Large-scale organized motions (commonly referred to coherent structures) and flow topology of a transitional separated-reattached flow have been visualised and investigated using flow visualisation techniques. Two geometrical shapes including two-dimensional flat plate with rectangular leading edge and three-dimensional square cylinder are chosen to shed a light on the flow topology and present coherent structures of the flow over these shapes. For both geometries and in the early stage of the transition, two-dimensional Kelvin-Helmholtz rolls are formed downstream of the leading edge. They are observed to be twisting around the square cylinder while they stay flat in the case of the two-dimensional flat plate. For both geometrical shapes, the two-dimensional Kelvin-Helmholtz rolls move downstream of the leading edge and they are subjected to distortion to form three-dimensional hairpin structures. The flow topology in the flat plate is different from that in the square cylinder. For the flat plate, there is a merging process by a pairing of the Kelvin-Helmholtz rolls to form a large structure that breaks down directly into many hairpin structures. For the squire cylinder case, the Kelvin-Helmholtz roll evolves topologically to form a hairpin structure. In the squire cylinder case, the reattachment length is much shorter and a forming of the three-dimensional structures is closer to the leading edge than that in the flat plate case.

1.
J.
Taghinia
,
M.
Rahman
, and
T.
Siikonen
,
European Journal of Mechanics B/Fluids
52
,
11
18
(
2015
).
2.
Z.
Yang
,
Int. Journal of Comp. Meth. and Exp. Meas.
1
(
2
),
116
131
(
2013
).
3.
S. J.
Schreck
and
M. C.
Robinson
,
IEEE Transactions on Energy Conversion
22
(
1
),
61
70
(
2007
).
4.
C.
Bak
,
H. A.
Madsen
,
P.
Fuglsang
, and
F.
Rasmussen
,
Wind Energy
2
(
1
),
195
210
(
1999
).
5.
U.
Rist
and
K.
Augustin
,
AIAA Journal
44
(
10
),
2217
2223
(
2006
).
6.
A. E.
Alving
and
H. H.
Fernholz
,
Journal of Fluid Mechanics
322
,
297
328
(
1996
).
7.
B. J.
Cantwell
,
Ann. Rev. Fluid Mech.
13
,
457
515
(
1981
).
8.
A. K. M. F.
Hussain
,
Journal of Fluid Mechanics
173
,
303
356
(
1986
).
9.
S. K.
Robinson
,
The Kinematics of Turbulent Boundary Layer Structure
,
NASA Technical Memorandum TM 103859
,
California
,
1991
.
10.
W. A.
McMullan
and
S. J.
Garrett
,
Phys. Fluids
28
,
015111
(
2016
).
11.
P.
Comte
,
J. H.
Silvestrini
, and
P.
Begou
,
Eur. J. Mech. B/Fluids.
17
(
4
),
615
637
(
1998
).
12.
F. K.
Browand
and
T. R.
Trout
,
Journal of Fluid Mechanics
158
,
489
509
(
1985
).
13.
G. L.
Brown
and
A.
Roshko
,
Journal of Fluid Mechanics
64
,
775
816
(
1974
).
14.
A. E.
Perry
and
M. S.
Chong
,
Journal of Fluid Mechanics
119
,
173
217
(
1982
).
15.
S. I.
Chernyshenko
and
M. F.
Baig
,
J. Fluid Mech.
544
,
99
131
(
2005
).
16.
P.
Schlatter
,
L.
Brandt
,
H. C.
de Lange
, and
D. S.
Henningson
,
Physics of Fluids
20
,
101505
(
2008
).
17.
J. H.
Watmuff
,
D. A.
Pook
,
T.
Sayadi
, and
X.
Wu
,
Proceedings of the Summer Program, Center for Turbulence Research
,
2010
,
97
106
.
18.
K.
Deguchi
and
P.
Hall
,
J. Fluid Mech.
778
,
451
484
(
2015
).
19.
F.
Scarano
,
C.
