CFD

1. CFD PROJECT
CFD Fundamental Study of Flow Past a Circular Cylinder
with Convective Heat Transfer
Presented by; Sammy Jamar Chemengich.
Presented to; Dr. Hesham M. Khalil
JANUARY 12, 2020
ALEXANDRIA UNIVERSITY
ALEXANDRIA, EGYPT.

2. 1
Table ofContents
1. LITERATURE REVIEW ON THE PROBLEM........................................................................................2
2. MODEL ANALYSIS AND SET UP .....................................................................................................3
2.1 Details of meshing and meshing quality. .....................................................................................3
Fig 1. A mesh representation of the domain with a zoomed in details around the cylinder ..............3
Table 1. Mesh metrics for different grid sizes................................................................................3
2.2 Problem setup and boundary conditions ......................................................................................4
Fig 2. A 2D representation of the experiment set up......................................................................4
2.3 Grid independent study. .............................................................................................................5
Fig3; Average velocityatdifferentgridsizes;30704 cellsforseriesFlu,8892 cellsforseriesFlu1
and 3648 cells for series Flu 2.......................................................................................................5
3. RESULTS AND DISCUSSION...........................................................................................................6
3.1 Variation of flow pattern in the wake region past the cylinder with flow Reynolds number.............6
Fig 5. Path lines of velocity magnitude for case 1 & 2.....................................................................8
Fig 6. Streamlines of velocity magnitude for case 3 & 4..................................................................8
3.2 Estimation of boundary layer separation angle at different flow Reynolds numbers........................9
Fig 7. Estimation of the separation angle for different Reynolds number ......................................10
Fig 8(b). A graphical representation of Cd Vs Re done by [8].........................................................10
Fig 9(a). A graphical representation of the change of Cdwith iterations for Re=10000...................11
Fig 9(b). A graphical representation of the change of force with iterations for Re=10000...............11
3.3 Evaluation of the relative importance of both convective and conductive heat transfer and variation
of Nusselt number with Reynolds numbers .....................................................................................11
Fig11. A graphical representationshowingthe effectof higherReynoldsnumberonheattransfer
via convection and conduction...................................................................................................12
3.4 Validation with available experimental data.........................................................................13
Fig12. A representationof velocity magnitude atthe wake regionfromcase 1a- 4a comparedwith
experimental data for case 1b-4b...............................................................................................14
4. CONCLUSION ............................................................................................................................15
5. REFERENCES..............................................................................................................................16

3. 2
1. LITERATURE REVIEW ON THE PROBLEM.
The flow of fluids and forced convection across a heated cylinder has been the subject of considerable
research interest because of its relevance in many engineering applications. For instance, a knowledge of
the hydrodynamic forces experienced by submerged cylindrical objects such as off-shore pipelines is
essential for the design of such structures. On the other hand, because of changing process and climatic
conditions, one also needs to determine the rate of heat transfer from such structures. Furthermore, present
economic and environmental concerns have raised an interest in methods to determine and control forced
convection heat transfer from horizontal cylindrical structures. Additional industrial processes where
heat/mass transfer from an isolated cylinder plays an important role include anemometry and chemical or
radioactive contamination/ purification, glass cooling, plastics and industrial devices, and other processes
from turbine blades to electronic circuits[1]–[7].
The Reynolds and Nusselt number, the rate of heat transfer from a cylinder to the streaming fluid is also
influenced by the type of thermal boundary condition prescribed at the surface of the cylinder, namely,
constant temperature or uniform heat flux. While in practice, the boundary conditions tend to be mixed and
complex, these two limiting thermal boundary conditions represent the cases of a generalthermalboundary
condition, i.e., linear heat transfer to a fixed temperature,isoflux for very low and isothermal for very high
heat transfer coefficients. Further complications arise from the variety of the possible flow regimes
depending upon the value of the Reynolds number. It is now generally agreed that the flow characteristics
past an confined cylinder transit from one regime to another as follows: steady flow without separation at
Re < 5, steady flow with two symmetric vortices up to Re <49, the onset of laminar vortex shedding at Re
≈ 49, three-dimensional (3D) wake-transition at Re ≈ 190–260, and shear-layer transition at Re ≈103
and
105
[5].
The present work is aimed at investigating the heat transfer in laminar and turbulent flows past a heated
circular cylinder confined between two parallel plates by using ANSYS Fluent to model and simulate the
flow and study the effects of Reynolds number and Nusselt number in heat transfer.

