Conference talks on turbulence and microswimmer collective behaviour

Viktor Skultety and James Hitchen will give dry-runs of their upcoming conference talks.

James' talk (~10 min):

The mechanism of long turbulent lifetimes in a low-dimensional model of plane Couette flow

Recently, our understanding of the transition to turbulence has significantly changed due to the discovery of exact solutions of the Navier-Stokes equations and the introduction of the self-sustaining process in parallel shear flows. This theory has been very successful in describing the main features of weakly turbulent states, including the metastable nature of turbulence close to the transition and the super-exponential dependence of its lifetime on the Reynolds number. The main strength of this approach is that it allows for a semi-analytical description of the turbulent dynamics in the form of a rather low-dimensional model. Here we systematically develop a novel low-dimensional model that allows us to investigate the origin of the very long turbulent life-times close to the transition. We find that there exists a particular periodic orbit that acts as a porous reflecting barrier between the laminar and turbulent states, and that serves to greatly increase the time before relaminarisation.

Viktor's talk (~30 min):

The role of correlations in the collective behaviour of microswimmer suspensions

The main distinction of 'active matter' from its passive counterpart is the ability to extract energy from the environment (consume food) and convert it into directed motion. One of the most striking consequences of this distinction is the appearance of collective motion in self-propelled particles suspended in a fluid observed in recent experiments and simulations: at low densities particles move around in an uncorrelated fashion, while at higher densities they organise into jets and vortices comprising many individual swimmers. Although this problem received significant attention in recent years, the precise origin of the transition is poorly understood. Here we present a novel kinetic theory that captures correlations below the transition, and is non-perturbative in the swimmer density. We show that correlations cannot be neglected at any density.