Benocci
, and
M. L.
Riethmuller
,
Phys. Fluids
11
(
12
),
3808
3818
(
1999
).
20.
F.
Giralt
and
J. A.
Ferre
,
Phys. Fluids A
5
(
7
),
1783
1789
(
1993
).
21.
A. K. M. F.
Hussain
and
M.
Hayakawa
,
Journal of Fluid Mechanics
180
,
193
229
(
1987
).
22.
F.
Hussain
and
M. V.
Melander
,
Studies in Turbulence
1992
,
157
178
.
23.
Z.
Yang
,
International Journal of Numerical Methods in Fluids
40
,
723
733
(
2002
).
24.
R. J.
Adrian
,
C. D.
Meinhart
, and
C. D.
Tomkins
,
J. Fluid Mech.
422
,
1
54
(
2000
).
25.
S.
Wei
and
T.
Ning
,
ACTA MECHANICA SINICA
4
(
4
),
291
296
(
1988
).
26.
S. V.
Gordeyev
and
F. O.
Thomas
,
J. Fluid Mech.
414
,
145
194
(
2000
);
S. V.
Gordeyev
and
F. O.
Thomas
,
J. Fluid Mech.
460
,
349
380
(
2002
).
27.
L. O.
Hellstrom
,
B.
Ganapathisubramani
, and
A. J.
Smits
,
J. Fluid Mech.
779
,
701
715
(
2015
).
28.
S.
Karaca
and
A. G.
Gungor
,
European Journal of Mechanics B/Fluids
56
,
71
81
(
2016
).
29.
A. G.
Gungor
and
M. P.
Simens
,
Procedia IUTAM, ScienceDirect
,
2015
, Vol.
14
, p.
69
.
30.
A.
Ducoin
,
J
,
Loiseau
, and
J.
Robinet
,
European Journal of Mechanics B/Fluids
57
,
231
248
(
2016
).
31.
M.
Kiya
and
K.
Sasaki
,
J of Fluid Mech.
154
,
463
491
(
1985
);
M.
Kiya
and
K.
Sasaki
,
J of Fluid Mech.
137
,
83
113
(
1983
).
32.
I. E.
Abdalla
and
Z.
Yang
,
AIAA Journal
43
(
12
),
2465
2474
(
2005
).
33.
Z.
Yang
and
P. R.
Voke
,
Journal of Fluid Mechanics
439
,
305
333
(
2001
).
34.
I. E.
Abdalla
,
Z.
Yang
, and
M.
Cook
,
International Journal of Computational Fluid Dynamics
23
(
1
),
25
57
(
2009
).
35.
Z.
Yang
,
Chinese Journal of Aeronautics
28
(
1
),
11
24
(
2015
).
36.
P.
Germano
,
U.
Piomelli
,
P.
Moin
, and
W. H.
Cabot
,
Journal of Physics of Fluids A
3
(
7
),
1760
1765
(
1991
).
37.
D. K.
Lilly
,
Physics of Fluids a-Fluid Dynamics
4
(
3
),
633
635
(
1992
).
38.
R. I.
Issa
,
Journal of Comput. Phys.
62
,
40
65
(
1986
).
39.
L. P.
Castro
and
E.
Epik
,
Journal of Fluid Mechanics
374
,
91
116
(
1998
).
40.
Z.
Yang
and
I. E.
Abdalla
,
International Journal of Heat and Fluid Flow
30
(
5
),
1026
1035
(
2009
).
41.
H.
Le
,
P.
Moin
, and
J.
Kim
,
Journal of Fluid Mechanics
330
,
349
374
(
1997
).
42.
N. J.
Cherry
,
R.
Hillier
, and
M. E. M. P.
Latour
,
Journal of Fluid Mechanics
144
13
46
(
1984
).
43.
J. C. R.
Hunt
,
A. A.
Wary
, and
P.
Moin
,
Proceeding of Summer Program, Center for Turbulence Research
,
1988
,
193
208
.
This content is only available via PDF.