The effects of elevated free-stream turbulence (FST) on natural and periodically excited separation bubbles were examined experimentally at low Reynolds numbers. The bubble was formed at the leading edge of a flat plate and the FST level was altered by placing a grid across the flow at different locations upstream of the plate. The mixing across the separated shear-layer increased due to the elevated FST and the two-dimensional periodic excitation, flattening, and shortening the bubble. Periodic excitation at frequencies that were at least an order of magnitude lower than the initial Kelvin–Helmholtz instability of the separated shear-layer were very effective at low FST. The fundamental excitation frequency and its first harmonic were amplified above the bubble. High frequency excitation $(F+≈3,$ based on the length of the natural low FST bubble that served as a reference baseline) affected the flow near the leading edge of the bubble but it rapidly decayed in the reattachment region. Lower frequencies $F+≈1$ were less effective and they decayed at a slower rate downstream of reattachment. An increase in FST level reduced the net effect of the periodic excitation on mixing enhancement and subsequent reattachment process. This was probably due to a destructive interference between the nominally 2D excitation and the random, in space and in time, FST. High FST is known to reduce the spanwise coherence in free shear layers [e.g., Chandrasuda, C., Mehta, R. D., Weir, A. D., and Bradshaw, P., 1978, “Effects of free-stream turbulence on large structures in turbulent mixing layers,” J. Fluid Mech., 85, pp. 693–704] and therefore also the effectiveness of the current control strategy, this not withstanding 2D periodic excitation accelerated the reattachment process and the recovery rate of the attached boundary layer, reducing its momentum loss.

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