This paper describes the change in the transition mechanism of a separated boundary layer formed from the semicircular leading-edge of a constant thickness airfoil as the free-stream turbulence (fst) increases. Experiments are carried out in a low-speed wind tunnel for three levels of fst (Tu = 0.65%, 4.6%, and 7.7%) at two Reynolds numbers (Re) 25,000 and 55,000 (based on the leading-edge diameter). Measurements of velocity and surface pressure along with flow field visualizations are carried out using a planar particle image velocimetry (PIV). The flow undergoes separation in the vicinity of leading-edge and reattaches in the downstream forming a separation bubble. The shear layer is laminar up to 20% of separation length, and then, the perturbations are amplified in the second-half attributing to breakdown and reattachment. The bubble length is highly susceptible to change in Tu. At low fst, the primary mode of instability of the shear layer is Kelvin–Helmholtz (K-H), although the local viscous effect may not be neglected. At high fst, the mechanism of shear layer rollup is bypassed with transient growth of perturbations along with evidence of spot formation. The predominant shedding frequency when normalized with respect to the momentum thickness at separation is almost constant and shows a good agreement with the previous studies. After reattachment, the flow takes longer length to approach a canonical boundary layer.

References

References
1.
Horton
,
H. P.
,
1968
, “
Laminar Separation Bubbles in Two and Three Dimensional Incompressible Flow
,” Ph.D. thesis, Department of Aeronautical Engineering, Queen Mary College, University of London, London.
2.
Gaster
,
M.
,
1967
, “
The Structure and Behaviour of Laminar Separation Bubbles
,” Aerodynamics Division N.P.L., Ministry of Technology, London, Reports and Memoranda No. 3595.
3.
Cherry
,
N. J.
,
Hiller
,
R.
, and
Latour
,
M. E. M. P.
,
1984
, “
Unsteady Measurements in a Separated and Reattaching Flow
,”
J. Fluid Mech.
,
144
, pp.
13
46
.
4.
Watmuff
,
J. H.
,
1999
, “
Evolution of a Wave Packet Into Vortex Loops in a Laminar Separation Bubble
,”
J. Fluid Mech.
,
397
, pp.
119
169
.
5.
Talan
,
M.
, and
Hourmouziadis
,
J.
,
2002
, “
Characteristic Regimes of Transitional Separation Bubbles in Unsteady Flow
,”
Flow, Turbul. Combust.
,
69
(
3–4
), pp.
207
227
.
6.
Dahnert
,
J.
,
Lyko
,
C.
, and
Peitsch
,
D.
,
2013
, “
Transition Mechanisms in Laminar Separated Flow Under Simulated Low Pressure Turbine Aerofoil Conditions
,”
ASME J. Turbomach.
,
135
(
1
), p.
011007
.
7.
Yarusevych
,
S.
,
Sullivan
,
E. P.
, and
Kawall
,
G. J.
,
2009
, “
On Vortex Shedding From an Airfoil in Low-Reynolds-Number Flows
,”
J. Fluid Mech.
,
632
, pp.
245
271
.
8.
Schobeiri
,
M. T.
,
Ozturk
,
B.
, and
Ashpis
,
E. D.
,
2007
, “
Effect of Reynolds Number and Periodic Unsteady Wake Flow Condition on Boundary Layer Development, Separation, and Intermittency Behavior Along the Suction Surface of a Low Pressure Turbine Blade
,”
ASME J. Turbomach.
,
129
(
1
), pp.
92
107
.
9.
Satta
,
F.
,
Simoni
,
D.
,
Ubaldi
,
M.
,
Zunino
,
P.
, and
Bertini
,
F.
,
2010
, “
Experimental Investigation of Separation and Transition Process on a High Lift Low-Pressure Turbine Profile Under Steady and Unsteady Inflow at Low Reynolds Number
,”
J. Therm. Sci.
,
19
(
1
), pp.
26
33
.
10.
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.
11.
Hain
,
R.
,
Kahler
,
C. J.
, and
Radespiel
,
R.
