Abstract

Separated and reattached turbulent flows induced by two-dimensional forward–backward-facing steps (FBFS) with different streamwise lengths submerged in a thick turbulent boundary layer (TBL) are investigated using time-resolved particle image velocimetry (TR-PIV). The aspect ratios (AR) of the step range from 1 to 8, and the Reynolds number based on the freestream velocity and step height is 13 200. The thickness of the incoming turbulent boundary layer is 6.5 times the step height. The effects of varying aspect ratio of the steps on the mean flow, principal stretching, Reynolds stresses, triple velocity correlation, two-point autocorrelation, and unsteadiness of turbulent separation bubbles are studied. The results indicate that the mean flow reattaches over the step for FBFS with aspect ratios of 2 and higher. Two local peaks of Reynolds stresses occur irrespective of AR, and for a sufficiently large AR, a third local peak of Reynolds stresses appear in the shear layer emanating from the trailing edge of the bluff bodies. The streamwise decay of Reynolds stresses is slower for smaller AR cases. Incoming coherent structure is strongly disturbed near an inclined edge where principal stretching switches orientation abruptly. The temporal variation of the first proper orthogonal decomposition (POD) mode and reverse flow area over the bluff bodies shows remarkable correlation, which signifies the flapping motion of separation bubble.

References

References
1.
Ji
,
M.
, and
Wang
,
M.
,
2010
, “
Sound Generation by Turbulent Boundary-Layer Flow Over Small Steps
,”
J. Fluid Mech.
,
654
, pp.
161
193
.10.1017/S0022112010000467
2.
Wang
,
H. F.
,
Zhou
,
Y.
, and
Mi
,
J.
,
2012
, “
Effects of Aspect Ratio on the Drag of a Wall-Mounted Finite-Length Cylinder in Subcritical and Critical Regimes
,”
Exp. Fluids
,
53
(
2
), pp.
423
436
.10.1007/s00348-012-1299-z
3.
Meinders
,
E. R.
,
Hanjalic
,
K.
, and
Martinuzzi
,
R. J.
,
1999
, “
Experimental Study of the Local Convection Heat Transfer From a Wall-Mounted Cube in Turbulent Channel Flow
,”
ASME J. Heat Transfer
,
121
(
3
), pp.
564
573
.10.1115/1.2826017
4.
Abu-Mulaweh
,
H. I.
,
2009
, “
Investigations on the Effect of Backward-Facing and Forward-Facing Steps on Turbulent Mixed-Convection Flow Over a Flat Plate
,”
Exp. Heat Transfer
,
22
(
2
), pp.
117
127
.10.1080/08916150902805927
5.
Sherry
,
M.
,
Lo Jacono
,
D.
, and
Sheridan
,
J.
,
2010
, “
An Experimental Investigation of the Recirculation Zone Formed Downstream of a Forward Facing Step
,”
J. Wind Eng. Ind. Aerodyn.
,
98
(
12
), pp.
888
894
.10.1016/j.jweia.2010.09.003
6.
Essel
,
E. E.
,
Nematollahi
,
A.
,
Thacher
,
E. W.
, and
Tachie
,
M. F.
,
2015
, “
Effects of Upstream Roughness and Reynolds Number on Separated and Reattached Turbulent Flow
,”
J. Turbul.
,
16
(
9
), pp.
872
899
.10.1080/14685248.2015.1033060
7.
Graziani
,
A.
,
Lippert
,
M.
,
Uystepruyst
,
D.
, and
Keirsbulck
,
L.
,
2017
, “
Scaling and Flow Dependencies Over Forward-Facing Steps
,”
Int. J. Heat Fluid Flow
,
67
, pp.
220
229
.10.1016/j.ijheatfluidflow.2017.08.009
8.
Hattori
,
H.
, and
Nagano
,
Y.
,
2010
, “
Investigation of Turbulent Boundary Layer Over Forward-Facing Step Via Direct Numerical Simulation
,”
Int. J. Heat Fluid Flow
,
31
(
3
), pp.
284
294
.10.1016/j.ijheatfluidflow.2010.02.027
9.
Nematollahi
,
A.
