Optimal streaks amplification in wakes and vortex shedding control.
Gerardo Del Guercio
Vendredi 7 novembre à 10 h 00 Amphithéâtre Nougaro
Sous réserve d’autorisation de soutenance par les rapporteurs
Abstract :
Wakes can sustain large transient energy growth. Optimal
perturbations are computed for the cases of parallel, weakly
non-parallel and the circular cylinder wakes. Streaks are found
to be the optimal amplified structures produced by the non normal
energy amplification. The level of energy increases with the
spanwise wavelength of the perturbations except in the circular
cylinder wake where the optimal is reached for lz=6D.
In parallel wakes these streaks are shown to suppress the
absolute instability. Furthermore the global instability of the
weakly non-parallel and the circular cylinder wakes can be
completely annihilate with moderate streaks amplitudes. The
comparison of these spanwise periodic (3D) optimal
perturbations with the spanwise uniform (2D) control showed
that the energy required to stabilize the wake is always smaller
for the 3D control. Moreover the sensitivity of the global mode
growth rate is discovered to be quadratic for 3D perturbations
while being linear for 2D ones meaning that usual first order
sensitivity analysis is unable to predict their larger
efficiency. The last part of this work offers an extension of the
control by streaks to the case of a turbulent wake past 3D
bodies. Streaks artificially forced in the absolute instability
region of the flow seem to have a relevant impact on the wake
dynamics.
Jury :
Pierre Brancher - IMFT, Toulouse
Olivier Cadot - UME, ENSTA, Palaiseau
Carlo Cossu - IMFT, Toulouse
Benoit Pier - LFMA, Ecully
Gregory Pujals - PSA Peugeot Citroen
Jean-christophe Robinet - DynFluid, Arts et Metiers, Paris