,
2009
, “
Dynamics of Laminar Separation Bubbles at Low-Reynolds-Number Aerofoils
,”
J. Fluid Mech.
,
630
, pp.
129
153
.
12.
Hu
,
H.
, and
Yang
,
Z.
,
2008
, “
An Experimental Study of the Laminar Flow Separation on a Low-Reynolds-Number Airfoil
,”
ASME J. Fluids Eng.
,
130
(
5
), p.
051101
.
13.
Lin
,
J. C. M.
, and
Pauley
,
L. L.
,
1996
, “
Low-Reynolds Number Separation on an Airfoil
,”
AIAA J.
,
34
(
8
), pp.
1570
1577
.
14.
Robert
,
S. K.
, and
Yaras
,
M. I.
,
2005
, “
Boundary Layer Transition Affected by Surface Roughness and Free-Stream Turbulence
,”
ASME J. Fluids Eng.
,
127
(
3
), pp.
449
457
.
15.
Sarkar
,
S.
, and
Voke
,
P.
,
2006
, “
Large-Eddy Simulation of Unsteady Surface Pressure Over a Low-Pressure Turbine Blade Due to Interactions of Passing Wakes and Inflectional Boundary Layer
,”
ASME J. Turbomach.
,
128
(
2
), pp.
221
231
.
16.
Spalart
,
P. R.
, and
Strelets
,
M. K.
,
2000
, “
Mechanisms of Transition and Heat Transfer in a Separation Bubble
,”
J. Fluid Mech.
,
403
, pp.
329
349
.
17.
Alam
,
M.
, and
Sandham
,
N. D.
,
2000
, “
Direct Numerical Simulation of Short Laminar Separation Bubbles With Turbulent Reattachment
,”
J. Fluid Mech.
,
410
, pp.
1
28
.
18.
Yang
,
Z. Y.
, and
Voke
,
P. R.
,
2001
, “
Large-Eddy Simulation of Boundary Layer Separation and Transition at a Change of Surface Curvature
,”
J. Fluid Mech.
,
439
, pp.
305
333
.
19.
Sarkar
,
S.
,
2008
, “
Identification of Flow Structures on a LP Turbine Blade Due to Periodic Passing Wakes
,”
ASME J. Fluids Eng.
,
130
(
6
), p.
061103
.
20.
Sarkar
,
S.
,
2009
, “
Influence of Wake Structure on Unsteady Flow in an LP Turbine Blade Passage
,”
ASME J. Turbomach.
,
131
(
4
), p.
041016
.
21.
Hiller
,
R.
, and
Cherry
,
N. J.
,
1981
, “
The Effects of Stream Turbulence on Separation Bubbles
,”
J. Wind Eng. Ind. Aerodyn.
,
8
(
1–2
), pp.
49
58
.
22.
Volino
,
J. R.
, and
Hultgren
,
S. L.
,
2001
, “
Measurements in Separated and Transitional Boundary Layers Under Low-Pressure Turbine Airfoil Conditions
,”
ASME J. Turbomach.
,
123
(
2
), pp.
189
197
.
23.
Volino
,
R. J.
,
2002
, “
Separated Flow Transition Under Simulated Low-Pressure Turbine Airfoil Conditions—Part 1: Mean Flow and Turbulence Statistics
,”
ASME J. Turbomach.
,
124
(
4
), pp.
645
655
.
24.
McAuliffe
,
B. R.
, and
Yaras
,
M. I.
,
2010
, “
Transition Mechanisms in Separation Bubbles Under Low and Elevated Free-Stream Turbulence
,”
ASME J. Turbomach.
,
132
(
1
), p.
011004
.
25.
Langari
,
M.
, and
Yang
,
Z.
,
2013
, “
Numerical Study of the Primary Instability in a Separated Boundary Layer Transition Under Elevated Free-Stream Turbulence
,”
Phys. Fluids
,
25
(
7
), p.
074106
.
26.
Halstead
,
D. E.
,
Wisler
,
D. C.
,
Okiishi
,
T. H.
,
Walker
,
G. J.
,
Hodson
,
H. P.
, and
Shin
,
H.-W.
