Strongly Nonlinear Effect in Unstable Wakes

We perform experiments and numerical simulations on the mean streamwise velocity field of an unstable cylinder wake. We show that the mean velocity exhibits two weak secondary minima as the consequence of nonlinear interactions, resulting in a strong mean flow correction of the unstable mode. This correction, dominating the basic flow, governs the decrease in the length of the recirculation region $\Delta L_r$ in the supercritical regime. This explains the early classical observations of M. Nishioka and H. Sato [J. Fluid Mech. 89, 49 (1978)].