Transient dynamics of laminar separation bubble formation and bursting

The unsteady dynamics of laminar separation bubble formation and bursting are studied on a NACA 0018 airfoil at an angle of attack of 6° using controlled ramp changes in freestream velocity between limiting Reynolds numbers of \(4.0\times 10^4\) and \(6.0\times 10^4\). Compared to the timescales associated with transition in the separated shear layer, the ramp changes are essentially quasi-steady with a duration of approximately 60 global convective timescales and a maximum non-dimensional acceleration of 0.015. Surface pressure and planar particle image velocimetry measurements are used to explore flow field development during laminar separation bubble formation and bursting transients. At the lower limiting Reynolds number, laminar separation occurs on the suction surface and the airfoil is stalled. At the higher limiting Reynolds number, transition to turbulence in the separated shear layer leads to reattachment in the mean sense and the formation of a short laminar separation bubble. During bubble formation, a steady-state condition is reached approximately 50 convective timescales after the initiation of the ramp change, with the most significant changes to the flow field occurring over a shorter period of time at the end of the transient. The duration of the bursting transient is 1.2 times longer than the formation transient, due to substantial oscillations in the position of the separated shear layer after cessation of reattachment that resemble the step response of a second-order under-damped system. During formation and bursting transients, the changes to the band of unstable frequencies and wavenumbers of the separated shear layer display a similar Reynolds number dependency to quasi-steady flows. Despite the quasi-steady nature of the imposed freestream accelerations, the movement of the separated shear layer exhibits substantial hysteresis.