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.

1.
Dovgal
,
A. V.
,
Kozlov
,
V. V.
, and
Michalke
,
A.
, 1994, “
Laminar Boundary Layer Separation: Instability and Associated Phenomena
,”
Prog. Aerosp. Sci.
0376-0421,
30
, pp.
61
94
.
2.
Estevadeordal
,
J.
, and
Kleis
,
S. J.
, 1999, “
High-Resolution Measurements of Two-Dimensional Instabilities and Turbulence Transition in Plane Mixing Layers
,”
Exp. Fluids
0723-4864,
27
, pp.
378
390
.
3.
Ho
,
C.
, and
Huerre
,
P.
, 1984, “
Perturbed Free Shear Layers
,”
Annu. Rev. Fluid Mech.
0066-4189,
16
, pp.
365
424
.
4.
Malkiel
,
E.
, and
Mayle
,
R. E.
, 1996, “
Transition in a Separation Bubble
,”
ASME J. Turbomach.
0889-504X,
118
, pp.
752
759
.
5.
Spalart
,
P. R.
, and
Strelets
,
M. K.
, 2000, “
Mechanisms of Transition and Heat Transfer in a Separation Bubble
,”
J. Fluid Mech.
0022-1120,
403
, pp.
329
349
.
6.
Yang
,
Z.
, and
Voke
,
P. R.
, 2001, “
Large-Eddy Simulation of Boundary-Layer Separation and Transition at a Change of Surface Curvature
,”
J. Fluid Mech.
0022-1120,
439
, pp.
305
333
.
7.
Abdalla
,
I. E.
, and
Yang
,
Z.
, 2004, “
Numerical Study of the Instability Mechanism in Transitional Separating-Reattaching Flow
,”
Int. J. Heat Fluid Flow
0142-727X,
25
, pp.
593
605
.
8.
McAuliffe
,
B. R.
, and
Yaras
,
M. I.
, 2005, “
Separation-Bubble-Transition Measurements on a Low-Re Airfoil Using Particle Image Velocimetry
,” ASME Paper No. GT2005-68663.
9.
McAuliffe
,
B. R.
, and
Yaras
,
M. I.
, 2008, “
Transition Mechanisms in Separation Bubbles Under Low and Elevated Freestream Turbulence
,”
ASME J. Turbomach.
0889-504X,
130
, in press.
10.
Volino
,
R. J.
, 2002, “
Separated Flow Transition Under Simulated Low-Pressure Turbine Airfoil Conditions: Part 1—Mean Flow and Turbulence Statistics
,”
ASME J. Turbomach.
0889-504X,
124
, pp.
645
655
.
11.
McAuliffe
,
B. R.
, and
Yaras
,
M. I.
, 2008, “
Numerical Study of Instability Mechanisms Leading to Transition in Separation Bubbles
,”
ASME J. Turbomach.
0889-504X,
130
, pp.
021006
.
12.
Gad-el-Hak
,
M.
, 1990, “
Control of Low-Speed Airfoil Aerodynamics
,”
AIAA J.
0001-1452,
28
, pp.
1537
1552
.
13.
Howell
,
R. J.
,
Ramesh
,
O. N.
,
Hodson
,
H. P.
,
Harvey
,
N. W.
, and
Schulte
,
V.
, 2001, “
High Lift and Aft-Loaded Profiles for Low-Pressure Turbines
,”
ASME J. Turbomach.
0889-504X,
123
, pp.
181
188
.
14.
Lake
,
J. P.
,
King
,
P. I.
, and
Rivir
,
R. B.
, 2000, “
Low Reynolds Number Loss Reduction on Turbine Blades With Dimples and V-Grooves
,” AIAA Paper No. 00-0738.
15.
Volino
,
R. J.
, 2003, “
Passive Flow Control on Low-Pressure Turbine Airfoils
,”
ASME J. Turbomach.
0889-504X,
125
, pp.
754
764
.
16.
Kerho
,
M.
,
Hutcherson
,
S.
,
Blackwelder
,
R. F.
, and
Liebeck
,
R. H.
, 1993, “
Vortex Generators Used to Control Laminar Separation Bubbles
,”
J. Aircr.
0021-8669,
30
, pp.