, and
Tachie
,
M. F.
,
2018
, “
Time-Resolved PIV Measurement of Influence of Upstream Roughness on Separated and Reattached Turbulent Flows Over a Forward-Facing Step
,”
AIP Adv.
,
8
(
10
), p.
105110
.10.1063/1.5063455
10.
Hillier
,
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
.10.1016/0167-6105(81)90007-6
11.
Saathoff
,
P. J.
, and
Melbourne
,
W. H.
,
1987
, “
Free Stream Turbulence and Wind Tunnel Blockage Effects on Streamwise Pressures
,”
J. Wind Eng. Ind. Aerodyn.
,
26
(
3
), pp.
353
370
.10.1016/0167-6105(87)90005-5
12.
Pearson
,
D.
,
Goulart
,
P.
, and
Ganapathisubramani
,
B.
,
2013
, “
Turbulent Separation Upstream of a Forward-Facing Step
,”
J. Fluid Mech.
,
724
, pp.
284
304
.10.1017/jfm.2013.113
13.
Graziani
,
A.
,
Kerhervé
,
F.
,
Martinuzzi
,
R. J.
, and
Keirsbulck
,
L.
,
2018
, “
Dynamics of the Recirculating Areas of a Forward-Facing Step
,”
Exp. Fluids
,
59
(
10
), p.
154
.10.1007/s00348-018-2608-y
14.
Fang
,
X.
, and
Tachie
,
M. F.
,
2020
, “
Spatio-Temporal Dynamics of Flow Separation Induced by a Forward-Facing Step Submerged in a Thick Turbulent Boundary Layer
,”
J. Fluid Mech.
,
892
, p.
A40
.10.1017/jfm.2020.209
15.
Essel
,
E. E.
, and
Tachie
,
M. F.
,
2015
, “
Roughness Effects on Turbulent Flow Downstream of a Backward Facing Step
,”
Flow, Turbul. Combust.
,
94
(
1
), pp.
125
153
.10.1007/s10494-014-9549-1
16.
Wu
,
Y.
,
Ren
,
H.
, and
Tang
,
H.
,
2013
, “
Turbulent Flow Over a Rough Backward-Facing Step
,”
Int. J. Heat Fluid Flow
,
44
, pp.
155
169
.10.1016/j.ijheatfluidflow.2013.05.014
17.
Hu
,
W.
,
Hickel
,
S.
, and
van Oudheusden
,
B.
,
2019
, “
Dynamics of a Supersonic Transitional Flow Over a Backward-Facing Step
,”
Phys. Rev. Fluids
,
4
(
10
), p.
103904
.10.1103/PhysRevFluids.4.103904
18.
Nakamura
,
Y.
, and
Ohya
,
Y.
,
1984
, “
The Effects of Turbulence on the Mean Flow Past Two-Dimensional Rectangular Cylinders
,”
J. Fluid Mech.
,
149
(
1
), pp.
255
273
.10.1017/S0022112084002640
19.
Akon
,
A.
, and
Kopp
,
G.
,
2016
, “
Mean Pressure Distributions and Reattachment Lengths for Roof-Separation Bubbles on Low-Rise Buildings
,”
J. Wind Eng. Ind. Aerodyn.
,
155
, pp.
115
125
.10.1016/j.jweia.2016.05.008
20.
Bergeles
,
G.
, and
Athanassiadis
,
N.
,
1983
, “
The Flow Past a Surface-Mounted Obstacle
,”
ASME J. Fluids Eng.
,
105
(
4
), pp.
461
463
.10.1115/1.3241030
21.
van der Kindere
,
J.
, and
Ganapathisubramani
,
B.
,
2018
, “
Effect of Length of Two-Dimensional Obstacles on Characteristics of Separation and Reattachment
,”
J. Wind Eng. Ind. Aerodyn.
,
178
, pp.
38
48
.10.1016/j.jweia.2018.04.018
22.
Fang
,
X.
, and
Tachie
,
M. F.
,
2019
, “
On the Unsteady Characteristics of Turbulent Separations Over a Forward-Backward-Facing Step
,”
J. Fluid Mech.
,
863
, pp.
994
1030
.10.1017/jfm.2018.962
23.
