Skin friction and coherent structures within a laminar separation bubble

Abstract

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.