study of Drag reduction by changing the size and position of upstream cylinder.

1. Unsteady Flow Simulation around a Square Cylinder
using Upstream Rod
By
Ramakant pandey
(2010FE13)
under the guidance
of
Er. Akshoy Ranjan Paul
Asst. Professor, Applied Mechanics Department
DEPARTMENT OF APPLIED MECHANICSDEPARTMENT OF APPLIED MECHANICS
MOTI LAL NEHRU NATIONAL INSTITUTE OF TECHNOLOGY, ALLAHABADMOTI LAL NEHRU NATIONAL INSTITUTE OF TECHNOLOGY, ALLAHABAD

2. INDEX
• Introduction
• Overview
• Importance of Drag reduction
• Literature Review
• Gaps in Literature
• Objectives
• Solution Methodology
• Result and Discussion
• Conclusion (with proposed equation)
• Future Scope
• References
• 3D Animations

3. INTRODUCTION
• As we know that when a body move in air (any fluid) or air move on
still body, body experience a force, That force is called drag force.
In engineering and practical applications, like automobile, aircrafts
and architectural structures, such as bridge decks and monuments,
etc., have either square or rectangular or circular Cross - sections are
subjected to drag force.
• The cross flow around such bodies is characterized by a large region
of flow separation with suction pressure, resulting in a large value of
the resistance force, In many engineering applications, for certain
purposes, it is desirable to diminish this large value of drag
coefficient, CD.

4. Overview
• Experience shows that there is resistance to motion of solid bodies
through real fluid. It depends on the shape of the bodies and velocity
of that body. It act in the direction opposite to incoming flow
velocity.
• Bodies have streamlined shape induce small region of wake
formation compare to blunt body due to this streamlined body have
lower value of pressure drag in comparison to bluff body.
• Total drag consists of pressure drag or form drag and friction drag or
skin drag.

5. Importance of drag reduction
• The ability to manipulate a flow field is of immense technological
importance. For example, if drags of vehicles and buildings can be
reduced, much fuel cost and materials for the buildings would be
saved. Flow control around bluff bodies is of importance and of
interest for wind engineering.
• When one structure is immersed in the wake of another, the
characteristics of the flow and the aerodynamic forces depend
strongly on the shape, spacing between the structures, arrangement
of the structures, and wind direction. It is therefore useful to
investigate these characteristics from a practical point of view.

6. Literature Review
Sr.
No.
Authors
Name
Title of paper Year of
Publish
Nature of
Work(Exp./Comp.)
List of
variables
Major
findings
Further
Scope of
work
1 Moon Kyoung
Kim, Dong
Keon Kim,
Soon Hyun
Yoon and Dae
Hee Lee
Measurements of
the flow fields
around two square
cylinders in a
tandem
arrangement
2008 Experimental
(using PIV)
Spacing
between two
cylinders
Drastic change in
flow pattern at
critical length
Size variation,
shape variation,
staggered
arrangement.
2 P.F. Zhanga,
J.J. Wanga,
L.X. Huangb
Numerical
simulation of flow
around cylinder
with an upstream
rod in
tandem at low
Reynolds numbers
2006 Computational variation of the
center-to-center
spacing ratio
It is found
that the mean drag
and the lift
fluctuation of the
cylinder can be
reduced by the
upstream rod,
Shape variation,
staggered angle
variation.
3 Baris
Gumusel and
Cengiz camci
Aerodynamic drag
characteristics and
shape design
of a radar antenna
used for airport
ground traffic
control.(ASDE-
airport surface
detection
equipment)
2010 computational shape of antena increase in fineness
ratio results in drag
reduction or gives
better relative
improvement.
RI--it is defined as
ratio of drag
cofficient reduction
divided by the drag
cofficient of ASDE-
X cross section.
may be variable
cross section
can be used, or
an aerofile
shape can be
used.

