Event-based reverse-flow intermittency analysis over a double-protuberance lifting-surface section

Near-stall flow separation over lifting surfaces is strongly unsteady and may affect hydrodynamic efficiency, load fluctuation, and maneuvering reliability in marine engineering applications. This study proposes an event-based reverse-flow area analysis to quantify flow-separation intermittency from raw time-resolved particle image velocimetry velocity fields. The method converts each instantaneous field into a reverse-flow area signal A r ( t ), from which event statistics, spectral organization, dynamic asymmetry, and event-triggered velocity-field evolution are extracted. Across seven threshold choices, the principal regime interpretation remains stable: root2 at the angle of attack (AoA) of 24° has the largest mean reverse-flow area signal ( A r = 0.433), root1 at AoA = 16° has the highest burst rate (20.6 – 45.2 s −1 ), and the baseline case at AoA = 18° shows the strongest spectral concentration with a dominant low-frequency peak near 1.56 Hz ( St = 0.0235). Comparison with conventional mean reverse-flow descriptors shows that mean topology, event recurrence, and spectral organization capture complementary aspects of separated-flow dynamics. The proposed framework provides a compact diagnostic approach for interpreting reverse-flow intermittency and spanwise dynamic non-equivalence over bio-inspired lifting-surface sections.