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Main Title: Airfoil in a high amplitude oscillating stream
Author(s): Strangfeld, Christoph
Müller-Vahl, Hanns
Nayeri, Christian Navid
Paschereit, Christian Oliver
Greenblatt, D.
Type: Article
Language Code: en
Abstract: A combined theoretical and experimental investigation was carried out with the objective of evaluating theoretical predictions relating to a two-dimensional airfoil subjected to high amplitude harmonic oscillation of the free stream at constant angle of attack. Current theoretical approaches were reviewed and extended for the purposes of quantifying the bound, unsteady vortex sheet strength along the airfoil chord. This resulted in a closed form solution that is valid for arbitrary reduced frequencies and amplitudes. In the experiments, the bound, unsteady vortex strength of a symmetric 18 % thick airfoil at low angles of attack was measured in a dedicated unsteady wind tunnel at maximum reduced frequencies of 0.1 and at velocity oscillations less than or equal to 50 %. With the boundary layer tripped near the leading edge and mid-chord, the phase and amplitude variations of the lift coefficient corresponded reasonably well with the theory. Near the maximum lift coefficient overshoot, the data exhibited an additional high-frequency oscillation. Comparisons of the measured and predicted vortex sheet indicated the existence of a recirculation bubble upstream of the trailing edge which sheds into the wake and modifies the Kutta condition. Without boundary layer tripping, a mid-chord bubble is present that strengthens during flow deceleration and its shedding produces a dramatically different effect. Instead of a lift coefficient overshoot, as per the theory, the data exhibit a significant undershoot. This undershoot is also accompanied by high-frequency oscillations that are characterized by the bubble shedding. In summary, the location of bubble and its subsequent shedding play decisive roles in the resulting temporal aerodynamic loads.
Issue Date: 2016
Date Available: 27-Oct-2017
DDC Class: 530 Physik
Subject(s): aerodynamics
general fluid mechanics
vortex shedding
Journal Title: Journal of fluid mechanics
Publisher: Cambridge University Press
Publisher Place: Cambridge
Volume: 793
Publisher DOI: 10.1017/jfm.2016.126
Page Start: 79
Page End: 108
EISSN: 1469-7645
ISSN: 0022-1120
Notes: Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.
This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.
Appears in Collections:FG Experimentelle Strömungsmechanik » Publications

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