Effects of elevated free-stream turbulence on active control of a separation bubble

Effects of elevated free-stream turbulence (FST) on natural and periodically excited separation bubble were studied experimentally, with relevance to lowpressure turbine blades at low Reynolds numbers. A 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 both elevated FST and Two-dimensional periodic excitation, flattening and shortening the bubble. Periodic excitation at frequencies that were at least an order of magnitude lower than the dominant shear-layer instability were very effective at low FST. The amplitudes of the fundamental excitation frequency and its harmonic were amplified over the bubble. High frequency excitation (F⁺≈z3, based on the length of the baseline low FST bubble) had a major effect close to separation, while the excitation fluctuating momentum rapidly decayed in the reattachment region. Low frequency excitation, that generated waves comparable to the length of the baseline bubble (F⁺≈I) were less effective and their magnitude decayed at a slower rate downstream of reattachment. An increase in the level of the FST reduced the net effect periodic excitation had on the mixing enhancement and subsequent reattachment process, probably due to a destructive interference between the nominally 2D excitation and the random, in space and time, FST reducing the span wise coherence and therefore the effectiveness of the current control strategy. However, even at the reduced effectiveness of 2D periodic excitation at elevated FST, it accelerated the reattachment process and the recovery rate of the attached boundary layer, enhancing the boundary layer resistance to repeat separation and reducing its momentum loss further downstream.