Structural changes of laminar separation bubbles induced by global linear instability*

Global modal linear instability analysis considers three- dimensional disturbances superimposed upon (essentially non- parallel) two- or three-dimensional basic flows. Here two- dimensional (BiGlobal) analysis of laminar separation bubbles embedded in a flat-plate boundary layer is performed. Results obtained show the presence of a stationary three-dimensional eigenmode, which is unstable for a finite range of spanwise wavenumbers, while the same steady basic flow is stable against two-dimensional disturbances of the Kelvin-Helmholtz/Tollmien- Schlichting class. Critical-point theory shows that 2D flow is ``structurally unstable'' and the presence of any 3D disturbance, like the aforementioned global mode will alter the complete topological description regardless of the disturbance amplitude. Critical-point theory is used here in order to characterize the different topological bifurcations exerted by global instability on the steady laminar two-dimensional bubble: a spanwise modulation of the separated region appears, eventually leading to the breakdown of the recirculation region into independent cellular structures, highly resemblant to the patterns observed experimentally on stalled airfoils.