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 \(1.8\times 10^5\) and angles of attack \(\mathrm{AoA} = [3^{\circ }\), \(5^{\circ }\), \(7^{\circ }\), \(10^{\circ }\)]. The thermal footprints \(T_\mathrm{w}(x,y,t)\) of the fluid unveil three different flow regimes whose complexity in time and space decreases when \(\mathrm{AoA}\) increases, up to \(10^{\circ }\) where the LSB-induced spatial gradients are linked to quasi-steady positions in time. At \(\mathrm{AoA} =7^{\circ }\) the LSB system undergoes a 3D destabilization, that induces C-shaped arcs at separation and weak bubble-flapping at reattachment. Structural changes occur at \(AoA=5^{\circ }\) and \(3^{\circ }\): bubble-flapping raises homogeneously at reattachment while intermittent, wedge-shaped events alter the LSB shape. The relative skin-friction vector fields \(\varvec {\tau }_\mathrm{w}(x,y,t)\), extracted from \(T_\mathrm{w}(x,y,t)\) 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 \({\mathfrak {S}}(y,t)\) and reattachment \({\mathfrak {R}}(y,t)\). 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 \({\mathfrak {S}}(y,t)\) 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 \({\mathfrak {R}}(y,t)\) move from diverging nodes to saddle points via the saddles stable manifolds. The triggering of a wedge-shaped event by a rising \(\varOmega\)-shaped vortex in the reverse LSB flow is captured and described in analogy to a simplified model.