being trapped horizontally.

physics.flu-dyn↗

arXiv2020

Effect of vegetation on the temperatures of Trappist-1 planets

TRAPPIST-1 is an ultra-cool dwarf hosting a system consisting of seven planets. While orbital properties, radii and masses of the planets are nowadays well constrained, one of the open fascinating issues is the possibility that an environment hospitable to life could develop on some of these planets. Here we use a simple formulation of an energy balance model that includes the vegetation coverage to investigate the possibility of life affecting the climate of the planets in the TRAPPIST-1 system. Results confirm that planet T-e has the best chance for a habitable world and indicate that vegetation coverage significantly affects the resulting temperatures and habitability properties. The influence of vegetation has been evaluated in different scenarios characterized by different vegetation types, land-sea distributions and levels of greenhouse effect. While changes in vegetation type produce small changes, about $0.1\%$, in the habitable surface fraction, different land-sea distributions, by also affecting the vegetation growth, produce different temperature distributions. Finally at latitudes where vegetation grows, the lowering of local albedo still represents a relevant contribution in settling the planetary temperature profiles even when levels of greenhouse effect higher than the Earth-like case are considered.

astro-ph.EP↗

arXiv2015

Sign singularity and flares in solar active region NOAA 11158

Solar Active Region NOAA 11158 has hosted a number of strong flares, including one X2.2 event. The complexity of current density and current helicity are studied through cancellation analysis of their sign-singular measure, which features power-law scaling. Spectral analysis is also performed, revealing the presence of two separate scaling ranges with different spectral index. The time evolution of parameters is discussed. Sudden changes of the cancellation exponents at the time of large flares, and the presence of correlation with EUV and X-ray flux, suggest that eruption of large flares can be linked to the small scale properties of the current structures.

astro-ph.SR↗