Through numerical simulations, this paper examines the nature of instability mechanisms leading to transition in separation bubbles. The results of two direct numerical simulations are presented in which separation of a laminar boundary layer occurs over a flat surface in the presence of an adverse pressure gradient. The primary difference in the flow conditions between the two simulations is the level of freestream turbulence with intensities of 0.1% and 1.45% at separation. In the first part of the paper, transition under a low-disturbance environment is examined, and the development of the Kelvin–Helmholtz instability in the separated shear layer is compared to the well-established instability characteristics of free shear layers. The study examines the role of the velocity-profile shape on the instability characteristics and the nature of the large-scale vortical structures shed downstream of the bubble. The second part of the paper examines transition in a high-disturbance environment, where the above-mentioned mechanism is bypassed as a result of elevated-freestream turbulence. Filtering of the freestream turbulence into the laminar boundary layer results in streamwise streaks, which provide conditions under which turbulent spots are produced in the separated shear layer, grow, and then merge to form a turbulent boundary layer. The results allow identification of the structure of the instability mechanism and the characteristic structure of the resultant turbulent spots. Recovery of the reattached turbulent boundary layer is then examined for both cases. The large-scale flow structures associated with transition are noted to remain coherent far downstream of reattachment, delaying recovery of the turbulent boundary layer to an equilibrium state.

1.
Mayle
,
R. E.
, 1991, “
The Role of Laminar-Turbulent Transition in Gas Turbine Engine
,”
ASME J. Turbomach.
0889-504X,
113
, pp.
509
537
.
2.
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
.
3.
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
.
4.
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
(
2
), p.
021006
.
5.
McAuliffe
,
B. R.
, 2007, “
Transition in Separation Bubbles: Physical Mechanisms and Passive Control Techniques
,” Ph.D. thesis, Carleton University.
6.
Roberts
,
S. K.
, and
Yaras
,
M. I.
, 2005, “
Boundary-Layer Transition Affected by Surface Roughness and Free-Stream Turbulence
,”
ASME J. Fluids Eng.
0098-2202,
127
, pp.
449
457
.
7.
Roberts
,
S. K.
, and
Yaras
,
M. I.
, 2006, “
Effects of Surface Roughness Geometry on Separation-Bubble Transition
,”
ASME J. Turbomach.
0889-504X,
128
, pp.
349
356
.
8.
Gostelow
,
J. P.
, and
Thomas
,
R. L.
, 2006, “
Interactions Between Propagating Wakes and Flow Instabilities in the Presence of a Laminar Separation Bubble
,” ASME Paper No. GT2006-91193.
9.
Watmuff
,
J. H.
, 1999, “
Evolution of a Wave Packet Into Vortex Loops in a Laminar Separation Bubble
,”
J. Fluid Mech.
0022-1120,
397
, pp.
119
169
.
10.
D’Ovidio
,
A.
,
Harkins
,
J. A.
, and
Gostelow
,
J. P.
, 2001, “
Turbulent Spots in Strong Adverse Pressure Gradients Part 1—Spot Behavior
,” ASME Paper No. 2001-GT-0194.
11.
Talan
,
M.
, and
Hourmouziadis
,
J.
, 2002, “
Characteristic Regimes of Transitional Separation Bubbles in Unsteady Flow
,”
Flow, Turbul. Combust.
1386-6184,
69
, pp.
207
227
.
12.
Volino
,
R. J.
, 2002, “
Separated Flow Transition Under Simulated Low-Pressure Turbine Air-Foil Conditions: Part 1—Mean Flow and Turbulence Statistics
,”
ASME J. Turbomach.
0889-504X,
124
, pp.
645
655
.
13.
Volino
,
R. J.
, 2002, “
Separated Flow Transition Under Simulated Low-Pressure Turbine Airfoil Conditions: Part 2—Turbulence Spectra
,”
ASME J. Turbomach.
0889-504X,
124
, pp.
656
664
.
14.
Roberts
,
S. K.
, and
Yaras
,
M. I.
, 2003, “
Effects of Periodic Unsteadiness, Free-Stream Turbulence and Flow Reynolds Number on Separation-Bubble Transition
,” ASME Paper No. GT2003-38626.
15.
Stieger
,
R. D.
, and
Hodson
,
H. P.
, 2004, “
The Transition Mechanism of Highly Loaded Low-Pressure Turbine Blades
,”
ASME J. Turbomach.
0889-504X,
126
, pp.
536
543
.
16.
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.
17.
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
.
18.
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
.
19.
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
.
20.
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.
21.
Wissink
,
J. G.
, and
Rodi
,
W.
, 2003, “
DNS of a Laminar Separation Bubble in the Presence of Oscillating External Flow
,”
Flow, Turbul. Combust.
1386-6184,
71
, pp.
311
331
.
22.
Wissink
,
J.
,
Michelassi
,
V.
, and
Rodi
,
W.
, 2004, “
Heat Transfer in a Laminar Separation Bubble Affected by Oscillating External Flow
,”
Int. J. Heat Fluid Flow
0142-727X,
25
, pp.
729
740
.
23.
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
.
24.
Roberts
,
S. K.
, and
Yaras
,
M. I.
, 2006, “
Large-Eddy Simulation of Transition in a Separation Bubble
,”
ASME J. Fluids Eng.
0098-2202,
128
, pp.
232
238
.
25.
Wissink
,
J.
, 2006, “
Separating, Transitional Flow Affected by Various Inflow Oscillation Regimes
,”
Appl. Math. Model.
