Cascades and Kolmogorov's lognormal scaling in two-dimensional bacterial turbulence

TL;DR

This study investigates two-dimensional bacterial turbulence, revealing inverse energy cascades and lognormal statistics consistent with Kolmogorov's turbulence theory, highlighting the complex interplay of active bacterial motion and fluid dynamics.

Contribution

It provides experimental evidence of inverse energy cascades and lognormal intermittency in bacterial turbulence, extending turbulence theory to active biological systems.

Findings

Inverse energy cascade observed at large scales.

Lognormal statistics verified for energy dissipation and enstrophy.

Scaling exponents match Kolmogorov 1962 refined theory.

Abstract

Collective movements of bacteria exhibit a remarkable pattern of turbulence-like vortices, in which the Richardson cascade plays an important role. In this work, we examine the energy and enstrophy cascades and their associated lognormal statistics using experimental velocity field data. The coherent structure observed on a large scale is due to the presence of the inverse energy cascade; while the kinetic energy is dissipated at all scales, since these active movements occur below the fluid viscosity scale. The forward enstrophy cascade occurs with injection at all scales and may be represented by other nonlinear interactions that are not captured by the existing experimental data. Furthermore, the lognormal statistics for both energy dissipation and enstrophy fields are verified in accordance with the Kolmogorov 1962 refined theory of turbulence. Their scaling exponents can be well described by the lognormal formula with intermittency parameters comparable with those of the three-dimensional hydrodynamic turbulence. The joint analysis of the multifractal measures of the energy dissipation rate and enstrophy follows an ellipse model from the lognormal statistics. Our results confirm the coexistence of the inverse energy cascade and the intermittency correction of the velocity scaling in this active fluid system. An inverse energy cascade diagram below the fluid viscosity is summarized to describe the observed two-dimensional bacterial turbulence. Our work provides an example of an active-flow model benchmark.