Direct measurement of skin friction using TSP data

Abstract

In this work, the focus is on the skin-friction data derived from time histories of temperature maps obtained using Temperature Sensitive Paint (TSP). These maps retain a detailed portrait of the footprints of the coherent flow structures close to the surface, under the hypothesis that the wall heat flux is mainly due to the convective action of the flow. We hypothesize that the temperature fluctuations, measured at the wall by means of TSP, can be used as tracers, so that the passage of the same event at close (but well separated) locations can be captured. A criterion to diagnose the streamwise profile of the time-averaged friction velocity is proposed as follows. Firstly, the best correlation distance (time lag) is identified between temperature time-histories acquired simultaneously at adjacent, streamwise aligned, locations. Under the hypothesis of strong locality of the heat fluxes between surface and fluid, the friction velocity is then estimated as the ratio between the streamwise distance of the two locations and the identified time lag. The proposed criterion is applied to the flow which develops on the surface of a NACA 0015 hydrofoil investigated at a Reynolds number of 180000 and angles of attack from 3° to 10°. The experiments were carried out in the cavitation tunnel CEIMM at CNR-INSEAN (Rome).