Coherent structures within the laminar separation bubble on a NACA 0015 hydrofoil

We report about the topology and the coherent structures inside the recirculating region of the Laminar Separation Bubble (LSB) that appear on the suction side of a NACA 0015 hydrofoil at Angles of Attack AoA = [3 o ,10o ] and at chord Reynolds number Re = 1.8 × 105 . The non-intrusive Temperature Sensitive Paint (TSP) technique provides the thermal footprints Tw(x, y,t) of the fluid on the model surface, unveiling three different flow regimes whose complexity in time and space decreases when AoA increases. Tw(x, y,t) maps are analyzed by applying an optical-flow-based method to extract the relative skin-friction fields; the obtained ?w lines provide a topological description of the flow at the wall. The analysis of the critical points shows that, with a fair level of generality, the separation region S is composed by a sequence of manifolds which connect saddle points to converging nodes via the saddles unstable manifolds whereas, in a complementary way, the reattachment region R is composed by a sequence of manifolds which connect saddle points to converging nodes via the saddles stable manifolds. This ubiquitous configuration is common to both the examined AoA and represents an essential portrait of the investigated LSB structure which fully agrees with 3D separation theory. 2D and 3D disturbances strongly alter this scenario. In the paper we report about: o the extraction of the ?w vector fields after the application of the optical-flow based algorithm; o the evolution of the flow topology at separation S at AoA = [3 o ,10o ] and reattachment R at AoA = 10o ; o the effect of weak 2D and 3D disturbances on the topology of the LSB at AoA = 10