Decades of research have established the ubiquity of inflectional instability in laminar separation bubbles (LSBs) and its dominant role in the laminar-turbulent transition process. However, this instability alone cannot fully explain the plethora of dynamics that are observed, including three-dimensionalisation of the mean flow and self-excited vortex shedding and transition. To fill these gaps, global instability mechanisms of LSBs were proposed in the past. Partial confirmation of some of these mechanisms has been obtained experimentally, but most low-turbulence wind tunnel experiments either did not find evidence of their presence, or attributed the possible evidence to by-products of the inflectional instability alone.
This talk will depart from the theoretically-predicted 3D global instability of LSBs, that distorts the separated flow along the spanwise direction. Recent results for non-linear and secondary instabilities of 3D separated flows will be presented. They show that the spanwise distortion strongly enhances inflectional instability, potentially leading to their absolute instability and to the appearance of a self-excited global oscillator. This sequence triggers the laminar–turbulent transition without requiring external disturbances or actuation, and also explains why the dynamics are analogous under low-intensity free-stream disturbances: the resulting LSBs are in good agreement with those reported for low-turbulence wind-tunnel experiments without explicit forcing. This indicates that the inherent dynamics described by the self-excited instability can have been present and disregarded in many experimental works to date.