# Séminaire de Mécanique d'Orsay

## Le Mercredi 12 octobre à 14h00 - Salle de conférences du LIMSI

### Travelling waves and large scale motions in turbulent shear flows

## Ashley Willis

School of Statistics and Mathematics, University of Sheffield (UK)

Due to their inherent nonlinearity, the vast majority of solutions for
shear flows have been discovered only in the last 10-15 years. These
solutions take the form of a travelling wave (TW). Their discovery has
permitted a new theoretical framework, where the TWs and their unstable
eigenvectors form a skeleton in phase space for the trajectories taken
by a chaotic turbulent flow.
At large Reynolds numbers, application of this approach is increasingly
difficult, since the number of degrees of freedom increases as the
turbulent eddies become small. Meanwhile, however, in experiments at
very large Reynolds numbers, mostly in pipe flow, it is the discovery of
large scale motions (LSMs) and very large scale motions (VLSMs) that has
attracted interest over the last decade.
In this talk we first review the progress in connecting turbulent
dynamics with travelling waves and periodic orbits in pipe flow. Then,
by parameterising the effect of the smallest eddies (via LES
simulation), we seek nonlinear solutions that may help us understand the
origin and dynamics of the LSMs and VLSMs.