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The present investigation shows that a triad of three vortices, twoco-rotating vortices (TCV) and a secondary vortex lies beneath them andcounter-rotating with them, is behind the quasi-periodic self-sustainedbursting and reformation of the laminar separation bubble (LSB) and itsassociated low-frequency flow oscillation (LFO). The upstream vortex of the TCV(UV) is driven by the gradient of the oscillating-velocity across the laminarportion of the separated shear layer and is faithfully aligned with it. Thesecondary vortex acts as a roller support that facilitates the rotation andorientation of the TCV. A global oscillation in the flow-field around theaerofoil is observed in all of the investigated angles of attack, including atzero angle of attack. The flow switches between an attached-phase against anadverse pressure gradient (APG) and a separated-phase despite a favourablepressure gradient (FPG) in a periodic manner with some disturbed cycles. Whenthe direction of the oscillating-flow is clockwise, it adds momentum to theboundary layer and helps it to remain attached against the APG and vice versa.The transition location along the separated shear layer moves upstream when theoscillating-flow rotates in the clockwise direction causing early-transitionand vice versa. The best description of the mechanism is that of a whirligig.When the oscillating-flow rotates in the clockwise direction, the UV whirls inthe clockwise direction and store energy until it is saturated, then theprocess is reversed. The present investigation paves the way for formulating atime-accurate physics-based model of the LFO and stall prediction, and opensthe door for control of its undesirable effects.

Bursting and reformation cycle of the laminar separation bubble over a NACA-0012 aerofoil: The underlying mechanism