  The present investigation shows that a triad of three vortices, two
co-rotating vortices (TCV) and a secondary vortex lies beneath them and
counter-rotating with them, is behind the quasi-periodic self-sustained
bursting and reformation of the laminar separation bubble (LSB) and its
associated low-frequency flow oscillation (LFO). The upstream vortex of the TCV
(UV) is driven by the gradient of the oscillating-velocity across the laminar
portion of the separated shear layer and is faithfully aligned with it. The
secondary vortex acts as a roller support that facilitates the rotation and
orientation of the TCV. A global oscillation in the flow-field around the
aerofoil is observed in all of the investigated angles of attack, including at
zero angle of attack. The flow switches between an attached-phase against an
adverse pressure gradient (APG) and a separated-phase despite a favourable
pressure gradient (FPG) in a periodic manner with some disturbed cycles. When
the direction of the oscillating-flow is clockwise, it adds momentum to the
boundary layer and helps it to remain attached against the APG and vice versa.
The transition location along the separated shear layer moves upstream when the
oscillating-flow rotates in the clockwise direction causing early-transition
and 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 in
the clockwise direction and store energy until it is saturated, then the
process is reversed. The present investigation paves the way for formulating a
time-accurate physics-based model of the LFO and stall prediction, and opens
the door for control of its undesirable effects.
