Structural sensitivity of the finite-amplitude vortex shedding behind a circular cylinder

Recirculating flows such as mixing layers, wakes and jets, may sustain synchronised periodic oscillations in a suitable parameter range. Under these conditions the whole flow field behaves like a global oscillator (``global mode''). A theoretical approach to this class of problems, in the context of weakly nonparallel quasi-onedimensional flows, was formulated by Chomaz, Huerre \& Redekopp (1991), Monkewitz, Huerre \& Chomaz (1993) and Le Diz\`es {\it et al.} (1996) who introduced the idea of a {\it wavemaker}. In the context of a fully twodimensional linear mode analysis of the wake behind a cylinder, a spatial visualisation of the {\it wavemaker} was obtained by Giannetti \& Luchini (2007), who determined the regions where the sensitivity of the frequency of oscillation to a localized feedback from velocity to force is maximal. In this contribution we apply a similar approach to nonlinear, finite-amplitude vortex shedding, in order to assess how unsteadiness and saturation modify the numerical results and how these compare to the predictions of the nonlinear weakly nonparallel theory.