Incipient stall characterization from skin friction maps

We experimentally characterize the incipient stall condition which develops on the suction side of a NACA 0015 hydrofoil model at Re = 180; 000, 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 twofold point of view. From one side, the relative skin-friction vector fields tw(x;y; t), extracted from temperature maps Tw(x;y; t) 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 S(y; t) and reattachment R(y; t). From the other side, a direct link between friction velocity Ut and celerity of propagation of temperature disturbances UT0 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 disturbancesUx;T0 is estimated after minimization of the dissimilarity between observed and ideal wave behavior suggested by the Taylor hypothesis. Eventually, a quantitative estimation of time averagedUx;t 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-ofattack 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°).