A transitional separation bubble develops on the suction side of an SD7003 airfoil. The transition process that forces the separated shear layer to reattach is governed by Kelvin-Helmholtz instabilities. Large scale vortices are formed due to this mechanism at the downstream end of the bubble. These vortices possess a three-dimensional structure and detach from the recirculation region, while other vortices are formed within the bubble. This detaching of the vortex is a highly unsteady process, which leads to a flapping of the separation bubble. The structure of these vortices and the flapping of the separation bubble due to these vortices are analyzed in time and space using time-resolved PIV measurements in a 2D and 3D set-up. The measurements are performed in a Goettingen type water tunnel using a rectangular SD7003 airfoil. The angle of attack is varied from 4 deg to 8 deg at a Reynolds number Rec=20,000. The light sheets are generated by an infrared laser-diode system and high-speed CMOS cameras are used. The 2D-time-resolved PIV measurements are performed in a single light-sheet. For the 3D-time-resolved PIV measurements a stereo-scanning PIV set-up with 10 light-sheet planes is used. The results of the time-resolved PIV measurements in a single light-sheet show the effect of a lower freestream turbulence level, i.e., an enlargement of the separation bubble. In general, the separation bubble moves upstream at increasing angle of attack, the vortex formation frequencies rise, which are frequencies of the bubble flapping. The instantaneous reattachment, which characterizes the bubble flapping, and the corresponding Strouhal number, is investigated. The stereo-scanning PIV measurements elucidate the three-dimensional character of the vortical structures, which evolve at the downstream end of the separation bubble. Typical structures are formed, e.g., the c-shape vortex and the screwdriver vortex.
Investigation of Kelvin-Helmholtz induced separation bubble flapping via time-resolved and scanning PIV measurements
Untersuchung von Kelvin-Helmholtz-induziertem Ablösungsblasenflattern durch zeitlich aufgelöste und abgetastete PIV-Messungen
2007
9 Seiten, 9 Bilder, 3 Tabellen, 23 Quellen
Aufsatz (Konferenz)
Datenträger
Englisch
Gasblase , Strömungsabriss , Flattern , Helmholtz-Gleichung , Teilchenabbildgeschwindigkeitsmessung , zeitliche Auflösung , Abtastung , Flügelprofil , Instabilität , Wirbelablösung , Rezirkulation , Wasserkanal , Anstellwinkel (Tragflügel) , Reynolds-Zahl , Infrarotlaser , Laserdiode , Strouhal-Zahl
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