Skin friction and coherent structures within a laminar separation bubble

We study the Laminar Separation Bubble (LSB) which develops on the suction side of a NACA 0015 hydrofoil by means of a Temperature-Sensitive Paint (TSP), at a Reynolds number of 1.8x10⁵ and angles of attack AoA = [3°, 5°, 7°, 10°]. The thermal footprints T_w(x,y,t) of the fluid unveil three different flow regimes whose complexity in time and space decreases when AoA increases, up to 10° where the LSB-induced spatial gradients are linked to almost fixed positions in time. At AoA = 7° the LSB system undergoes through a 3D destabilization, that induces C-shaped arcs at separation and weak bubble-flapping at reattachment. Structural changes occur at AoA = 5° and 3°: bubble-flapping raises homogeneously at reattachment while intermittent, wedge- shaped events alter the LSB shape. The relative skin-friction vector fields τ_w(x,y,t), extracted from T_w(x,y,t) by means of an optical-flow-based algorithm, provide the topology of the flow at the wall and feed a physics-based criterion for the identification of flow separation S(y,t) and reattachment R(y,t) which fulfill, in average, a ground-truth validation. The obtained S(y,t) is composed of several manifolds that extend spanwise from saddle points to converging nodes via the saddles unstable manifold, while, at least at higher AoA, manifolds that compose R(y,t) move from diverging nodes to saddle points via the saddles stable manifolds. The trigger of a wedge-shaped event by a rising Omega-shaped vortex in the reverse LSB flow is captured and described.