Instabilities and emergence of coherent structures in the turbulent channel flow

Conf’luence Jean Christophe Robinet

Professeur DYNFLUID
Laboratory Arts et Métiers Sciences and Technologies

Vendredi 27 mars à 10h30 • Amphithéâtre Nougaro

Turbulent flows are characterised by a marvellous phenomenology combining chaos and order, fluctuations on multiple scales and recurrent patterns named coherent structures. The aim of this talk is to elucidate certain physical processes involving coherent structures in the turbulent channel flow.
Different numerical techniques are implemented towards this aim, with a common ground: investigate how coherent structures become unstable and, eventually, generate new coherent structures on different scales. This investigation is divided in two parts.
In the first part, the instability of streaks is addressed more directly but in a different direction. A linear stability analysis is performed on an array of periodic streaks in order to show that these structures undergo sub-harmonic or detuned instabilities with unstable modes characterized by large wavelengths (several times larger than the streaks wavelength). These instabilities are related to the large-scale motions (LSMs) observed in experiments and direct numerical simulations of high-Reynolds number flows.
In the second part, the large-scale linear instability of streaks is revisited at low Reynolds number in order to capture the wavelengths and the critical Reynolds number of the instability leading to laminar-turbulent patterns.

Fig. : Direct numerical simulation (DNS) in channel,
iso-surface of the streamwise velocity colored
with the distance from the wall. Re_tau=1000.[/caption]