Bursting and reformation cycle of the laminar separation bubble over a NACA-0012 aerofoil: Characterisation of the flow-field

Abstract

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.