4. 3
2. MODEL ANALYSIS AND SET UP
2.1 Details of meshing and meshing quality.
The grid structure of the computational domain used in the present investigation is shown in Fig. 2. The
best mesh for easy computation was found to be mesh 2 with element size of 0.04mm spread uniformly
within the fluid domain with some inflation around the cylinder and the plate walls.
Fig 1. A mesh representation ofthe domain with a zoomed in details around the cylinder
Table 1. Mesh metrics for different grid sizes
Mesh grids Minimum Maximum Average Standard
deviation
Grid 1 element size 0.04mm
Nodes 2422; elements 3648
Aspect ratio 1.0021 7.6625 2.3218 2.0158
skewness 1.3057e-010 0.6187 4.5593e-002 5.591e-002
Grid 2 element size 0.02mm
Nodes 5334; elements 8892
Aspect ratio 1.009 7.665 1.6657 1.8635
skewness 1.3057e-010 0.55288 4.358e-002 5.2368e-002
Grid 3 element size 0.01mm
Nodes 16820; elements 30704
Aspect ratio 1.008 7.665 1.5087 1.1999
skewness 0.25609 1.001 0.92196 0.16181

5. 4
2.2 Problem setup and boundary conditions
Fig 2. A 2D representation ofthe experiment set up
Table 2. inflow velocity at each Reynolds number
Case Inlet velocity Reynolds number
Case 1 0.002mm/s 0.038
Case 2 3.65mm/s 50
Case 3 14.64mm/s 200
Case 4 730mm/s 10000
The experiment was performed using ANSYS fluent. The energy option in the fluent solver was activated
and the set up was initialized with constant values for the solid and fluid systems i.e. aluminum and air. The
method of solver was PISO algorithm with second order priority since its more stable and computationally
cheap. Several monitors were created to specifically monitor variables like velocity, pressure, drag
coefficient and drag force. Case 1-3 were solved under the laminar flow solver as the Reynolds number is
small.
Case 4 was solved under the k-epsilon model, realizable epsilon with standard wall functions. This
turbulence model was chosen mainly because of its suitability to solve simple confined flows and its cheap
computation cost. Both of the cases were performed under default system settings for outflow parameters.
The set up for the cylinder was at 573K for temperature and the plate walls and surface was assigned 293K.
The system tests the effects of air flow in conduction and convection of heat. The properties of air were
assumed to be; density of 1.225kg/m3, temperature of 293K and kinematic viscosity of 1.78x10-6
.
AsAnsys requires thatthe boundary conditions be assigned some pressure values, zero pressure wasapplied
to the outflow boundary in this analysis. On the inflow boundary, a uniform velocity was assigned to the x-
direction. The inflow velocity may be obtained based on Reynolds number using equation 1.

6. 5
𝑹𝒆 =
𝝆𝒗𝑫
𝝁
(1)
Where 𝜌=density of air
𝑣= velocity of air
𝐷 =hydraulic diameter of the plates
𝑢=kinematic viscosity of air
2.3 Grid independent study.
In this study, three different mesh sizes (Grid 1-3648, Grid 2-8892 and Grid 3-30704) are adopted in order
to check the mesh independence. A detailed grid independence study has been performed and results are
obtained for the average velocity but there are no considerable changes between Grid 1 and Grid 2 (the
results are shown below). Thus, a grid size of 3648 is found to meet the requirements of the both grid
independence and computation time limit.
The grid independence study was conducted under the highest inlet velocity of 730mm/s since its where
the turbulence occurs. Through the experiment we establish that a grid of 3648 cells is the best due to its
low computation cost and its ability to converge fast.
Fig 3; Average velocity at different grid sizes; 30704 cells for series Flu, 8892 cells for series Flu 1
and 3648 cells for seriesFlu 2