Cook
,
N. J.
,
1978
, “
Wind Tunnel Simulation of the Adiabatic Atmospheric Boundary Layer by Roughness Barrier and Mixing Device Methods
,”
J. Wind Eng. Ind. Aerodyn.
,
3
(
2–3
), pp.
157
176
.10.1016/0167-6105(78)90007-7
24.
Lim
,
H. C.
,
Castro
,
I. P.
, and
Hoxey
,
R. P.
,
2007
, “
Bluff Bodies in Deep Turbulent Boundary Layers: Reynolds-Number Issues
,”
J. Fluid Mech.
,
571
, pp.
97
118
.10.1017/S0022112006003223
25.
Fang
,
X.
, and
Tachie
,
M. F.
,
2019
, “
Flows Over Surface-Mounted Bluff Bodies With Different Spanwise Widths Submerged in a Deep Turbulent Boundary Layer
,”
J. Fluid Mech.
,
877
, pp.
717
758
.10.1017/jfm.2019.617
26.
Samimy
,
M.
, and
Lele
,
S. K.
,
1991
, “
Motion of Particles With Inertia in a Compressible Free Shear Layer
,”
Phys. Rev. Fluids, A
,
3
(
8
), pp.
1915
1923
.10.1063/1.857921
27.
Kumar
,
P. P.
, and
Dey
,
J.
,
2019
, “
Shape Factor of the Turbulent Boundary Layer on a Flat Plate and the Reynolds Shear Stress in the Outer Region
,”
Phys. Rev. Fluids
,
4
(
2
), p.
02460
.10.1103/PhysRevFluids.4.024605
28.
Adrian
,
R. J.
,
Meinhart
,
C. D.
, and
Tomkins
,
C. D.
,
2000
, “
Vortex Organization in the Outer Region of the Turbulent Boundary Layer
,”
J. Fluid Mech.
,
422
, pp.
1
54
.10.1017/S0022112000001580
29.
Pope
,
S. B.
,
2000
,
Turbulent Flows
,
Cambridge University Press
,
Cambridge, UK
.
30.
Jimenez
,
J.
,
1991
, “
Kinematic Alignment Effects in Turbulent Flows
,”
Phys. Rev. Fluids, A
,
4
(
4
), pp.
652
654
.10.1063/1.858282
31.
Moin
,
P.
, and
Kim
,
J.
,
1985
, “
The Structure of the Vorticity Field in Turbulent Channel Flow—Part 1. Analysis of Instantaneous Fields and Statistical Correlations
,”
J. Fluid Mech.
,
155
, pp.
441
464
.10.1017/S0022112085001896
32.
Blackburn
,
H. M.
,
Mansour
,
N. N.
, and
Cantwell
,
B. J.
,
1996
, “
Topology of Fine-Scale Motions in Turbulent Channel Flow
,”
J. Fluid Mech.
,
310
, pp.
269
292
.10.1017/S0022112096001802
33.
Sirovich
,
L.
,
1987
, “
Turbulence and the Dynamics of Coherent Structures Part I
,”
Q. Appl. Math.
,
45
(
3
), pp.
561
571
.10.1090/qam/910462
34.
Meyer
,
K. E.
,
Pedersen
,
J. M.
, and
Ozcan
,
O.
,
2007
, “
A Turbulent Jet in Crossflow Analysed With Proper Orthogonal Decomposition
,”
J. Fluid Mech.
,
583
, pp.
199
227
.10.1017/S0022112007006143
35.
Thacker
,
A.
,
Aubrun
,
S.
,
Leroy
,
A.
, and
Devinant
,
P.
,
2013
, “
Experimental Characterization of Flow Unsteadiness in the Centerline Plane of an Ahmed Body Rear Slant
,”
Exp. Fluids
,
54
(
3
), p.
1479
.10.1007/s00348-013-1479-5
36.
Mohammed-Taifour
,
A.
, and
Weiss
,
J.
,
2016
, “
Unsteadiness in a Large Turbulent Separation Bubble
,”
J. Fluid Mech.
,
799
, pp.
383
412
.10.1017/jfm.2016.377
You do not currently have access to this content.