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

Abstract

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 a model’s boundary layer in high speed flows. Besides pure transition detection, cryoTSP result images provide information on the transition zone, the characteristics of transition and on the underlying instability. Because of the small boundary layer thickness given in cryogenic testing at high Reynolds numbers, the surface quality of the model must be very high. Therefore it is necessary to investigate the influence of the paint layer on resulting aerodynamic coefficients measured in parallel to transition detection by cryoTSP. Since temperature-sensitive paint has not to be oxygen permeable like pressure-sensitive paint (PSP), commercial three component polyurethan binders or clear coating can be used as the binder for cryoTSP. Such kind of paint layer can be polished to a desired level of roughness. We investigated the influence of surface roughness of cryoTSP on the free boundary-layer transition on 2D and 3D models in transonic wind tunnels. There are some criteria for surface finishing of cryogenic wind tunnel models. However, it can be shown that there may still be some effect of roughness on transition location for roughness of the order proposed by these values. As a rule of thumb it can be said, that surface finishing in cryogenic, high Reynolds number testing should be “as smooth as possible” when performing transition detection measurement. Using high resolution scientific CCD cameras for TSP image acquisition, a transition zone can be resolved rather than a sharp transition point in chord-wise direction, which represents the physical mechanism behind transition more accurately than specifying a transition point only. Additionally, laminar separation bubbles and flow separation can be detected by the cryoTSP method. Using a special technique when spraying the paint, it is possible to operate the pressure tappings on a model also during cryoTSP testing. 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 pressure values compared to the model without TSP. Furthermore, the thickness of the cryoTSP on the model (approx. 100 μm) can influence the pressure distribution. We experimentally investigated the effect of cryoTSP on pressure tapping on a natural laminar flow (NLF) airfoil at Reynolds numbers of 6 Mio and 15 Mio and for Mach numbers of M = 0.30 and 0.62, respectively.