ABSOLUTE AND CONVECTIVE SECONDARY INSTABILITIES IN SPATIALLY PERIODICSHEAR FLOWS

The generic problem of the spatiotemporal instability of a periodic ba sic flow (Stuart vortices) is considered in order to interpret the seq uence of bifurcations observed in open shear flows. Using a novel nume rical technique, we show that the more concentrated the vortices, the smaller the backflow needed to trigger absolute instability. These res ults allow us to propose an alternative interpretation for the subharm onic resonance observed in forced shear flows, which is classically at tributed to an acoustic feedback.