Stretching and breaking of particles in compressible random flows

TL;DR

This paper investigates how small-scale flow compressibility influences the stretching and breaking of particles in turbulent suspensions, revealing that compressibility affects strain fluctuations and particle deformation dynamics.

Contribution

It provides a theoretical and numerical analysis of particle extension in compressible chaotic flows, including an exact solution for the Kraichnan model and insights into extreme strain effects.

Findings

Large deviations in strain rate cause significant particle stretching.

Stiff particles break faster in more compressible flows due to extreme strain events.

Elastic particles tend to stretch less and break slower in compressible flows.

Abstract

A key feature of turbulent suspensions that involve floating particles on the surface or inertial particles in the bulk is the compressibility of the effective particle-phase velocity field. Little, however, is known about the effects of small-scale flow compressibility on the stretching and breaking of particles. Here, we gain insight into the nature of these effects by studying the deformation of tiny particles in model fluctuating flows. We consider a generic particle with extensional dynamics that are governed by a vector model, which accounts for elasticity, internal viscosity, and non-affine deformation. Applying the dynamical systems approach of Balkovsky, Fouxon & Lebedev (2000), we first obtain general results for the stationary statistics of particle extension in compressible chaotic flows. We then specialize to a time-decorrelated Gaussian random flow and derive an exact solution for the Batchelor regime of the compressible Kraichnan model. We also perform numerical simulations for a time-correlated renewing flow. While straining is suppressed on the average in compressible flows, our results show that large deviations of the strain rate strongly stretch particles and give rise to a power-law distribution of extensions. Extreme straining events are particularly important for stiff particles and, in the examples considered here, give rise to a counter-intuitive effect: stiff particles stretch more and break faster in flows of increasing compressibility. Highly-elastic particles, whose deformation is dictated by the mean straining, stretch less and break slower. Though based on specific random flows, our work shows how compressibility can affect the extensional dynamics of particles by altering the fluctuations of the strain rate, including its large deviations.