0307-904X,
30
, pp.
1134
1142
.
26.
McAuliffe
,
B. R.
, and
Yaras
,
M. I.
, 2008, “
Numerical Study of Turbulent-Spot Development in a Separated Shear Layer
,”
ASME J. Turbomach.
0889-504X,
130
(
4
), p.
041018
.
27.
Raw
,
M.
, 1996, “
Robustness of Coupled Algebraic Multigrid for the Navier-Stokes Equations
,” AIAA Paper No. 96-0297.
28.
Hutchinson
,
B. R.
, and
Raithby
,
G. D.
, 1986, “
A Multigrid Method Based on the Additive Correction Strategy
,”
Numer. Heat Transfer
0149-5720,
9
, pp.
511
537
.
29.
Na
,
Y.
, and
Moin
,
P.
, 1998, “
Direct Numerical Simulation of a Separated Turbulent Boundary Layer
,”
J. Fluid Mech.
0022-1120,
374
, pp.
379
405
.
30.
Kalitzin
,
G.
, 2003, “
DNS of Fully Turbulent Flow in a LPT Passage
,”
Int. J. Heat Fluid Flow
0142-727X,
24
, pp.
636
644
.
31.
Roach
,
P. E.
, 1987, “
The Generation of Nearly Isotropic Turbulence by Means of Grids
,”
Int. J. Heat Fluid Flow
0142-727X,
8
, pp.
82
92
.
32.
Ho
,
C.
, and
Huerre
,
P.
, 1984, “
Perturbed Free Shear Layers
,”
Annu. Rev. Fluid Mech.
0066-4189,
16
, pp.
365
424
.
33.
Ripley
,
M. D.
, and
Pauley
,
L. L.
, 1993, “
The Unsteady Structure of Two-Dimensional Steady Laminar Separation
,”
Phys. Fluids A
0899-8213,
5
, pp.
3099
3106
.
34.
Muti Lin
,
J. C.
, and
Pauley
,
L. L.
, 1996, “
Low-Reynolds-Number Separation on an Airfoil
,”
AIAA J.
0001-1452,
34
, pp.
1570
1577
.
35.
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
.
36.
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
.
37.
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
.
38.
Huang
,
L. S.
, and
Ho
,
C. M.
, 1990, “
Small-Scale Transition in a Plane Mixing Layer
,”
J. Fluid Mech.
0022-1120,
210
, pp.
475
500
.
39.
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
.
40.
Winant
,
C. D.
, and
Browand
,
F. K.
, 1974, “
Vortex Pairing: The Mechanism of Turbulent Mixing-Layer Growth at Moderate Reynolds Number
,”
J. Fluid Mech.
0022-1120,
63
, pp.
237
255
.
41.
Ho
,
C.
, and
Huang
,
L. S.
, 1982, “
Subharmonics and Vortex Merging in Mixing Layers
,”
J. Fluid Mech.
0022-1120,
119
, pp.
443
473
.
42.
Malkiel
,
E.
, and
Mayle
,
R. E.
, 1996, “
Transition in a Separation Bubble
,”
ASME J. Turbomach.
0889-504X,
118
, pp.
752
759
.
43.
Pierrehumbert
,
R. T.
, and
Widnall
,
S. E.
, 1982, “
The Two-and Three-Dimensional Instabilities of a Spatially Periodic Shear Layer
,”
J. Fluid Mech.
0022-1120,
114
, pp.
59
82
.
44.
Schlichting
,
H.
, and
Gersten
,
K.
, 2000,
Boundary Layer Theory
,
8th ed.
,
Springer-Verlag
,
Berlin
.
45.
Panton
,
R. L.
, 2001, “
Overview of the Self-Sustaining Mechanisms of Wall Turbulence
,”
Prog. Aerosp. Sci.
0376-0421,
37
, pp.
341
383
.
46.
Kiya
,
M.
, and
Sasaki
,
K.
, 1985, “
Structure of Large-Scale Vortices and Unsteady Reverse Flow in the Reattaching Zone of a Turbulent Separation Bubble
,”
J. Fluid Mech.
0022-1120,
154
, pp.
463
491
.
47.
Emmons
,
H. W.
, 1951, “
The Laminar-Turbulent Transition in a Boundary Layer—Part 1
,”
J. Aeronaut. Sci.
0095-9812,
18
, pp.
490
498
.
48.
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
.
49.
Jacobs
,
R. G.
, and
Durbin
,
P. A.
, 2001, “
Simulations of Bypass Transition
,”
J. Fluid Mech.
0022-1120,
428
, pp.
185
212
.
50.
Asai
,
M.
,
Minagawa
,
M.
, and
Nishioka
,
M.
, 2002, “
The Instability and Breakdown of a Near-Wall Low-Speed Streak
,”
J. Fluid Mech.
0022-1120,
455
, pp.
289
314
.
51.
Konishi
,
Y.
, and
Asai
,
M.
, 2004, “
Experimental Investigation of the Instability of Spanwise-Periodic Low-Speed Streaks
,”
Fluid Dyn. Res.
0169-5983,
34
, p.
299
315
.
52.
Wu
,
X.
,
Jacobs
,
R. G.
,
Hunt
,
J. C. R.
, and
Durbin
,
P. A.
, 1999, “
Simulation of Boundary Layer Transition Induced by Periodically Passing Wakes
,”
J. Fluid Mech.
0022-1120,
398
, pp.
109
153
.
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