# Laboratoire de Mécanique des Fluides et d’Acoustique - UMR 5509

LMFA - UMR 5509
Laboratoire de Mécanique des Fluides et d’Acoustique
Lyon
France

## Wouter Bos

Équipe de Recherche : Turbulence & Instabilités
tél : 04.72.18.62.04
email

Research in the spotlight :

### Turbulent versus laminar mixing, what is most efficient ? [With Benjamin Kadoch and Kai Schneider]

A turbulent flow mixes in general more rapidly a passive scalar than a laminar flow does. From an energetic point of view, for statistically homogeneous or periodic flows, the laminar regime is more efficient. However, the presence of walls may change this picture. We consider in this investigation mixing in two-dimensional laminar and turbulent wall-bounded flows using direct numerical simulation. We show that for sufficiently large Schmidt number, turbulent flows more efficiently mix a wall-bounded scalar field than a chaotic or laminar flow does. The mixing efficiency is shown to be a function of the Péclet number, and a phenomenological explanation yields a scaling law, consistent with the observations.

Pour en savoir plus :

### Power fluctuations in turbulence [with Rémi Zamansky (IMFT)]

How universal are the power-fluctuations measured in the electricity output of a windmill ? Indeed, the power fluctuation experienced by a windmill blade are determined by the power absorbed by the incoming, turbulent airflow. The universal character of turbulent power fluctuations is, in general, of major importance in the modeling and understanding of fluid flows in industry, geophysics and astrophysics. However, the manner in which the turbulence is generated can be radically different in different types of flows, so that it seems improbable that universality is observed among different types of flow. Nevertheless, we show that different turbulence generating mechanisms can lead to similar statistics of the energy input since the input is not only determined by the forcing mechanism, but also by the turbulent flow itself.

### Football players and turbulent trajectories (with B. Kadoch and K. Schneider)

(...) Indeed, even though a football match does not seem to be ergodic, and some may argue that the trajectories of football players are not completely random, the long-time PDF of the directional change converged to a shape close to the one for fluid particles (...)

Recent experiments and simulations have shown that unsteady turbulent flows display a universal behavior at short and intermediate times, different from classical scaling relations. The origin of these observations is explained and the exact form of the observed universal scaling is derived. The derived scaling for the normalized dissipation rate $C_\epsilon$ as a function of the Reynolds number $R_\lambda$ is,
$$C_\epsilon\sim R_\lambda^{-15/14},$$