arXiv subjects

Stefano Brizzolara

Publications and source records attributed to Stefano Brizzolara.

5 recordsLinked to original sources

arXiv2026

Lagrangian Proper Orthogonal Decomposition

We introduce a modal representation for Lagrangian trajectories in turbulence, termed Lagrangian Proper Orthogonal Decomposition (LPOD). An ensemble of particle trajectories is used to construct velocity time series, which are normalized independently for each trajectory to isolate fluctuations. Principal Component Analysis is then applied to the resulting dataset, with temporal instances defining the feature space. The method is tested on trajectories from both direct numerical simulations of homogeneous isotropic turbulence and three-dimensional particle-tracking experiments, showing that the leading modes exhibit similar structures and energy distributions in both cases. Truncated reconstructions are obtained by combining modes and coefficients, rescaling the fluctuations, and integrating in time. For trajectories of the order of the integral time scale, single-particle dispersion and curvature statistics are accurately reproduced using a limited number of modes (c.a. 10), whereas capturing the tails of acceleration distributions requires a larger set (c.a. 30-60). Longer trajectories require progressively more modes for accurate reconstruction. These results suggest a possible route to data-driven generation of synthetic particle trajectories via stochastic sampling of the modal Lagrangian dynamics.

physics.flu-dyn↗

arXiv2024

Numerical Modeling of Liquid Wall Flows for Fusion Energy Applications Using Maxwell-Navier-Stokes Equations

During the Z-Pinch fusion process, electric current is injected into liquid metal from the plasma column, generating Lorentz forces that deform the liquid metal's free surface. Modeling this phenomenon is essential for assessing the feasibility of using liquid metal as an electrode wall in fusion devices. Traditionally, such problems, where liquid metal is exposed to electromagnetic forces, are modeled using magneto-hydrodynamic (MHD) formulation, which is more suitable for cases without external electric current penetration into liquid metals. MHD formulation typically models situations where liquid metal flows in the presence of an external magnetic field, with the initial magnetic field known and evolving over time via the magnetic induction equation. However, in Z-Pinch fusion devices, the electric current penetrates and traverses through the liquid metal, necessitating numerical calculations for the initial magnetic field. Additionally, the deformation of the liquid metal surface alters the current path's geometry and the resulting magnetic field, rendering traditional MHD formulations unsuitable. This work addresses this issue by directly solving Maxwell's equations, instead of the magnetic induction equation, in combination with Navier-Stokes equations, making it possible to predict the magnetic field even when the fluid is in motion. The Maxwell equations are solved in potential formulation alongside Navier-Stokes equations using a finite volume numerical method on a collocated grid arrangement. This proposed numerical framework successfully captures the deformation of the liquid metal's free surface due to the applied electric current.

physics.comp-ph↗

arXiv2023

Universal alignment in turbulent pair dispersion

Countless processes in nature and industry, from rain droplet nucleation to plankton interaction in the ocean, are intimately related to turbulent fluctuations of local concentrations of advected matter. These fluctuations can be described by considering the change of the separation between particle pairs, known as pair dispersion, which is believed to obey a cubic in time growth according to Richardson's theory. Our work reveals a universal, scale-invariant alignment between the relative velocity and position vectors of dispersing particles at a mean angle that we show to be a universal constant of turbulence. We connect the value of this mean angle to Richardson's traditional theory and find agreement with data from a numerical simulation and a laboratory experiment. While the Richardson's cubic regime has been observed for small initial particle separations only, the constancy of the mean angle manifests throughout the entire inertial range of turbulence. Thus, our work reveals the universal nature of turbulent pair dispersion through a geometrical paradigm whose validity goes beyond the classical theory, and provides a novel framework for understanding and modeling transport and mixing processes.

physics.flu-dyn↗

arXiv2021

Fiber Tracking Velocimetry for two-point statistics of turbulence

We propose and validate a novel experimental technique to measure two-point statistics of turbulent flows. It consists in spreading rigid fibers in the flow and tracking their position and orientation in time and therefore been named ``Fiber Tracking Velocimetry'' (FTV). By choosing different fiber lengths, i.e. within the inertial or dissipative range of scales, the statistics of turbulence fluctuations at the selected lengthscale can be probed accurately by simply measuring the fiber velocity at its two ends, and projecting it along the transverse-to-the-fiber direction. By means of fully-resolved direct numerical simulations and experiments, we show that these fiber-based transverse velocity increments are statistically equivalent to the (unperturbed) flow transverse velocity increments. Moreover, we show that the turbulent energy dissipation rate can be accurately measured exploiting sufficiently short fibers. The technique has been tested against standard Particle Tracking Velocimetry (PTV) of flow tracers with excellent agreement. Our technique overcomes the well-known problem of PTV to probe two-point statistics reliably because of the fast relative diffusion in turbulence that prevents the mutual distance between particles to remain constant at the lengthscale of interest. This problem, making it difficult to obtain converged statistics for a fixed separation distance, is even more dramatic for natural flows in open domains. A prominent example are oceanic currents, where drifters (i.e.~the tracer-particle counterpart used in field measurements) disperse quickly, but at the same time their number has to be limited to save costs. Inspired by our laboratory experiments, we propose pairs of connected drifters as a viable option to solve the issue.

physics.flu-dyn↗

arXiv2021

Transition from shear-dominated to Rayleigh-Taylor turbulence

Turbulent mixing layers in nature are often characterized by the presence of a mean shear and an unstable buoyancy gradient between two streams of different velocity. Depending on the relative strength of shear versus buoyancy, either the former or the latter may dominate the turbulence and mixing between the two streams. In this paper, we present a phenomenological theory that leads to the identification of two distinct turbulent regimes: an early regime, dominated by the mean shear, and a later regime dominated by the buoyancy. The main theoretical result consists of the identification of a cross-over time-scale that discerns between the shear- and the buoyancy-dominated turbulence. This cross-over time depends on three large-scale constants of the flow, namely the buoyancy difference, the velocity difference between the two streams, and the gravitational acceleration. We validate our theory against direct numerical simulations (DNSs) of a temporal turbulent mixing layer compounded with an unstable stratification. We observe that the cross-over time correctly predicts the transition from shear to buoyancy driven turbulence, in terms of turbulent kinetic energy production, energy spectra scaling and mixing layer thickness.

physics.flu-dyn↗