Time-resolved and volumetric PIV measurements of a transitional separation bubble on an SD7003 airfoil |
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Authors: | S Burgmann J Dannemann W Schröder |
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Institution: | (1) Department of Mechanical Engineering, Institute of Aerodynamics, RWTH Aachen University, Wüllnerstrasse 5a, 52062 Aachen, Germany |
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Abstract: | To comprehensively understand the effects of Kelvin–Helmholtz instabilities on a transitional separation bubble on the suction
side of an airfoil regarding as to flapping of the bubble and its impact on the airfoil performance, the temporal and spatial
structure of the vortices occurring at the downstream end of the separation bubble is investigated. Since the bubble variation
leads to a change of the pressure distribution, the investigation of the instantaneous velocity field is essential to understand
the details of the overall airfoil performance. This vortex formation in the reattachment region on the upper surface of an
SD7003 airfoil is analyzed in detail at different angles of attack. At a Reynolds number Re
c < 100,000 the laminar boundary layer separates at angles of attack >4°. Due to transition processes, turbulent reattachment
of the separated shear layer occurs enclosing a locally confined recirculation region. To identify the location of the separation
bubble and to describe the dynamics of the reattachment, a time-resolved PIV measurement in a single light-sheet is performed.
To elucidate the spatial structure of the flow patterns in the reattachment region in time and space, a stereo scanning PIV
set-up is applied. The flow field is recorded in at least ten successive light-sheet planes with two high-speed cameras enclosing
a viewing angle of 65° to detect all three velocity components within a light-sheet leading to a time-resolved volumetric
measurement due to a high scanning speed. The measurements evidence the development of quasi-periodic vortex structures. The
temporal dynamics of the vortex roll-up, initialized by the Kelvin–Helmholtz (KH) instability, is shown as well as the spatial
development of the vortex roll-up process. Based on these measurements a model for the evolving vortex structure consisting
of the formation of c-shape vortices and their transformation into screwdriver vortices is introduced. |
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