The underlying mechanism behind the bursting and reformation cycle of the laminar separation bubble over a NACA-0012 at the inception of stall

The mechanism behind the bursting and reformation cycle of the laminar separation bubble (LSB) over a NACA-0012 at Re_c = 5 x 10⁴, Ma_∞ = 0.4, and various angles of attack at the inception of stall is investigated numerically using Large Eddy Simulation (LES). It is shown that a triad of three vortices is behind the quasi-periodic self-sustained bursting and reformation of the LSB and its associated low-frequency flow oscillation (LFO). A global oscillation in the flow-field around the aerofoil is observed in all of the investigated angles 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 dynamics of the flow and how the triad of vortices drives and sustains the global oscillation in the flow-field are discussed^a.

^aarXiv:1807.09199