Unsteady boundary-layer transition measurements with Temperature-Sensitive Paint under cryogenic conditions

Kurzfassung

The study presents the first measurement of boundary-layer transition on a harmonically-pitching airfoil under cryogenic conditions. The experiments were conducted in the cryogenic wind tunnel of the German-Dutch Wind Tunnels in Cologne using a test rig especially designed for harmonic pitch oscillations of a two-dimensional model equipped with the laminar NLF(2)-0415 airfoil. The tests were performed at free stream Reynolds and Mach numbers of Re = 6 × 10^6 and M = 0.34. Unsteady pitch oscillations were investigated at different pitch frequencies (up to 40 Hz, i.e. a reduced frequency of 0.540) and pitch amplitudes, as well as for a steady angle-of-attack polar for comparison. The boundary-layer transition movement was captured by means of spatially high-resolving temperature-sensitive paint using a surface-integrated heating layer of carbon nanotubes (cntTSP) and fast-response pressure transducers. The cntTSP data was processed according to the already established 'Differential Thermography' (DT) method as well as using a recently presented method based on the evaluation of the qualitative distribution of the heat-transfer coefficient (HTC). The techniques are described in detail and results are evaluated with respect to measurement-based thermal hysteresis as well as the influence of varying pitch frequency and amplitude. The latter could successfully be measured by all applied methods. The findings further reveal significant improvements in the detection of unsteady boundary-layer transition when applying the HTC method compared to DT. These are: A reduction of the measurement error in terms of the thermal hysteresis component in the results and an increased result density at pitch phases, where the DT method inherently fails to yield results.