7. Literature Review
4 Yoichi
Yamagishi,
Shigeo Kimura,
Makoto Oki and
Chisa Hatayama
Effect of corner
cutoffs on flow
characteristics
around a square
cylinder
international
conf., 2009,
moscow,
russia.
Experimental,
numerical
analysis and
visualizatin
changing
chamfer shape,
dimensions and
angles of
attack.
variations in the drag
coefficients CD with
the angle of attack α
for cylinders.
5 A. Prasad,
C.H.K.
Wi|liamson
A method for the
reduction of bluff
body drag
1997 Experimental upstream plate
width and
distance
between plate
and cylinder
it is possible to
reduce bluff body
drag dramatically
with the use of small
flat plates placed
upstream
Shape and size
variation,
staggered
arrangement with
angle variation
6 KWANGMIN SON,
JIN CHO,
WOO-PYUNG
JEON AND
HAECHEON CHOI
Mechanism of drag
reduction by a
surface trip wire on
a sphere
2011 Experimental Location of
wire and
diameter of
wire.
three different flow
characteristics are
observed above the
sphere surface,
7 Tamotsu
Igarashi,
Nobuaki Terachi
Drag reduction of
flat plate normal to
airstream by flow
control using a rod.
2002 Experimental Rod diameter
And distance
between axes
of rod and
plate.
The maximum
reduction of the total
drag coefficient is
about 20–30%
compared to the drag
without the rod in the
same range of the
Reynolds
number
Staggered
arrangement,
multi- ple rods can
be use, shape
change of
upstream rod.

8. Literature Review
8 T. Tsutsui, T.
Igarashi
Drag reduction
of a circular
cylinder in an
air-stream
2002 Experimental Upstream rod diameter
and distance between
cylinder and upstream
rod
The optimum
conditions of the drag reduction
are d/D=0.25, L/D=1.75 -2.0.
The reduction of the total drag
including the drag of the rod is
63% compared with that of a
single cylinder
`
9 Ming Zhao,
Liang Cheng,
Bin Teng,
Dongfang
Liang
Numerical
simulation of
viscous flow
past two
circular
cylinders of
different
diameters
2005 Computational The gap between the
small cylinder and the
large cylinder and The
position angle of the
small cylinder relative to
the flow direction
the shedding flow behind the
two cylinders can
be classified into three types, For
the very small gap ratio, there is
only one wake behind the two
cylinders, At medium gap ratios,
there exist strong interactions
between the vortex
shedding from the large cylinder
and the shedding from the small
cylinder, For very
large gap ratios, the interaction
between the shedding from
the two cylinders becomes very
weak
upstream
cylinder
diameter and
shape variation,
staggered angle.

9. Literature Review
Literature review
10 D. Sumner,
O. O.
Akosile
Behaviour of a
closely spaced pair
of circular cylinders
in cross-flow
CSME
2004
Forum 1
Experimental At two Pitch
Ratio,
staggered
angle.
The general behavior of the
force coefficients
and the Strouhal number was
similar for
both pitch ratios, since the
flow pattern for
closely spaced staggered
cylinders is similar to a
single bluff body, with a
single vortex shedding
process.
Two different shapes
can be used,
Reynolds no.
variation.
11 Shun C.
Yen , Jung
H. Liu
Wake flow behind
two side-by-side
square cylinders
2011 Experimental Reynolds No.,
Gap ratio.
Results classified into three
modes single mode, gap-flow
mode, couple vortex-
shedding.

10. Research Gap Identified from Literature
• Lots of studies have been done on drag reduction of circular cylinder but
paid little attention to square cylinder.
• Variation in center distance and size is done for drag reduction but no
findings on the drag reduction analysis by using the rod of different cross
sections.
• Effects of upstream rod shapes are not found in literature review.
• Use of multiple upstream rods not found in literature, which can be used in
staggered arrangement.
• Use of multiple upstream rods of different cross-section simultaneously.

11. Objectives
• To calculate drag, when staggered angle α is varying at
constant L/D and d/D.
• To calculate drag, when staggered angle α is varying at
constant d/D and variable L/D.
• To calculate drag, when staggered angle α, d/D and L/D are
varying.
• To calculate drag, when two upstream rods are used in
staggered arrangement.
• To obtain the least drag condition (optimized condition).
Note:- Multiple upstream rods can be arranged in tandem or staggered
arrangement, In tandem arrangement rods are placed one after another on
an axis while in staggered arrangement rods are placed with some stagger
angle along one side of axis or may be along both the sides of an axis.

