Investigation of the vortex induced unsteadiness of a separation bubble via time-resolved and scanning PIV measurements |
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Authors: | S Burgmann W Schröder |
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Institution: | (1) Chair of Fluid Mechanics and Institute of Aerodynamics, RWTH Aachen University, Wüllnerstrasse 5a, 52062 Aachen, Germany |
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Abstract: | A transitional separation bubble on the suction side of an SD7003 airfoil is considered. The transition process that forces
the separated shear layer to reattach seems to be 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 separation of the vortex is a highly
unsteady process, which leads to a bubble flapping. The structure of these vortices and the flapping of the separation bubble
due to these vortices are temporally and spatially analyzed at angles of attack from 4° to 8° and chord-length based Reynolds
numbers Re
c = 20,000–60,000 using time-resolved PIV measurements in a 2D and a 3D set-up, i.e., stereo-scanning PIV measurements are
done in the latter case. These measurements complete former studies at a Reynolds number of Re
c = 20,000. The results of the time-resolved PIV measurements in a single light-sheet show the influence of the angle of attack
and the Reynolds number. The characteristic parameters of the separation bubble are analyzed focusing on the unsteadiness
of the separation bubble, e.g., the varying size of the main recirculation region, which characterizes the bubble flapping,
and the corresponding Strouhal number are investigated. Furthermore, the impact of the freestream turbulence is investigated
by juxtaposing the current and former results. The stereo-scanning PIV measurements at Reynolds numbers up to 60,000 elucidate
the three-dimensional character of the vortical structures, which evolve at the downstream end of the separation bubble. It
is shown that the same typical structures are formed, e.g., the c-shape vortex and the screwdriver vortex at each Reynolds
number and angle of attack investigated and the occurrence of these patterns in relation to Λ-structures is discussed. To
evidence the impact of the freestream turbulence, these results are compared with findings of former measurements. |
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