STRONGLY NONLINEAR EFFECT IN UNSTABLE WAKES

We perform experiments and numerical simulations on the mean streamwis e velocity field of an unstable cylinder wake. We show that the mean v elocity exhibits two weak secondary minima as the consequence of nonli near interactions, resulting in a strong mean flow correction of the u nstable mode. This correction, dominating the basic flow, governs the decrease in the length of the recirculation region Delta L-r in the su percritical regime. This explains the early classical observations of M. Nishioka and H. Sate [J. Fluid Mech. 89, 49 (1978)].