315
319
.
17.
McAuliffe
,
B. R.
, 2003, “
An Experimental Study of Flow Control Using Blowing for a Low-Pressure Turbine Airfoil
,” MS thesis, Carleton University, Ottawa, ON, Canada.
18.
Robarge
,
T. W.
,
Stark
,
A. M.
,
Min
,
S. -K.
,
Khalatov
,
A. A.
, and
Byerley
,
A. R.
, 2004, “
Design Considerations for Using Indented Surface Treatments to Control Boundary Layer Separation
,” AIAA Paper No. 2004-425.
19.
Zhang
,
X. -F.
, and
Hodson
,
H.
, 2005, “
The Combined Effects of Surface Trips and Unsteady Wakes on the Boundary Layer Development of an Ultra-High-Lift LP Turbine Blade
,”
ASME J. Turbomach.
0889-504X,
127
, pp.
479
488
.
20.
Zhang
,
X. -F.
,
Vera
,
M.
, and
Hodson
,
H.
, 2006, “
Separation and Transition Control on an Aft-Loaded Ultra-High-Lift LP Turbine Blade at Low Reynolds Numbers: Low-Speed Investigation
,”
ASME J. Turbomach.
0889-504X,
128
, pp.
517
527
.
21.
Bohl
,
D. G.
, and
Volino
,
R. J.
, 2005, “
Experiments With Three Dimensional Passive Flow Control Devices on Low-Pressure Turbine Airfoils
,” ASME Paper No. GT2005-68969.
22.
Merchant
,
A.
, 2003, “
Aerodynamic Design and Performance of Aspirated Airfoils
,”
ASME J. Turbomach.
0889-504X,
125
, pp.
141
148
.
23.
Bons
,
J. P.
,
Sondergaard
,
R.
, and
Rivir
,
R. B.
, 2001, “
Turbine Separation Control Using Pulsed Vortex Generator Jets
,”
ASME J. Turbomach.
0889-504X,
123
, pp.
198
206
.
24.
Bons
,
J. P.
,
Sondergaard
,
R.
, and
Rivir
,
R. B.
, 2002, “
The Fluid Dynamics of LPT Blade Separation Control Using Pulsed Jets
,”
ASME J. Turbomach.
0889-504X,
124
, pp.
77
85
.
25.
Culley
,
D. E.
,
Bright
,
M. M.
,
Prahst
,
P. S.
, and
Strazisar
,
A. J.
, 2004, “
Active Flow Separation Control of a Stator Vane Using Surface Injection in a Multistage Compressor Experiment
,”
ASME J. Turbomach.
0889-504X,
126
, pp.
24
34
.
26.
McAuliffe
,
B. R.
, and
Sjolander
,
S. A.
, 2004, “
Active Flow Control Using Steady Blowing for a Low-Pressure Turbine Cascade
,”
ASME J. Turbomach.
0889-504X,
126
, pp.
560
569
.
27.
Hanff
,
E. S.
, 2004, “
PIV Application in Advanced Low Reynolds Number Facility
,”
IEEE Aerosp. Electron. Syst. Mag.
0885-8985,
40
, pp.
310
319
.
28.
Bao
,
F.
, and
Dallmann
,
U. C.
, 2004, “
Some Physical Aspects of Separation Bubble on a Rounded Backward-Facing Step
,”
Aerosp. Sci. Technol.
1270-9638,
8
, pp.
83
91
.
29.
Saric
,
W. S.
,
Reed
,
H. L.
, and
Kerschen
,
E. J.
, 2002, “
Boundary-Layer Receptivity to Freestream Disturbances
,”
Annu. Rev. Fluid Mech.
0066-4189,
34
, pp.
291
319
.
30.
Scarano
,
F.
, and
Riethmuller
,
M. L.
, 2000, “
Advances in Iterative Multigrid PIV Image Processing
,”
Exp. Fluids
0723-4864,
29
, pp.
S51
S60
.
31.
Westerweel
,
J.
, 2000, “
Theoretical Analysis of the Measurement Precision in Particle Image Velocimetry
,”
Exp. Fluids
0723-4864,
29
, pp.
