Transitional separation bubbles over swept wings

Direct numerical simulations and global linear stability analysis have been performed to investigate the effect of sweep on the behaviour of transitional separation bubbles developing over a NACA0012 airfoil at moderate Reynolds numbers. The results indicate that the laminar-turbulent transition inside the bubble is greatly affected by the sweep angle, with the onset of two-dimensional vortex shedding being favoured over three-dimensional vortex shedding as sweep is increased. Both the DNS and global stability results show that the laminar-turbulent transition of the separation bubble is dominated by instability modes developing in the detached shear layer induced by the bubble and sustained by an acoustic feedback loop. An additional set of modes with relatively low frequencies and concentrated inside the separation bubble were also uncovered by a Dynamic Mode Decomposition of the DNS data. These modes are globally unstable and lead to three-dimensional oscillation of the separations bubble.