AN EXPERIMENTAL DATABASE FOR THE ANALYSIS OF BURSTING OF A LAMINAR SEPARATION BUBBLE

The bursting phenomenon consists in the switch of a laminar separation bubble from a short to a long configuration. In the former case, reduced effects on the profile pressure distribution are typically observed with respect to the attached condition. On the contrary, long bubbles provoke significant variations of the loading coefficient upstream of the separation position, with increased risk of stall. The present work presents an experimental database on separated-flow transitional boundary layers under different Reynolds numbers, adverse pressure gradients and turbulence intensity levels (Tu=1.5%, 2.5%, 3.5%). Overall, more than 90 flow conditions were tested concerning short and long bubbles for the characterization of separated flows under turbine-like conditions. Measurements were performed on a flat plate geometry using a fast response particle image velocimetry (PIV) system. For each flow case, two sets of 6000 snapshots were acquired at a sampling rate of 300 and 1000 Hz. Based on existing criteria for the identification of the bursting phenomenon, the flow cases were clustered in terms of short and long bubble states. Additionally, the kind of instability (i.e. convective or absolute) developing into the separated boundary layer was identified based on time-mean flow quantities. The present study highlights the existing link between the bursting of a laminar separation bubble and the onset of the absolute instability of the separated shear layer, with stationary vortices forming in the dead air region.