S3
S12
.
32.
Stanislas
,
M.
,
Okamoto
,
K.
, and
Kähler
,
C. J.
, 2003, “
Main Results of the First International PIV Challenge
,”
Meas. Sci. Technol.
0957-0233,
14
, pp.
R63
R89
.
33.
Stanislas
,
M.
,
Okamoto
,
K.
, and
Kähler
,
C. J.
, 2005, “
Main Results of the Second International PIV Challenge
,”
Exp. Fluids
0723-4864,
39
, pp.
170
191
.
34.
Raffel
,
M.
,
Willert
,
C.
, and
Kompenhans
,
J.
, 1998, “
Particle Image Velocimetry: A Practical Guide
,”
Experimental Fluid Mechanics
,
Springer-Verlag
,
Berlin
.
35.
Meunier
,
P.
, and
Leweke
,
T.
, 2003, “
Analysis and Treatment of Errors Due to High Velocity Gradients in Particle Image Velocimetry
,”
Exp. Fluids
0723-4864,
35
, pp.
408
421
.
36.
Hall
,
S. D.
,
Behnia
,
M.
,
Fletcher
,
C. A. J.
, and
Morrison
,
G.
, 2003, “
Investigation of the Secondary Corner Vortex in a Benchmark Turbulent Backward-Facing Step Using Cross-Correlation Particle Imaging Velocimetry
,”
Exp. Fluids
0723-4864,
35
, pp.
139
151
.
37.
Schlichting
,
H.
, and
Gersten
,
K.
, 2000,
Boundary Layer Theory
,
8th ed.
,
Springer-Verlag
,
Berlin
.
38.
Rist
,
U.
, and
Maucher
,
U.
, 2002, “
Investigations of Time-Growing Instabilities in Laminar Separation Bubbles
,”
Eur. J. Mech. B/Fluids
0997-7546,
21
, pp.
495
509
.
39.
Kelly
,
R. E.
, 1967, “
On The Stability of an Inviscid Shear Layer Which is Periodic in Space and Time
,”
J. Fluid Mech.
0022-1120,
27
, pp.
657
689
.
40.
Wissink
,
J. G.
, and
Rodi
,
W.
, 2002, “
DNS of Transition in a Laminar Separation Bubble
,”
Advances in Turbulence IX, Proceedings of the Ninth European Turbulence Conference
, Southampton, UK., I. P. Castro, and P. E. Hancock, eds.
41.
White
,
F. M.
, 1991,
Viscous Fluid Flow
,
2nd ed.
,
McGraw-Hill
,
Boston
.
42.
Panton
,
R. L.
, 2001, “
Overview of the Self-Sustaining Mechanisms of Wall Turbulence
,”
Prog. Aerosp. Sci.
0376-0421,
37
, pp.
341
383
.
43.
Ho
,
C.
, and
Huang
,
L. S.
, 1982, “
Subharmonics and Vortex Merging in Mixing Layers
,”
J. Fluid Mech.
0022-1120,
119
, pp.
443
473
.
44.
Alam
,
M.
, and
Sandham
,
N. D.
, 2000, “
Direct Numerical Simulation of ‘Short’ Laminar Separation Bubbles With Turbulent Reattachment
,”
J. Fluid Mech.
0022-1120,
403
, pp.
223
250
.
45.
Roberts
,
S. K.
, and
Yaras
,
M. I.
, 2005, “
Modeling Transition in Separated and Attached Boundary Layers
,”
ASME J. Turbomach.
0889-504X,
127
, pp.
402
411
.
46.
Praisner
,
T. J.
, and
Clark
,
J. P.
, 2007, “
Predicting Transition in Turbomachinery, Part I—A Review and New Model Development
,”
ASME J. Turbomach.
0889-504X,
129
, pp.
1
13
.
47.
Lang
,
M.
,
Rist
,
U.
, and
Wagner
,
S.
, 2004, “
Investigations on Controlled Transition Development in a Laminar Separation Bubble by Means of LDA and PIV
,”
Exp. Fluids
0723-4864,
36
, pp.
43
52
.
You do not currently have access to this content.