On 3D instabilities of 2D time-periodic flows

We consider how time-periodic 2D flows may become unstable to 3D
perturbations. The Karman vortex street, the 2D periodically shedding
wake of a circular cylinder, is the prototypical example. We shall
consider this as well as a periodically forced enclosed flow as
examples to illustrate the abstract problem in equivariant
bifurcations describing the 2D to 3D transitions. The talk will
present results from normal form analysis, Floquet stability analysis,
computations of the 3D Navier-Stokes equations, and laboratory
experiments.