Laminar separation bubbles are relevant in various applications, from unmanned aerial vehicles to turbomachinery blades, as they have significant impact on the performance of the respective airfoils.

This work investigates the three-dimensional, spatio-temporal flow development in the aft portion of a laminar separation bubble, formed on a flat plate by means of a pressure body. Time-resolved planar and tomographic PIV measurements are performed, complemented by linear stability theory analysis.

Our analysis shows that the flow becomes unstable upstream of separation, where both normal and oblique modes undergo amplification. While the shear layer roll up is linked to the amplification of the fundamental normal mode, the oblique modes at angles lower than approximately 30° are also amplified substantially at the fundamental frequency. A model based on stability analysis and experimental measurements is employed to demonstrate that the spanwise deformations of rollers are produced due to the superposition of normal and oblique instability modes, initiating upstream of separation. The degree of the initial spanwise deformations depends on the relative amplitude of the dominant normal and oblique waves. This is confirmed by forcing the normal mode through a controlled impulsive perturbation introduced by a spanwise invariant dielectric-barrier-discharge plasma actuator, resulting in the formation of spanwise coherent vortices.

This research has been carried out in collaboration with Dr. S. Yarusevych from the depratment of Mechanical and Mechatronics Engineering of the University of Waterloo.