12. Staggered arrangement set-up
D
L
α
d
/

13. Computational setup

14. Computational geometry
• Test section: 2500 mm × 1500 mm × 1500 mm
• Square cylinder: 60 mm × 60 mm × 1000 mm
• Upstream rod:
Length: 1000 mm
Diameter: 16.02 mm

15. Computational specification
Solver: 3D,pressure based, Unsteady.
Viscous model: Realizable K-ε, Standard wall function.
Boundary conditions:
Velocity inlet has taken as inlet boundary condition and zero gauge pressure used as
exit condition. upstream rod and downstream cylinder are given wall as two different
entities.
-velocity inlet: 15 m/s ,Re. No.61,500
. To indicate the turbulence quantities at the inlet, like turbulent kinetic energy (k) and
turbulence dissipation rate (ε), the following relation is used-
Where L – turbulent length scale
I – turbulent intensity = 0.16 (Re)-1/8
No slip boundary condition is specified at the wall.

16. Variation in Geometry
• Staggered angle(α)= 0, 1, 2, 3, 5, 9, 9, 31, 45.
• d/D=0.1, 0.167, 0.267, 0.5
• L/D=1.5, 1.7, 1.9, 2.3, 3.0, 3.5, 4.0

17. • Conservation of momentum for compressible turbulent flow with no body forces and
source terms can be written as
.( ) 0V
t
ρ
ρ
∂
+∇ =
∂
ur
.
V
V V p
t
ρ τ
 ∂
+ ∇ =−∇ −∇ ÷
∂ 
ur
uur ur
. . . 0
e
V e p V V
t
ρ τ
∂ 
+ ∇ + ∇ − ∇ = ÷
∂ 
uur ur r uur
• The equation for conservation of mass for an compressible flow in vector
notation can be written as
•Equation for the conservation of Energy for the compressible flow can be
written as
Governing Equations of Fluid Flow

18. ( ) ( ) t
j k b M k
j j k j
k
k ku G G Y S
t x x x
µ
ρ ρ µ ρε
σ
  ∂ ∂ ∂ ∂
+ = + + + + − +  ÷
∂ ∂ ∂ ∂   
Model specification
The modelled transport equations for k and in the realizable k- model areԑ ԑ
( ) ( )
2
1 2 1 3
t
j b
j j j
u C S C C C G S
t x x x kk v
ε ε ε
ε
µ ε ε ε
ρε ρε µ ρ ε ρ
σ ε
  ∂ ∂ ∂ ∂
+ = + + − + +  ÷
∂ ∂ ∂ ∂ +   
j
k i j
i
u
G u u
x
ρ
∂
′ ′= −
∂
The model constants taken for analysis are 1 21.45, 1.8, 1.0, 1.2kC Cε εσ σ= = = =
In above equation represents the generation of turbulence kinetic energy due to mean
velocity gradient
is the generation of turbulence kinetic energy due to buoyancy
kG
Pr
t
b i
t i
T
G g
x
µ
β
∂
=
∂
bG
YM represents the contribution of the fluctuating dilatation in compressible
turbulence to the overall dissipation rate. and are constants. and are the
turbulent Prandtl numbers for k and respectively.ԑ
1C2C kσ εσ

19. Meshing

20. Grid Independency Checking
Any CFD solution heavily depends on the size and fitness of meshing. Therefore, care
must be taken in selecting the grid types (coarse, medium or fine) so not to affect the
solution. In the present case, the solutions of bare cylinder are carried out for different
sizes of grids and coefficient of drag was monitored for each grid types.
Case Elements
% Error in successive cases
1 109643 2.137
-
2 194426 2.063
3.46
3 426552 2.021
2.03
4 594600 1.996
1.237
5 867785 1.979
0.85
6 1184524 1.976
0.15
DC

