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J. Romblad, D. Ohno, T. Nemitz, W. Würz, E. Krämer
Within the Luftfahrtforschungsprogramm (LuFo), measurements on a laminar flow airfoil in flight as well as in a wind tunnel are combined with direct numerical simulations (DNS) to provide the base for future improvements of transition prediction under unsteady inflow conditions. Two regions of turbulent length scales of the inflow are of particular interest for investigating the effect on boundary layer transition on straight wings: large length scales leading to unsteady pressure distributions of the airfoil and small length scales corresponding to the streamwise wave lengths of Tollmien-Schlichting (TS) waves in the boundary layer. The current paper focuses on wind tunnel measurements of the effect of small scale turbulence. In the Laminar Wind Tunnel of the IAG, small scale turbulence is investigated using an active grid in the settling chamber, enabling measurements at turbulence levels (Tu) ranging from 0.01% (15-5000 Hz) without grid up to 0.11%. The turbulence spectra compare very well with flight measurements in the frequency range relevant for TS-instability. Hot-wire anemometry is used for detailed boundary layer measurement while modified piezoresistive pressure transducers are utilized to measure surface pressures fluctuations up to 8 kHz. In the range 0.01% <= Tu <= 0.11% the transition location on the pressure side of the MW-166-39-44-43 airfoil shifts upstream with increasing Tu, the frequency of the most amplified TS waves increases and the transition process change gradually from a quasi-uniform nature to a structure with isolated wave packets. Simultaneously the frequency wavenumber spectrum indicates an increased amount of energy at higher spanwise wave numbers. The Tu dependent n-factor of the MW-166-39-44-43 airfoil was found to match very well with the modified e^9 method of Mack, while the corresponding n-factor of a XIS40mod airfoil showed a much lower sensitivity, and a close to linear dependency on Tu. A variation in contribution of the receptivity coefficient in the modified e^9 method did not capture the difference in Tu-dependency observed for the two airfoils.
Deutscher Luft- und Raumfahrtkongress 2018, Friedrichshafen
Verlag, Ort:
Deutsche Gesellschaft für Luft- und Raumfahrt - Lilienthal-Oberth e.V., Bonn, 2018
Conference Paper
21,0 x 29,7 cm, 9 Seiten
Stichworte zum Inhalt:
Transition, Turbulent inflow
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Romblad, J.; Ohno, D.; et al. (2018): Laminar to Turbulent Transition due to unsteady Inflow Conditions: Wind Tunnel Experiments at increased Turbulence Levels. Deutsche Gesellschaft für Luft- und Raumfahrt - Lilienthal-Oberth e.V.. (Text). https://doi.org/10.25967/480138. urn:nbn:de:101:1-2018110914395849216046.
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