Instability and Transition in a Laminar Separation Bubble

In this numerical study, a laminar separation bubble is simulated by imposition of suction to create an adverse pressure gradient. The DNS elucidates the entire transition process over the separation bubble leading to turbulence. Several important conclusions are drawn from the simulations regarding the origins of transition and evolution of turbulence. Break down to turbulence, preceded by three-dimensional motions and non-linear interactions, occurs in the second half of the mean bubble length. Two topological structures of the bubble causing vortex shedding are suggested; one for the normal shedding and the other for the low frequency flapping. The normal shedding frequency can be attributed to the regular shedding of smaller vortices while shedding of large vortices formed due to coalescence of smaller vortices results in the low-frequency flapping. Due to the shedding of bigger vortices, the instantaneous reattachment point varies greatly resulting in large variation in the instantaneous bubble length. Break down of longitudinal streaks, appearing via Λ-vortices and vortex stretching mechanism, characterizes the transition process. Low values of reverse flow suggest that a convective instability is involved. The instability analysis indicates that the initial amplification of disturbances is due to T-S mechanism while the roll-up of the shear layer takes place due to Kelvin-Helmholtz instability.