2012 Laufer Lecture
Hydrodynamics Laboratory – LadHyx
CNRS – Ecole Polytechnique
Title: “Amplifiers and Oscillators: A Hydronamic Stability Perspective”
Complex flows of interest in aeronautical applications may often be considered as involving a combination of simpler prototypical free shear flows such as mixing layers, wakes, jets, etc. which prevail in different spatial subdomains. Such archetypical shear flows are in a turbulent state but they are known to exhibit large-scale vortical structures which dominate the dynamics up to very large Reynolds numbers. Experimental observations indicate that some flows are extrinsic amplifiers of external noise while others are intrinsic oscillators which beat at a specific frequency. In this lecture, such structures will be described as instablity waves living on a suitable mean flow. Hydrodynamic stability concepts such as convective/absolute instability, local/ global instability, will then be shown to account for the extrinsic or intrinsic spatio-temporal dynamics of coherent structures in several shear flows. The classical locally parallel asymptotic analyses and the more recently developed fully non-parallel numerical approaches will be critically assessed.