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 degrees, 5 degrees, 7 degrees, 10 degrees]. The thermal footprints Tw(x,y,t) of the fluid unveil three different flow regimes whose complexity in time and space decreases when AoA increases, up to 10 degrees where the LSB-induced spatial gradients are linked to quasi-steady positions in time. At AoA=7 degrees the LSB system undergoes a 3D destabilization, that induces C-shaped arcs at separation and weak bubble-flapping at reattachment. Structural changes occur at AoA=5 degrees and 3 degrees: bubble-flapping raises homogeneously at reattachment while intermittent, wedge-shaped events alter the LSB shape. The relative skin-friction vector fields w(x,y,t), extracted from Tw(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). This criterion fulfills, in average, a novel skin-friction ground-truth estimation grounded on the determination of the propagation velocity of temperature fluctuations. 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 triggering of a wedge-shaped event by a rising -shaped vortex in the reverse LSB flow is captured and described in analogy to a simplified model.