Effect of angle of attack on vortex dynamics in laminar separation bubbles

The dynamics of coherent structures that develop within laminar separation bubbles over a NACA 0018 airfoil at Re-c = 100 000 and Angles of Attack (AOA) of 0 degrees, 5 degrees, 8 degrees, and 10 degrees are investigated experimentally using a high-speed flow visualization technique and vortex tracking via embedded microphones. The results identify vortex shedding within the separation bubble for all the cases investigated. The vortices form upstream of mean transition location and break down in the vicinity of mean reattachment, with irregular vortex merging events detected in the aft portion of the separation bubble. As the mean size of the separation bubble decreases with increasing angle of attack, vortex shedding characteristics also change appreciably, with shedding frequency increasing and characteristic streamwise wavelength decreasing. However, vortex roll up and salient aspects of vortex development take place within approximately the same regions downstream of separation location in terms of percentage of the bubble length. For all the cases examined, significant cycle-to-cycle variability is observed in salient vortex characteristics, being particularly pronounced in the aft portion of the bubble where vortex merging occurs and significant spanwise deformations of vortex filaments are expected. Merging of the shear layer vortices is shown to occur irregularly between the mean transition and reattachment location, with merging events separated by a relatively wide range of shedding cycles. As the angle of attack increases from AOA = 5 degrees to 10 degrees, the fraction of primary vortices involved in merging increases up to about 25%. However, enhancement in vortex merging is also noted at lower AOA where shedding occurs close to the trailing edge, which is speculated to be linked to upstream scattering of periodic pressure fluctuations induced at the trailing edge.