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Time-resolved topology of turbulent boundary layer separation over the trailing edge of an airfoil

Published online by Cambridge University Press:  18 March 2020

Austin Ma ,
Bradley Gibeau  and
Sina Ghaemi Open the ORCID record for Sina Ghaemi [Opens in a new window]
Austin Ma
Affiliation:
Department of Mechanical Engineering, University of Alberta, Edmonton, AlbertaT6G 2R3, Canada
Bradley Gibeau
Affiliation:
Department of Mechanical Engineering, University of Alberta, Edmonton, AlbertaT6G 2R3, Canada
Sina Ghaemi*
Affiliation:
Department of Mechanical Engineering, University of Alberta, Edmonton, AlbertaT6G 2R3, Canada
*
†Email address for correspondence: ghaemi@ualberta.ca
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Abstract

The unsteady organization of a separated turbulent boundary layer was investigated upstream from the trailing edge of a NACA 4418 airfoil. The angle of attack was $9^{\circ }$ in the pre-stall regime. Two particle image velocimetry fields of view were of interest: a streamwise–wall-normal plane at midspan of the airfoil and a streamwise–spanwise plane parallel to and near the surface of the airfoil. In the near-surface streamwise–spanwise plane, the mean velocity field revealed a saddle point near midspan and a pair of counter-rotating foci at the sides. This pattern is reminiscent of a stall cell, which has been traditionally associated with flow separation on thick airfoils at and slightly beyond the angle of attack of maximum lift. Isolating the low frequencies showed that the instantaneous separation front consisted of several smaller structures that also resembled a stall cell pattern, but they were an order of magnitude smaller than the one found in the mean pattern. These instantaneous stall cells were of two types: forward and backward. The forward stall cells were formed by strong high-speed streaks from upstream, while backward stall cells formed as a result of strong backflow just downstream from the separation front, resulting in a foci pair and a saddle point on their upstream side. In both cases, the foci pairs acted to mobilize high-speed momentum of the associated streak into a rotational motion, causing these streaks to dissipate. Finally, proper orthogonal decomposition revealed that low-order modes were associated with the movement and distortion of the separation front.

JFM classification

Boundary Layers: Boundary layer separation Turbulent Flows: Turbulent boundary layers
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 891 , 25 May 2020 , A1
Copyright
© The Author(s), 2020. Published by Cambridge University Press

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Ma et al. supplementary movie 1

Time-resolved animation of the skin-friction lines over 1.5 s. The flow is slowed down by a factor of 12 in the video and the background contours shows the instantaneous streamwise velocity.

Download Ma et al. supplementary movie 1(Video)
Video 24.2 MB

Ma et al. supplementary movie 2

A video of the formation of a forward stall cell due to local concentration of downstream momentum. The colors in the background correspond to instantaneous streamwise velocity.

Download Ma et al. supplementary movie 2(Video)
Video 4.3 MB

Ma et al. supplementary movie 3

A video of the formation of a backward stall cell due to a strong backflow. The colors in the background correspond to instantaneous streamwise velocity.

Download Ma et al. supplementary movie 3(Video)
Video 6.7 MB