Streamwise and doubly-localised periodic orbits in plane Poiseuille flow

Abstract

We study localised exact coherent structures in plane Poiseuille flow that are relative periodic orbits. They are obtained from extended states in smaller, periodically continued domains, by increasing the length to obtain streamwise localization and then by increasing the width to achieve spanwise localisation. The states maintain the travelling wave structure of the extended states, which is then modulated by a localised envelope on larger scales. In streamwise direction, the envelope shows exponential localization, with different exponents on the upstream and downstream side. The upstream exponent increases linearly with Reynolds number Re, but the downstream exponent is essentially independent of Re. In the spanwise direction the decay is compatible with a power-law localisation. As the width increases the localised state undergoes further bifurcations which add additional unstable directions, so that the edge state the relative attractor on the boundary between the laminar and turbulent motions, in the system becomes chaotic.