Fig. 2: LV2F-airfoil with a hot-film array in the adaptive test section of the DNW-KRG. Up-stream view into the Ludwieg tube
Fig. 3: Layer system of a hot-film suitable for cryogenic temperatures
It can be shown that in addition to the simulation of the entire Reynolds number range, also the turbulence level of the wind tunnels incoming flow has to be comparable to free flight conditions. Flow quality measurements have been conducted by AS-HK in the DNW-KRG, where the time and spatial development of single fluctuation quantities were investigated. Here the total pressure, static pressure, total temperature and mass flux were recorded in the test section and in the Ludwieg tube of the DNW-KRG. The dependence of the flow quality on the fluid temperature was of special interest, as an influence of the temperature is to be expected when the fluid is cooled down to 100 K for the simulation of flight Reynolds numbers in the DNW-KRG (Fig. 1).
A LV2F laminar type airfoil was equipped with a hot-film sensor array to investigate the transi-tion behaviour on a 2D profile in the DNW-KRG (Fig. 2). The thin boundary layers created by the high Reynolds number flow and low test temperatures required a special hot-film system. The hot-film sensor system was directly deposited on the model surface and characterised by its low surface roughness; guaranteeing a disturbance free transition measurement. A special elastic substrate was used for the sensors, which made the use of this system at low temperatures possible. The sensor design used here is a system of different layers (Fig. 3) as suggested by Johnson et al., with altered layer thicknesses and composition of the silicon dioxide layer for the tests in the DNW-KRG.
The point of transition, length of intermittent flow, and a range of other boundary layer properties could be extracted from the hot-film data. This allowed the characterisation of the temperature influence on transition at constant Mach and Reynolds number (Fig. 4).
Fig. 4: Determination of the transition location on the LV2F laminar type profile with the help of the rms of the installed hot-film A.C. voltage signals. The data here shows that the transition location is identical (x/c=30%) for different flow temperatures in the DNW-KRG
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