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Research Papers: Fundamental Issues and Canonical Flows

Passive Manipulation of Separation-Bubble Transition Using Surface Modifications

[+] Author and Article Information
Brian R. McAuliffe1

Department of Mechanical and Aerospace Engineering, Carleton University, 1125 Colonel By Drive, Ottawa, ON, K1S 5B6, Canadabrian.mcauliffe@nrc-cnrc.gc.ca

Metin I. Yaras

Department of Mechanical and Aerospace Engineering, Carleton University, 1125 Colonel By Drive, Ottawa, ON, K1S 5B6, Canadametiṉyaras@carleton.ca

1

Present address: Aerodynamics Laboratory, Institute for Aerospace Research, National Research Council of Canada, Building M-2, 1200 Montreal Road, Ottawa, ON, K1A 0R6, Canada.

J. Fluids Eng 131(2), 021201 (Jan 08, 2009) (16 pages) doi:10.1115/1.2978997 History: Received March 15, 2007; Revised July 18, 2008; Published January 08, 2009

Through experiments using two-dimensional particle-image velocimetry (PIV), this paper examines the nature of transition in a separation bubble and manipulations of the resultant breakdown to turbulence through passive means of control. An airfoil was used that provides minimal variation in the separation location over a wide operating range, with various two-dimensional modifications made to the surface for the purpose of manipulating the transition process. The study was conducted under low-freestream-turbulence conditions over a flow Reynolds number range of 28,000–101,000 based on airfoil chord. The spatial nature of the measurements has allowed identification of the dominant flow structures associated with transition in the separated shear layer and the manipulations introduced by the surface modifications. The Kelvin–Helmholtz (K-H) instability is identified as the dominant transition mechanism in the separated shear layer, leading to the roll-up of spanwise vorticity and subsequent breakdown into small-scale turbulence. Similarities with planar free-shear layers are noted, including the frequency of maximum amplification rate for the K-H instability and the vortex-pairing phenomenon initiated by a subharmonic instability. In some cases, secondary pairing events are observed and result in a laminar intervortex region consisting of freestream fluid entrained toward the surface due to the strong circulation of the large-scale vortices. Results of the surface-modification study show that different physical mechanisms can be manipulated to affect the separation, transition, and reattachment processes over the airfoil. These manipulations are also shown to affect the boundary-layer losses observed downstream of reattachment, with all surface-indentation configurations providing decreased losses at the three lowest Reynolds numbers and three of the five configurations providing decreased losses at the highest Reynolds number. The primary mechanisms that provide these manipulations include: suppression of the vortex-pairing phenomenon, which reduces both the shear-layer thickness and the levels of small-scale turbulence; the promotion of smaller-scale turbulence, resulting from the disturbances generated upstream of separation, which provides quicker transition and shorter separation bubbles; the elimination of the separation bubble with transition occurring in an attached boundary layer; and physical disturbance, downstream of separation, of the growing instability waves to manipulate the vortical structures and cause quicker reattachment.

Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 6

Instability growth in the separated shear layer for Re=39,000

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Figure 7

Fluctuation growth rates and transition onset location for Re=39,000

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Figure 12

Mean streamlines and contour plots of velocity correlations at Re=39,000 (dividing streamline overlaid on contour plots)

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Figure 13

Dominant wavelengths observed in the separated shear layer for all configurations

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Figure 14

Separation, transition onset, and reattachment locations for all configurations

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Figure 15

Transition onset and reattachment lengths, relative to separation, for all configurations

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Figure 16

Momentum-thickness Reynolds numbers for all configurations at x∕C=0.85

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Figure 9

Secondary pairing phenomenon observed at Re=39,000

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Figure 10

Streamwise correlation function of wall-normal velocity fluctuations at x∕C=0.426 and y∕C=0.061 for Re=39,000

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Figure 11

Streamwise correlation of dominant wavelengths for Re=39,000; (a) wavelength distribution and (b) correlation peak distribution

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Figure 8

Vortex-pairing phenomenon observed at Re=28,000

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Figure 20

Velocity-vector profiles and ⟨u′v′⟩ distributions at a Reynolds number of 51,000 for (a) the clean airfoil and (b) the LG configuration

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Figure 21

Unsteady separation observed downstream of the groove of the LG configuration at Re=101,000

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Figure 22

Unsteady separation observed downstream of the surface modification for the SN configuration at Re=101,000

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Figure 17

Velocity-vector profiles and ⟨u′v′⟩ distributions at a Reynolds number of 39,000 for (a) the clean airfoil and (b) the ST configuration

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Figure 18

Streamwise correlation of dominant wavelengths for SG and CL configurations for Re=28,000; (a) wavelength distributions and (b) correlation peak distributions

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Figure 19

Variation in instability growth and wavelength for the SR configuration at Re=39,000 for uncorrelated instants in time

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Figure 1

Schematic of low-Reynolds-number tow-tank facility

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Figure 2

Airfoil geometry

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Figure 3

Geometry of surface modifications (dashed line indicates extent of flexible vinyl strip)

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Figure 4

Ensemble-averaged streamlines over airfoil

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Figure 5

Shear layer property distributions over airfoil; (a) shear-layer edge velocity, (b) displacement and momentum thicknesses, and (c) shape factor

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