Conference dry run

Renato: Lattice model of bacterial turbulence

One of the most striking difference between active and passive systems  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.

Recent work (Stenhammar et al, PRL 119, 028005 (2017)) suggests that  this transition is caused by mutual reorientation of the swimmers and is insensitive to their translational degrees of freedom.

In this work we propose a lattice-based model of collective motion. We consider dipolar swimmers pinned to lattices of various symmetries,  and study their dynamics in the presence of long-ranged hydrodynamic interactions.

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James: Understanding the finite lifetimes of Newtonian Turbulence

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. The exact form of such models is typically guided by one's intuition and DNS. In this talk we present a systematic way of deriving low-dimensional models for plane Couette flow that requires no previous intuition of the system in question or its dynamics. We find that the model exhibits a subcritical transition to turbulent dynamics, contains stable periodic orbits, exact coherent structures and finite turbulent lifetimes. We demonstrate that the super-exponential nature of the lifetimes requires interactions between exact coherent structures of different symmetries and discuss the implications of this discovery for the transition.