Incipient stall characterization from skin friction maps

Kurzfassung

We experimentally characterize the incipient stall condition which develops on the suction side of a NACA 0015 hydrofoil model at Re = 180000, by observing the skin friction evolution on the model surface. Grounding on temperature data acquired via Temperature Sensitive Paint (TSP), the proposed approach adopts a two-fold point of view. From one side, the relative skin-friction vector fields, extracted from temperature maps by means of an optical-flow-based algorithm, provide flow topology at the wall and feed a physics-based criterion for the identification of flow separation and reattachment. From the other side, a direct link between friction velocity and celerity of propagation of temperature disturbances is established on the basis of currently available results in scientific literature. Then, the time averaged streamwise component of velocity of passive transport of temperature disturbances is estimated after minimization of the dissimilarity between observed and ideal wave behavior suggested by the Taylor hypothesis. Eventually, a quantitative estimation of time averaged friction-velocity maps is gained. We describe, through the synergistic application of the proposed methods, the incipient stall evolution by firstly identifying the trailing edge separation at an angle-of-attack AoA = 11.5° and then by capturing the onset of upstream oriented mushroom-like structures at AoA = 13°. The concomitant occurrence of both scenarios is reported as well (AoA = 12.2°).