  This study examines the effects of angle of attack on the characteristics of
the laminar separation bubble (LSB), its associated low-frequency flow
oscillation (LFO), and the flow-field around a NACA-0012 aerofoil at $Re_c =
5\times 10^4$ and $M_{\infty} = 0.4$. In the range of the investigated angles
of attack, statistics of the flow-field suggest the existence of three distinct
angle-of-attack regimes: 1) the flow and aerodynamic characteristics are not
much affected by the LFO for angles of attack $\alpha < 9.25^{\circ}$; 2) the
flow-field undergoes a transition regime in which the bubble bursting and
reformation cycle, LFO, develops until it reaches a quasi-periodic switching
between separated and attached flow in the range of angles of attack
$9.25^{\circ} \leq \alpha \leq 9.6^{\circ}$; and 3) for the angles of attack
$\alpha > 9.6^{\circ}$, the flow-field and the aerodynamic characteristics are
overwhelmed by a quasi-periodic and self-sustained LFO until the aerofoil
approaches the angle of full stall. A trailing-edge bubble (TEB) forms at
$\alpha > 9.25^{\circ}$ and grows with the angle of attack. The LSB and TEB
merge and continue to deform until they form an open bubble at $\alpha =
10.5^{\circ}$. On the suction surface of the aerofoil, the pressure
distribution shows that the presence of the LSB induces a gradual and continues
adverse pressure gradient (APG) when the flow is attached. The bursting of the
bubble causes a gradual and continues favourable pressure gradient (FPG) when
the flow is separated. This is indicative that a natural forcing mechanism
keeps the flow attached against the APG and separated despite the FPG. The
present investigation suggests that most of the observations reported in the
literature about the LSB and its associated LFO are neither thresholds nor
indicators for the inception of the instability, but rather are consequences of
it.