21. Validation
Two models K-ε RNG and K-ε
Realizable were tested in present
computational work, out of which K-
ε realizable shows better agreement
With reference Experimental paper of
Zhang and wang(2005).
% Error for K-ε RNG and Exp.
(P.F.Zhang -2005)= 16.75%
% Error for K-ε Realizable and Exp.
(P.F.Zhang -2005)= 4.31%
0 5 10 15 20
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Cp
Length along cylinder(mm)
0deg,L/D=1.9,d/D=0.267
Exp.(P.F.Zhang-2005)
Computational(Reliaziable)
Computational(RNG)

22. Validation for the bare cylinder
• For the case of Bare
cylinder Drag given in
reference paper is 2.21
and from present
computational work it
found 1.98.
• Percentage Error in
Drag is 10.40%.
0 5 10 15 20 25
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
Cp
Length along the cylinder (mm)
Exp.(P.F.Zhang-2005)
Present (computational)
Uncertainity=0.26

23. Flow pattern 3D Bluff Body

24. Flow pattern 3D circular cylinder

25. pattern of static pressure contour around square cylinder
Boundary layer
pattern
visualization in
presence of
upstream rod
and in bare
cylinder case.
In bare cylinder flow is coming
directly on front face but when
upstream rod is introduced, due to
wake of rod pressure at front face
of cylinder decreases and reduces
half of the drag acting on square
cylinder.

26. Flow
visualization
Static Pressure
Contour
At L/D=1.9, d/D=0.267
Bare cylinder and
α=1°
α=4° and α=6°
α=7° and α=19°

27. At L/D=1.9,d/D=0.267
Bare cylinder, α=1°,
α=4°, α=6°, α=7°, α=19°
In this figure
as the staggered angle α is
increasing the “shielding
effect” is decreasing which
results in the reduction of
drag, due to decrease in the
shielding effect, the back
suction pressure decreases
which is the main cause of
drag reduction.
At α ≥ 20, the
rod cylinder arrangement
starts acting like a bare
cylinder arrangement.
In wake
independent region rod and
cylinder gives the drag
similar to that in bare
cylinder case, which is 1.98
in the present
computational work.
Flow visualization Velocity pattern

28. Streamlines
Streamlines pattern for bare cylinder and varying
staggered angles at α= 0°, 1°, 4°, 6°, 7°, 19°

29. Results and discussion
Coefficient of pressure
• The case of the single cylinder is
also presented for comparison. For
a<2, the pressure distribution on the
upper and lower sides of the square
cylinder is roughly symmetrical
about the center line. The upward
side has a low pressure like that at
a=0, which implies that the shield
effect of the rod on the square
cylinder also exists, and that the flow
is in cavity flow mode.
•In wake merging mode (2 ≤ α ≤9),
the enhanced asymmetrical flow
results in the asymmetrical pressure
distribution on the square cylinder.
• Variation in staggered angle α at constant L/D and
d/D
α=0°-9°
L/D=1.9
d/D=0.267

30. Variation in staggered angle α at constant L/D and d/D
α=9°-45°, L/D=1.9, d/D=0.267
• In the weak boundary layer
interaction mode, the effect
of the rod’s wake on the
cylinder is reduced, and the
pressure on the upper and
lower sides tend to that of the
square cylinder alone in a
cross flow. This implies that
the separation bubble on the
lower side of the square
cylinder begins to disappear.
At last in the negligible
interaction mode, the
pressure distribution of the
square cylinder is the same as
that of a single cylinder.
0 5 10 15 20 25
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
Cp
Length along the cylinder (mm)
Bare
9°
19°
31°
45°

31. Drag Calculation For α=0°-45°
L/D=1.9
d/D=0.267
bare 0° 1° 2° 3° 5° 9° 19° 31° 45°
Cd 1.98 0.95 0.99 1.05 1.18 1.32 1.36 1.43 1.50 1.74
% reduction w.r.t bare
cylinder 51.83 49.63 46.67 40.27 33.36 31.33 27.63 24.03 12.29 12.29
Cd
Staggered angle (α)

32. Variation in staggered angle α at constant d/D and variable L/D
α=2°-9°, L/D=1.5-3.5, d/D=0.267
0 5 10 15 20 25
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
Cp
Length along the cylinder (mm)
d/D=0.267
angle L/D
2° 1.5
2° 1.7
3° 1.9
4° 2.3
7° 3.0
9° 3.5
9° 4.0

