Temperature-Sensitive Paint Application in Cryogenic Wind Tunnels: Transition Detection at High Reynolds Numbers and Influence of the Technique on Measured Aerodynamic Coefficients

The method of temperature-sensitive paint (TSP) adapted to industry-scale, cryogenic wind tunnels (i.e. cryoTSP) is used to detect the natural, laminar-to-turbulent transition of the boundary layer in high speed flows. Because of the small boundary layer thickness given in cryogenic testing at high Reynolds numbers, the influence of roughness of a TSP layer on the natural boundary-layer transition is analyzed for Tollmien-Schlichting and for crossflow instability. Using a special technique when spraying the paint, it is possible to operate pressure tappings on a model in parallel to transition detection by cryoTSP. Since the pressure orifices have to penetrate the paint layer, their shape and curvature are changed by the painting and polishing, which may lead to a deviation in the measured C_p distributions compared to the model without TSP. Furthermore, the thickness of the cryoTSP on the model can influence the pressure distribution and drag coefficient. The effect of the TSP paint layer on pressure tappings and drag measurement of a laminar type airfoil is experimentally investigated for Reynolds numbers of 6 Mio and 15 Mio and for Mach numbers of M = 0.30 and 0.62 in the Ludwieg-tube cryogenic wind tunnel DNW-KRG.