Cryogenic wind tunnel model with starboard wing painted by TSP
Cryogenic wind tunnels such as the European-Transonic Windtunnel (ETW) can be cooled down by liquid nitrogen to temperatures as low as 100 K. At this rather low temperature, the standard IR technique no longer works.
With the help of the temperature senitive paint (TSP) technique, the laminar-to-turbulent boundary-layer transition on wind tunnel models can nevertheless be detected. To visualize laminar and turbulent regions on a TSP painted surface, the method of temperature steps can be used: the oncoming flow is heated or cooled with respect to the model and the temperature change in the fluid is transfered faster to the painted surface in areas where the boundary layer is turbulent. This is caused by the different heat transfer coefficients in the turbulent and laminar boundary layers. Hence, the transition line occurs as a borderline between dark and light areas of a TSP image taken during the step change. In addition, methods like heating the model can be used to establish a temperature difference between flow and model.
The working principle of TSP is based on the thermal quenching mechanism of molecules which are embedded in the paint. These so-called luminophores are excited by incident light of a certain wavelength (for example UV or blue light) which sends the molecules to an excited state. Subsequently the excited molecules drop back to the ground state by emission of light of a longer wavelength (for example red). In addition to that, there exists a process of deactivation without light emission whose rate is dependent on the heat content of the paint. The higher the temperature of TSP, the more molecules drop back without light emission and the paint appears darker in comparison to colder regions.
The TSP-paint used by our institute for transition detection in cryogenic testing was developed by the Japanese Aerospace Exploration Agency (JAXA) and is optimized for large, industry-scale wind tunnels. The DLR Institute of Aerodynamics and Flow Technology continously improves the TSP technique and develops new paints for other applications as well.
Transition detection on various models, gathered in different cryogenic wind tunnels.
A colour map was applied to the original b/w result images. Bright areas represent the laminar, and darker (red) areas represent the turbulent part of the boundary layer. The corresponding Reynolds numbers were in the range of 2 Mio to 12 Mio and the Mach numbers covered the range of 0.2 to 0.8.
Train in the Side Wind Test Facility Göttingen