Experimental investigation of the structure and dynamics of laminar separation bubbles at the onset of bursting

A two-dimensional laminar separation bubble on a flat plate is studied experimentally using Particle Image Velocimetry (PIV) and flow visualization. The separation bubble was generated on a flat plate by an imposed adverse pressure gradient. The adverse pressure gradient was generated by using an inverted wing with a NACA 64₃-618 airfoil mounted above the flat plate. A parametric study of the effect of the upstream flow velocity and the induced pressure gradient on the mean flow topology and the unsteady behaviour of the separation bubble was carried out in the low-speed water tunnel of the Hydrodynamics Laboratory at the University of Arizona. The structure and dynamics of the laminar separation bubble were found to depend strongly on the aforementioned parameters. As the flow velocity is reduced, at very low flow velocities the bubble is seen to undergo a drastic change in geometry, resulting in bubble bursting. An attempt is made in this work at understanding the physics of bubble bursting. For certain flow conditions, strong vortex shedding near the reattachment region of the bubble was observed, which is a characteristic behaviour of short bubbles. High-resolution spatio-temporal PIV measurements were made to analyze the formation and breakdown of these flow structures.