Spontaneous Laminar Separation Bubble Bursting on an Airfoil

An experimental investigation was conducted on a NACA 0018 airfoil at near-stall conditions, with stalled, stable laminar-separation bubble, and intermediate flow conditions were considered. Planar particle image velocimetry and surface pressure measurements were used to quantify the flow development at the midspan of the model. For the intermediate conditions, the airfoil is stalled in the time-average sense. However, conditionally averaged velocity field measurements reveal that reattachment of the separated flow and laminar separation bubble formation occur intermittently on the suction surface. The characteristics of the intermittently forming separation bubble and fully separated flow are analyzed based on conditional statistics, revealing variations in the transition process that may underpin the observed low-frequency flow oscillations. The growth rates of the most amplified wavenumbers in the separated shear layer vary with the angle between the separation streamline and the suction surface, leading to earlier transition when the separation angle increases. This change in transition location may play a role in spontaneous changes between fully separated and reattaching flow states.