33. case
Case 1 2 3 4 5 6 7 8
α 2° 2° 3° 4° 5° 7° 9° 9°
L/D
1.5
1.7
1.9 2.3
2.7
3.0 3.5 4.0
CD 1.03 0.81
1.18 1.12
1.11
1.36 1.33 1.69
% Reduction in drag 47.97 59.1 40.4 43.4 43.9 31.8 32.8 14.62

34. Variation in staggered angle α at variable d/D and L/D
α=2°-23°,L/D=1.7-3.5, d/D=0.1, 0.5
0 5 10 15 20 25
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
Cp
Length along the cylinder(mm)
d/D=0.1 d/D=0.267 d/D=0.5
angle L/D angle L/D angle L/D
2° 1.7 4° 2.3 11° 2.5
2° 2.3 5° 2.7 23° 3.5
2° 3.5 9° 3.5 23° 4.0

35. Variation in d/D
0 5 10 15 20 25
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
Cp
Length along the cylinder(mm)
d/D=0.1
angle L/D
2° 1.7
2° 2.3
2° 3.5
0 5 10 15 20 25
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
Cp
Length along the cylinder(mm)
d/D=0.267
angle L/D
4° 2.3
5° 2.7
9° 3.5
0 5 10 15 20 25
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
Cp
Length along the cylinder(mm)
d/D=0.5
angle L/D
11° 2.5
23° 3.5
23° 4.0
Graphs shows the
variation of staggered
angle, L/D distance and
d/D ratio simultaneously.

36. Variation in staggered angle α at variable d/D and L/D
α=2°,11°,23°,L/D=1.7-3.5, d/D=0.1, 0.5
d/D
0.1
0.267
0.5
case 1 2 3 4 5 6 7 8 9
L/D 1.7 2.3 3.5 2.3 2.7 3.5 2.5 3.5 4.0
α 2° 2° 2° 4° 5° 9° 11° 23° 23°
CD 1.69 1.65 1.75 1.12 1.11 1.33 1.45 1.69 1.74
CD
Case

37. Use of two upstream rods
0 5 10 15 20 25
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
Cp
Length along the cylinder(mm)
angle L/D d/D
5° 1.9 0.1
5° 1.9 0.267
9° 1.9 0.5
Results obtained from above
three conditions are used to
choose the best three conditions
to visualize the effect of multiple
upstream rods to reduce the drag
of square cylinder, taking into
consideration total drag of the
system.
Pressure distribution along
square cylinder in case of 5°
staggered angle at d/D=0.5
gives better results as compared
to single upstream rod as in this
analysis total drag of system is
taken into consideration.

38. Use of two upstream rods
Cd
case
d/D 0.1 0.167 0.267
α 5° 9° 5°
Cd 1.85 1.19 0.65
% reduction w.r.t
bare cylinder 6.76 39.89 67.27

39. Conclusion
• In tandem arrangement, the reduction of drag was mainly caused by the increase of
the rear suction pressure. When the staggered angle was introduced, the shield and
the disturbance effect of the rod on the square cylinder diminished which results in
the increase of the cylinder drag. The side force induced by the staggered angle is
small.
• Drag coefficients have been calculated for staggered angle α,various combinations
of L/D and d/D ratios. The results obtained from the simulations are similar with
the reference paper(P.F.Zhang-2005).
• Cd keeps a low value for α<3–5 at different L/D. Afterwards, it increases quickly
to the value 1.98 (Bare cylinder drag coefficient in the present study). For α>20, Cd
has little variation and remains around the constant value of 1.98.
• The flow modes take place in the following regular order: the cavity mode or the
wake impinge mode (depends on L) occurs first and the wake splitting mode
follows. Then, the wake merging mode appears, and the next one is the weak
boundary layer interaction and the negligible interaction mode terminates the whole
process.
• For two upstream rods case 3 gives the maximum 67.27% reduction in drag with
respect to bare cylinder.

