The effects of three-dimensional forcing on flow development within a laminar separation bubble

This work examines flow development in a laminar separation bubble (LSB) undergoing natural transition and transition controlled with two-dimensional and spanwise modulated disturbances. The investigation is carried out in a series of wind tunnel tests, with the separation bubble formed over a flat plate subjected to an adverse pressure gradient. Velocity field measurements are performed using time-resolved, two-component Particle Image Velocimetry (PIV). Disturbances are produced using surface-mounted plasma actuators in a novel configuration that allows for the introduction of controlled disturbances that are two-dimensional or of a prescribed spanwise wavelength. The natural transition process is dominated by shear layer vortex shedding which is characterized by significant spanwise deformations in the aft portion of the bubble. When the flow is subjected to either two or three-dimensional forcing, vortex formation within the separation bubble is rendered two-dimensional. However, while the two-dimensionally forced perturbations remain largely two-dimensional until breakdown, a clear spanwise wavelength that matches the input wavelength of the forcing develops when the flow is subjected to the spanwise modulated forcing. The reported findings point to the presence of a secondary instability in the separation bubble, which leads to the amplification of the initially weak spanwise component of input disturbances, causing the shear layer vortices to develop significant spanwise undulations.