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Airfoil in a high amplitude oscillating stream

Published online by Cambridge University Press:  15 March 2016

C. Strangfeld ,
H. Müller-Vahl ,
C. N. Nayeri ,
C. O. Paschereit  and
D. Greenblatt
C. Strangfeld*
Affiliation:
Bundesanstalt für Materialforschung und -prüfung, Unter den Eichen 87, 12205 Berlin, Germany Hermann-Föttinger Institut, Institute of Fluid Dynamics and Technical Acoustics, Technische Universität Berlin, Müller-Breslau-Str. 8, 10623 Berlin, Germany
H. Müller-Vahl
Affiliation:
Hermann-Föttinger Institut, Institute of Fluid Dynamics and Technical Acoustics, Technische Universität Berlin, Müller-Breslau-Str. 8, 10623 Berlin, Germany Faculty of Mechanical Engineering, Technion – Israel Institute of Technology, 32000 Haifa, Israel
C. N. Nayeri
Affiliation:
Hermann-Föttinger Institut, Institute of Fluid Dynamics and Technical Acoustics, Technische Universität Berlin, Müller-Breslau-Str. 8, 10623 Berlin, Germany
C. O. Paschereit
Affiliation:
Hermann-Föttinger Institut, Institute of Fluid Dynamics and Technical Acoustics, Technische Universität Berlin, Müller-Breslau-Str. 8, 10623 Berlin, Germany
D. Greenblatt
Affiliation:
Faculty of Mechanical Engineering, Technion – Israel Institute of Technology, 32000 Haifa, Israel
*
†Email address for correspondence: christoph.strangfeld@bam.de
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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.

JFM classification

Aerodynamics: Aerodynamics Mathematical Foundations: General fluid mechanics Vortex Flows: Vortex shedding
Type
Papers
Information
Journal of Fluid Mechanics , Volume 793 , 25 April 2016 , pp. 79 - 108
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Copyright
© 2016 Cambridge University Press 

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