40. Future scope
• Experiments and simulations can be performed for more than one thin rod placed
upstream. Thus various combinations can be tested for different combinations of
diameters of rods and different combination of the distances between them.
• Various geometries of the upstream rod can be tested like triangular cross section
or an aero foil shaped rod.
• Upstream rods can be used with the change in cross-section of downstream cylinder
also.
• In the case of upstream rods, two rods of different cross-sections can be used.
• Slots of various shapes can be used in downstream cylinder to reduce pressure
separation.

41. References
• BLUFF-BODY AERODYNAMICS,Lecture Notes By Guido Buresti,Department of
Aerospace Engineering,University of Pisa, Italy
• Roshko,1960, “Experiments on the flow past a circular cylinder at very high Reynolds
number”, pp 345-356
• Bearman PW 1965, “Investigation of the flow behind a two-dimensional model with a blunt
trailing edge and fitted with splitter plates”, Journal of Fluid Mech. Vol. 21 pp.241–255.
• Bearman PW, Obasaju ED 1982, “An experimental study of pressure fluctuation on fixed and
oscillating square-section cylinder”, J Fluid Mech. Vol.119 pp. 297–321.
• Lesage F, Gartshore IS 1987, “A method of reducing drag and fluctuating side force on bluff
bodies”, Journal of Wind Engg.vol.2, pp. 229–245.
• Sakamoto and Haniu 1994, “Optimum suppression of fluid forces acting on a circular
cylinder”, ASME Journal of Fluids Eng. Vol.116, pp. 221–227.
• Williamson and Prasad, 1997, “A method for the reduction of bluff body drag” Journal of
Wind Engineering and Industrial Aerodynamics, vol. 69- 71, pp.155 167.
• Tamura, t,1998, “Numerical prediction of unsteady pressures on a square cylinder with
various corner shapes”, Journal of Wind Engineering and Industrial Aerodynamics, pp- 531-
542.
• T. Tamura, 1998, “Numerical prediction of unsteady pressures on a square cylinder with
various corner shapes” Journal of Wind Engineering and Industrial Aerodynamics vol. 74-76,
pp. 531-542.
• Lemay and Bouak, 1997, “Passive control of the aerodynamic forces acting on a circular
cylinder” Experimental thermal and fluid sciences, vol.16, Pages 112-121.

42. References
• Igarashi, T., 1997,” Drag reduction of a square prism by flow control using a small rod”,
Journal of Wind Engineering and Industrial Aerodynamics, vol. 67, pp-141-153.
• Liu, Chia-Hung, 2002, “Observations of hysteresis in flow around two square cylinders in a
tandem arrangement” Journal of Wind Engineering and Industrial Aerodynamics, vol. 90, pp.
1019–1050
• Igarashi, T., “Drag reduction of flat plate normal to airstream by flow control using a rod”,
Journal of Wind Engineering and Industrial Aerodynamics, vol. 90, page. 359–376, 2002
• Tsutsui, T. 2002, “Drag reduction of a circular cylinder in an air-stream”, Journal of Wind
Engineering and Industrial Aerodynamics, vol. 90, page. 527–541
• P. F. Zhang, J. J. Wang, 2005, “Aerodynamic characteristics of a square cylinder with a rod in
a staggered arrangement”, Experiments in Fluids, vol.38, pp 494–502.
• J. J. Wang, P. F. Zhang in, 2006, “Drag Reduction of a Circular Cylinder Using an Upstream
Rod Flow”, Turbulence and Combustion, vol.76, pp.-83–101.
• S.C. Yen, K.C. San, 2007, “Interactions of tandem square cylinders at low Reynolds
numbers”, Experimental Thermal and Fluid Science, vol. 32, pp-927–938
• Moon Kyoung Kim, Dong Keon Kim, 2008, “Measurements of the flow fields around two
square cylinders in a tandem arrangement”, Journal of Mechanical Science and Technology,
vol. 22, pp- 397-407

43. Upstream rod in Tandem arrangement

44. Upstream rod in 9° staggered angle

45. Few 3D visualisations

46. 3D visualizations

47. Velocity contour

48. 3D visualizations

49. 3D visualization

50. Thankyou