Determination of the response time of TSP in cryogenic conditions

The capability of the Temperature-Sensitive Paint (TSP) method for application in unsteady flows in cryogenic wind tunnels was investigated by carrying out experiments in the Cryogenic Ludwieg-Tube Göttingen (KRG). The 2-D airfoil model (LV2F) was coated with a Ruthenium-based paint and tested in a nitrogen atmosphere at temperatures in the range 120 – 283 K. TSP images were acquired with a framing rate up to 50 kHz at different Mach and Reynolds numbers. With the help of the intensity-based TSP method, the transition location on the airfoil model under cryogenic conditions could be measured, and the model-characteristic transitional oscillations of the order of 100 Hz could be detected and resolved. In addition, intermittent turbulent wedges, which are often seen on wind tunnel model surfaces when using thermographic measurements methods, and which occur naturally mainly towards the beginning or end of a KRG wind tunnel run, provided another unique possibility to further analyze the response time of the TSP layer. Since the Ruthenium-based TSP used here was able to capture the footprints of these naturally occurring intermittent turbulent wedges under cryogenic conditions, one can conclude that the TSP coating procedure presented here can be successfully applied to detect unsteady flow phenomena with a time resolution better than 1 ms.