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On vortex shedding from an airfoil in low-Reynolds-number flows

Published online by Cambridge University Press:  27 July 2009

SERHIY YARUSEVYCH ,
PIERRE E. SULLIVAN  and
JOHN G. KAWALL
SERHIY YARUSEVYCH*
Affiliation:
Department of Mechanical & Mechatronics Engineering, University of Waterloo, Waterloo, N2L 3G1, Canada
PIERRE E. SULLIVAN
Affiliation:
Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, M5S 3G8, Canada
JOHN G. KAWALL
Affiliation:
Department of Mechanical and Industrial Engineering, Ryerson University, Toronto, M5B 2K3, Canada
*
†Email address for correspondence: syarus@uwaterloo.ca
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Abstract

Development of coherent structures in the separated shear layer and wake of an airfoil in low-Reynolds-number flows was studied experimentally for a range of airfoil chord Reynolds numbers, 55 × 103 ≤ Rec ≤ 210 × 103, and three angles of attack, α = 0°, 5° and 10°. To illustrate the effect of separated shear layer development on the characteristics of coherent structures, experiments were conducted for two flow regimes common to airfoil operation at low Reynolds numbers: (i) boundary layer separation without reattachment and (ii) separation bubble formation. The results demonstrate that roll-up vortices form in the separated shear layer due to the amplification of natural disturbances, and these structures play a key role in flow transition to turbulence. The final stage of transition in the separated shear layer, associated with the growth of a sub-harmonic component of fundamental disturbances, is linked to the merging of the roll-up vortices. Turbulent wake vortex shedding is shown to occur for both flow regimes investigated. Each of the two flow regimes produces distinctly different characteristics of the roll-up and wake vortices. The study focuses on frequency scaling of the investigated coherent structures and the effect of flow regime on the frequency scaling. Analysis of the results and available data from previous experiments shows that the fundamental frequency of the shear layer vortices exhibits a power law dependency on the Reynolds number for both flow regimes. In contrast, the wake vortex shedding frequency is shown to vary linearly with the Reynolds number. An alternative frequency scaling is proposed, which results in a good collapse of experimental data across the investigated range of Reynolds numbers.

Type
Papers
Information
Journal of Fluid Mechanics , Volume 632 , 10 August 2009 , pp. 245 - 271
Copyright
Copyright © Cambridge University Press 2009

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