On the evolution of particle-puffs in turbulence

TL;DR

This study uses high-resolution simulations to analyze how turbulent particle puffs evolve in shape and dispersion, revealing the influence of wind fluctuations and robustness of shape approximations.

Contribution

It provides detailed characterization of particle puff evolution in turbulence and assesses the impact of shape approximations on hit probability analysis.

Findings

Particle puff shapes are strongly influenced by wind fluctuations.

Shape approximations (sphere, ellipsoid) are robust for hit probability.

High-resolution simulations enable detailed turbulence analysis.

Abstract

We study the evolution of turbulent puffs by means of high-resolution numerical simulations. Puffs are bunches of passive particles released from an initially spherical distribution at regular time intervals of the order of the Kolmogorov time. The instantaneous shapes of particle puffs, in particular their asphericity and prolateness, are characterized by measuring the gyration tensor. Analysis has been performed by following, up to one large scale eddy-turn-over time, more than $10^4$ different puffs, each made of 2000 particle tracers, emitted from different places in a homogeneous and isotropic turbulent fluid with Taylor-scale Reynolds number $Re_\lambda \sim 300$. We also analyze the probability of hitting a given target placed downstream with respect to the local wind at the time of emission, presenting data for three different cases: (i) without any reconstruction of the shape, i.e. considering the bunch of point tracers, and approximating the particle-puff as a (ii) sphere or as an (iii) ellipsoid. The results show a strong dependence on the fluctuations of the instantaneous wind at the moment of the emission and appear to be robust with respect to the approximations (i)-(iii).