Laminar-turbulent Transition in a Laminar Separation Bubble in the Presence of Free-stream Turbulence

Direct numerical simulations (DNS) are employed to investigate the hydrodynamic instability mechanisms and transition to turbulence in laminar separation bubbles (LSBs) on a flat plate. A set of numerical simulat2013-0264ions has been carried out to investigate the transition process, and in particular to shed light on the development of the large coherent structures, which arise during transition. Particular focus is directed towards understanding and identifying the relevant physical mechanisms governing the interaction of separation and transition in laminar separation bubbles in the presence of free-stream turbulence (FST). For the natural flow. i.e. zero FST, the transition mechanism involves a Kelvin-Helmholtz instability and a growth of three-dimensional very low-frequency disturbances of the shear layer. With the inclusion of FST, transition is accelerated. For the separation bubbles investigated, the transition process is the result of two different mechanisms: i) Strong amplification of high-frequency (order of the shedding frequency), essentially two-dimensional or weakly oblique fluctuating disturbances and ii) low-frequency, three- dimensional Klebanoff perturbations caused by FST. Depending on the intensity of the FST, one of these mechanisms would dominate the transition process, or both mechanisms are blended together and contribute simultaneously.