,
1997
, “
Boundary Layer Development in Axial Compressors and Turbines: Part 1 of 4—Composite Picture
,”
ASME J. Turbomach.
,
119
(
1
), pp.
114
127
.
27.
Solomon
,
W. J.
,
2000
, “
Effects of Turbulence and Solidity on the Boundary Layer Development in a Low Pressure Turbine
,”
ASME
Paper No. 2000-GT-0273.
28.
Tennekes
,
H.
, and
Lumley
,
J. L.
,
1972
,
A First Course in Turbulence
,
The MIT Press
, Cambridge, MA.
29.
Oberlack
,
M.
,
2002
, “
On the Decay Exponent of Isotropic Turbulence
,”
Proc. Appl. Math. Mech.
,
1
(
1
), pp.
294
297
.
30.
Sharma
,
D. M.
, and
Poddar
,
K.
,
2010
, “
Investigations on Quasi-Steady Characteristics for an Airfoil Oscillating at Low Reduced Frequencies
,”
Int. J. Aerosp. Eng.
,
2010
, p.
940528
.
31.
Yavuzkurt
,
S.
,
1984
, “
A Guide to Uncertainty Analysis of Hot-Wire Data
,”
ASME J. Fluids Eng.
,
106
(
2
), pp.
181
186
.
32.
Bradshaw
,
P.
,
1971
,
An Introduction to Turbulence and Its Measurement
,
Pergamon Press
,
Oxford
.
33.
Roshko
,
A.
, and
Lau
,
J. K.
,
1965
, “
Some Observations on Transition and Reattachment of a Free Shear Layer in Incompressible Flow
,”
Proc. Heat Transfer, Fluid Mech., Inst.
,
18
, pp.
157
167
.
34.
Gerakopulous
,
R.
,
Boutilier
,
M. S. H.
, and
Yarusevych
,
S.
,
2010
, “
Aerodynamic Characterisation of a NACA 0018 Airfoil at Low Reynolds Numbers
,”
AIAA
Paper No. 2010-4629.
35.
Stevenson
,
J. P. J.
,
Walsh
,
E. J.
,
Nolan
,
K. P.
, and
Davies
,
M. R. D.
,
2014
, “
Separation & Free-Stream Turbulence: Implications for Surface Aerodynamics & Heat Transfer
,”
J. Phys.: Conf. Ser.
,
525
(
1
), p.
012018
.
36.
Dovgal
,
A.
,
Kozlov
,
V.
, and
Michalke
,
A.
,
1994
, “
Laminar Boundary Layer Separation: Instability and Associated Phenomena
,”
Prog. Aerosp. Sci.
,
30
(
1
), pp.
61
94
.
37.
Kaun
,
C. L.
, and
Wang
,
T.
,
1990
, “
Investigation of the Intermittent Behavior of Transitional Boundary Layer Using a Conditional Averaging Technique
,”
Exp. Therm. Fluid Sci.
,
3
(
2
), pp.
157
173
.
38.
Hedley
,
T. B.
, and
Keffer
,
J. F.
,
1974
, “
Turbulent/Non-Turbulent Decisions in an Intermittent Flow
,”
J. Fluid Mech.
,
64
(
4
), pp.
625
644
.
39.
Dhawan
,
S.
, and
Narasimha
,
R.
,
1958
, “
Some Properties of Boundary Layer Flow During the Transition From Laminar to Turbulent Motion
,”
J. Fluid Mech.
,
3
(
4
), pp.
418
436
.
40.
Butler
,
K. M.
, and
Farrell
,
B. F.
,
1992
, “
Three-Dimensional Optical Disturbances in Viscous Shear Flow
,”
Phys. Fluids A
,
4
(
8
), pp.
1637
1650
.
41.
Jacobs
,
R. G.
, and
Durbin
,
P. A.
,
2001
, “
Simulations of Bypass Transition
,”
J. Fluid Mech.
,
428
, pp.
185
212
.
42.
Emmons
,
H. W.
,
1951
, “
The Laminar-Turbulent Transition in a Boundary Layer—Part I
,”
J. Aerosp. Sci.
,
18
(
7
), pp.
