Irreversibility in bacterial turbulence: Insights from the mean-bacterial-velocity model

We use the mean-bacterial-velocity model to investigate the irreversibility of two-dimensional (2D) bacterial turbulence and to compare it with its 2D fluid-turbulence counterpart. We carry out extensive direct numerical simulations of Lagrangian tracer parti-cles that are advected by the velocity field in this model. We demonstrate how the statistical properties of these particles help us to uncover an important, qualitative way in which irreversibility in bacterial turbulence is different from its fluid-turbulence counterpart: For large but negative (or large and positive) values of the activity (or friction) parameter, the probability distribution functions of energy increments, along tracer trajectories, or the power are positively skewed; so irreversibility in bacterial turbulence can lead, on average, to particles gaining energy faster than they lose it, which is the exact opposite of what is observed for tracers in 2D fluid turbulence.

Automated summary

We use the mean-bacterial-velocity model to study the irreversibility of 2D bacterial turbulence and compare it with 2D fluid turbulence. Through extensive numerical simulations, we find that the statistical properties of particles in bacterial turbulence differ qualitatively from those in fluid turbulence. Particularly, for certain parameter values, particles in bacterial turbulence can gain energy faster than they lose it, which is opposite to what is observed in 2D fluid turbulence.