arXiv subjects

Ludovico Fossà

Publications and source records attributed to Ludovico Fossà.

5 recordsLinked to original sources

arXiv2026

Elastic pseudoturbulence induced by low-Galilei settling spheres

In this Letter, we show how a suspension of light solid spheres settling through a polymer solution results in a chaotic and highly-intermittent state. By leveraging particle-resolved direct numerical simulations, we investigate the effect of increasing polymer relaxation time and Deborah number $De$ on viscoelastic sedimentation at a low density ratio $ρ_s/ρ_f=5$ and a low Galilei number $Ga=3.16$. Even at moderate $De$, the spheres form gravity-aligned clusters and settle faster, while the polymer stresses energize the large scales of motion. The onset of elastic turbulence and intermittency is signaled by a $-4$ spectral scaling in the high-wavenumber range and by nonlinear high-order exponents of the velocity structure functions. These results indicate that viscoelastic effects induce pseudoturbulence in the presence of viscous-dominated sedimentation.

physics.flu-dyn↗

arXiv2026

A suspension of heavy Kolmogorov-size spheres suppresses the inertial cascade in homogeneous and isotropic turbulence

The effect of Kolmogorov-size spherical particles on homogeneous and isotropic turbulence is investigated using particle-resolved direct numerical simulations at an unladen Taylor-scale Reynolds number of $150$. Four monodisperse suspensions of particles with identical diameter and volume fraction $10^{-3}$ are considered, while the particle-to-fluid density ratio varies between $100$ and $1500$ and the mass fraction between $0.1$ and $0.6$. As particle inertia increases, the energy spectrum departs from the canonical Kolmogorov $κ^{-5/3}$ scaling and approaches a peculiar regime with $κ^{-1}$. In this limit, the nonlinear energy transfer is strongly suppressed and the kinetic energy balance is dominated by the fluid-solid interaction and the viscous dissipation. Consistently, the second-order structure function shows logarithmic scaling at separations larger than the particle diameter, indicating velocity decorrelation. Increasing particle inertia promotes axial strain and vortex compression in the vicinity of the particles and enhances the particle-fluid relative velocity. Particle clustering is maximum when the Stokes number based on the Kolmogorov time scale is $O(1)$ and weakens as the density ratio and the Stokes number increase, with the volume and the population of the clusters decreasing when inertia is enhanced. When clustering occurs, particles preferentially sample regions of high strain and low vorticity.

physics.flu-dyn↗

arXiv2025

Compressible boundary layers over isotropic porous surfaces

A compressible laminar boundary layer developing over an isotropic porous substrate is investigated by asymptotic and numerical methods. The substrate is modeled as an array of cubes. The momentum and enthalpy balance equations are derived by volume averaging. The self-similar solution proposed by Tsiberkin (2018) [Transp. Porous Media 121(1):109-120] for streamwise-growing permeability is extended to include compressibility, heat conduction and a nonlinear drag. The velocity profile shows an inflection point at the free fluid-porous interfacial layer, below which it decreases to zero. A marked reduction of the adiabatic recovery temperature of the fluid and the velocity gradient at the interface is observed for high porosity, large grains and relatively high Mach numbers. The temperature imposed at the bottom of the porous substrate has a negligible influence on the shear stresses.

physics.flu-dyn↗

arXiv2023

Receptivity of compressible boundary layers over porous surfaces

Supersonic pre-transitional boundary layers flowing over porous flat and concave surfaces are studied using numerical and asymptotic methods. The porous wall is composed of thin equally-spaced cylindrical microcavities. The flow is perturbed by small-amplitude, free-stream vortical disturbances of the convected gust type. From the proximity of the leading edge, these external agents generate the compressible Klebanoff modes, i.e. low-frequency disturbances of the kinematic and thermal kind that grow algebraically downstream. For Klebanoff modes with a spanwise wavelength comparable with the boundary-layer thickness, the porous surface has a negligible effect on their growth. When the spanwise wavelength is instead larger than the boundary-layer thickness, these disturbances are effectively attenuated by the porous surface. For a specified set of frequency and wavelengths, the Klebanoff modes evolve into oblique Tollmien-Schlichting waves through a leading-edge-adjustment receptivity mechanism. The wavenumber of these waves is only slightly modified over the porous surface, while the growth rate increases, thus confirming previous experimental results. An asymptotic analysis based on the triple-deck theory confirms these numerical findings. When the wall is concave, the amplitude of the Klebanoff modes is enhanced by the wall curvature and is attenuated by the wall porosity during the initial development.

physics.flu-dyn↗

arXiv2022

Small-Scale air turbulence structure, microphysical time scales and local supersaturation balance at a warm Cloud Top Boundary

Recent results have shown that there is an acceleration in the spread of the size distribution of droplet populations in the region bordering the cloud and undersaturated ambient. We have analyzed the supersaturation balance in this region, which is typically a highly intermittent shearless turbulent mixing layer, under a condition where there is no mean updraft. We have investigated the evolution of the cloud-clear air interface and of the droplets therein via direct numerical simulations. We have compared horizontal averages of the phase relaxation, evaporation, reaction and condensation times within the cloud-clear air interface for the size distributions of the initial monodisperse and polydisperse droplets. For the monodisperse population, a clustering of the values of the reaction, phase and evaporation times, that is around 20-30 seconds, is observed in the central area of the mixing layer, just before the location where the maximum value of the supersaturation turbulent flux occurs. This clustering of values is similar for the polydisperse population but also includes the condensation time. The mismatch between the time derivative of the supersaturation and the condensation term in the interfacial mixing layer is correlated with the planar covariance of the horizontal longitudinal velocity derivatives of the carrier air flow and the supersaturation field, thus suggesting that a quasi-linear relationship may exist between these quantities.

physics.ao-ph↗