490
498
.
43.
Mayle
,
R. E.
,
1991
, “
The Role of Laminar–Turbulent Transition in Gas Turbine Engines
,”
ASME J. Turbomach.
,
113
(
4
), pp.
509
536
.
44.
Chen
,
K. K.
, and
Thyson
,
N. A.
,
1971
, “
Extension of Emmon's Spot Theory to Flows on Blunt Bodies
,”
AIAA J.
,
9
(
5
), pp.
821
825
.
45.
Ellsworth
,
R. H.
, and
Mueller
,
T. J.
,
1991
, “
Airfoil Boundary Layer Measurements at Low Re in an Accelerating Flow From a Nonzero Velocity
,”
Exp. Fluids
,
11
(
6
), pp.
368
374
.
46.
Luchini
,
P.
,
2000
, “
Reynolds-Number-Independent Instability of the Boundary Layer Over a Flat Surface
,”
J. Fluid Mech.
,
404
, pp.
289
309
.
47.
Matsubara
,
M.
, and
Alfredsson
,
H.
,
2001
, “
Disturbance Growth in Boundary Layers Subjected to Free-Stream Turbulence
,”
J. Fluid. Mech.
,
430
, pp.
149
168
.
48.
Ol
,
M. V.
,
Hanff
,
E.
,
McAuliffe
,
B.
,
Scholz
,
U.
, and
Kahler
,
C.
,
2005
, “
Comparison of Laminar Separation Bubble Measurements on a Low Reynolds Number Airfoil in Three Facilities
,”
AIAA
Paper No. 2005-5149.
49.
Chandrasekar
,
S.
,
1961
,
Hydrodynamic and Hydromagnetic Stability
,
Clarendon Press
,
Oxford
.
50.
Walker
,
G. J.
,
1989
, “
Transitional Flow on Axial Turbomachine Blading
,”
AIAA J.
,
27
(
5
), pp.
595
602
.
51.
Abu-Ghannam
,
B. J.
, and
Shaw
,
R.
,
1980
, “
Natural Transition of Boundary Layers—The Effects of Turbulence, Pressure Gradient, and Flow History
,”
J. Mech. Eng. Sci.
,
22
(
5
), pp.
213
228
.
52.
Gostelow
,
J. P.
,
Blunden
,
A. R.
, and
Blunden
,
W. R.
,
1994
, “
Effects of Free-Stream Turbulence and Adverse Pressure Gradients on Boundary Layer Transition
,”
ASME J. Turbomach.
,
116
(
3
), pp.
392
404
.
53.
Curle
,
N.
, and
Skan
,
S. W.
,
1957
, “
Approximate Methods for Predicting Separation Properties of Laminar Boundary Layers
,”
Aeronaut. Q.
,
8
, pp.
257
268
.
54.
Thwaites
,
B.
,
1949
, “
Approximate Calculation of the Laminar Boundary Layer
,”
Aeronaut. Q.
,
14
, pp.
61
85
.
55.
Roberts
,
W. B.
,
1980
, “
Calculation of Laminar Separation Bubbles and Their Effect on Airfoil Performance
,”
AIAA J.
,
18
(
1
), pp.
25
31
.
56.
Hinze
,
J. O.
,
1959
,
Turbulence
,
McGraw-Hill
,
New York
.
57.
Hatman
,
A.
, and
Wang
,
T.
,
1999
, “
A Prediction Model for Separated-Flow Transition
,”
ASME J. Turbomach.
,
121
(
3
), pp.
594
602
.
58.
Praisner
,
T. J.
, and
Clark
,
J. P.
,
2007
, “
Predicting Transition in Turbomachinery—Part I: A Review and New Model Development
,”
ASME J. Turbomach.
,
129
(
1
), pp.
1
13
.
59.
Davis
,
R. L.
,
Carter
,
J. E.
, and
Reshotko
,
E.
,
1987
, “
Analysis of Transitional Separation Bubbles on Infinite Swept Wings
,”
AIAA J.
,
25
(
3
), pp.
421
428
.
You do not currently have access to this content.