Embed Size (px)
Citation preview
Unsteady Flow Simulation around a Square Cylinder using Upstream Rod
ByRamakant pandey
(2010FE13)
under the guidance of
Er. Akshoy Ranjan PaulAsst. Professor, Applied Mechanics Department
DEPARTMENT OF APPLIED MECHANICSDEPARTMENT OF APPLIED MECHANICSMOTI LAL NEHRU NATIONAL INSTITUTE OF TECHNOLOGY, ALLAHABADMOTI LAL NEHRU NATIONAL INSTITUTE OF TECHNOLOGY, ALLAHABAD
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
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 .
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.
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.
Literature ReviewSr. 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.
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.
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.
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.
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.
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.
Staggered arrangement set-up
D
L
d/
Computational setup
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
Computational specificationSolver: 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 (), 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.
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
• Conservation of momentum for compressible turbulent flow with no body forces and source terms can be written as
.( ) 0Vt
��������������
.V
V V pt
������������������������������������������
. . . 0e
V e p V Vt
��������������������������������������������������������
• 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
tj k b M k
j j k j
kk ku G G Y S
t x x x
Model specificationThe modelled transport equations for k and ԑ in the realizable k- ԑ model are
2
1 2 1 3t
j bj j j
u C S C C C G St x x x kk v
jk i j
i
uG u u
x
The model constants taken for analysis are 1 21.45, 1.8, 1.0, 1.2kC C
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
Prt
b it i
TG g
x
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 ԑ respectively.
1C2C k
Meshing
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 2.0633.46
3 426552 2.0212.03
4 594600 1.9961.237
5 867785 1.9790.85
6 1184524 1.9760.15
DC
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)
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
Flow pattern 3D Bluff Body
Flow pattern 3D circular cylinder
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.
Flow visualization Static Pressure ContourAt L/D=1.9, d/D=0.267
Bare cylinder and α=1°
α=4° and α=6°
α=7° and α=19°
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
Streamlines
Streamlines pattern for bare cylinder and varying staggered angles at α= 0°, 1°, 4°, 6°, 7°, 19°
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
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°
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)
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
case
Case 1 2 3 4 5 6 7 8α 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
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
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.
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 9L/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
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.
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
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.
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.
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.
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
Upstream rod in Tandem arrangement
Upstream rod in 9° staggered angle
Few 3D visualisations
3D visualizations
Velocity contour
3D visualizations
3D visualization
Thankyou