7. 6
3. RESULTS AND DISCUSSION
3.1 Variation of flow pattern in the wake region past the cylinder with flow Reynolds number.
The set up was run through the ANSYS software to determine the possible flow patterns under the different
flow velocities. The time taken to perform the analysis duration increased with an increase in the Reynolds
number, with the analysis for case 4 taking about streamlines derived from the analysis are shown below.
Some typical cases compared with the experimental results [1].
Case 1 corresponds to flow with sufficiently low Reynolds number (slug flow or laminar flow). And their
streamlines appear symmetrically front to rear. When the Reynolds number exceed 50 as seen in case 2,a
pair of upper and lower vortices were generated within the wake region of the cylinder.as the Reynolds
number exceeded 200, the von Karman vortex street occurred. When the Reynolds number increased
further, the von Karman vortices lasted for a shorter period. As a result, the distance of the vortices
decreased. The results of the analysis are in good agreement with the visualized outcome derived from the
experiments.
Case 1; Contour of Velocity magnitude at Re=0.038
Case 2; Contour of Velocity magnitude at Re=50

8. 7
Case 3; Contour of Velocity magnitude at Re=200
Case 4; Contour of Velocity magnitude at Re=10000
Fig 4. Contour of velocity for different cases from case 1-4.
Case 1, path line of velocity magnitude at Re=0.038

9. 8
Case 2, Path line of velocity magnitude at Re=50
Fig 5. Path lines ofvelocity magnitude for case 1 & 2
Case 3, stream lines for velocity magnitude at Re=200
Case 4, streamline of velocity magnitude at Re=10000
Fig 6. Streamlines ofvelocity magnitude for case 3 & 4

10. 9
3.2 Estimation of boundary layer separation angle at different flow Reynolds numbers .
In fluid mechanics flow, when the Re<1, the fluid does not leave the cylinder but flows uniformly along its
surface. This state is called laminar flow. As the Reynolds number gradually increases, the flow separates
from the surface behind the cylindrical surface, and vortices begin to form a wake region.
Furthermore, when Reynolds number gets between 50-500, separated vortices are generated regularly
above and below the cylinder. This is called Karman vortex. Finally, when Re> 1000, the vortex behind the
cylinder is randomly generated, forming a turbulent flow.
Taking case 2 for example, where Reynolds number is 50, the behavior of the flow was traced. The
distributions of the velocity vectors in the vicinity of the cylinder are plotted as shown below. Measuring
the angle clockwise starting from the center of the front edge of the cylinder, it was found that the flow
begins to separate from the wall surface at an angle of about 130 degrees and that reverse flow region
formed[1].
To determine this, the figures below show the flow streamline around the cylinder at various Reynolds
numbers.
Flow velocity in the vicinity of the cylinder for Reynolds number of 50
Flow velocity in the vicinity of the cylinder for Reynolds number of 200

11. 10
Flow velocity in the vicinity of the cylinder for Reynolds number of 10000
Fig 7. Estimation ofthe separation angle for different Reynolds number
Fig 8(a). A graphical representation of Cd Vs Re for the experiment
Fig 8(b). A graphical representation ofCd Vs Re done by [8]

12. 11
The experiment gives very high values of Coefficient drag vs Reynolds number. However it achieves the
principle of fluid flow as experimentally proved by [8] research. For flow of Re=10000 the fluid achieves
almost close value to the experimental data value of Cd= 0.3 for Re of 10000.
Fig 9(a). A graphical representation ofthe change ofCd with iterations for Re=10000
Fig 9(b). A graphical representation ofthe change offorce with iterations for Re=10000
3.3 Evaluation of the relative importance of both convective and conductive heat transfer and
variation of Nusselt number with Reynolds numbers.
A Nusselt number of value one represents heat transfer by pure conduction. A value between one and 10 is
characteristic of slug flow or laminar flow. Alarger Nusselt number corresponds to more active convection,
with turbulent flow typically in the 100–1000 range.
The Nusselt number equation is given by;

13. 12
𝑵𝒖 =
𝒉𝑫
𝒌
(2)
Where h is the convective heat transfer coefficient of flow
D is the hydraulic diameter
K is the thermal conductivity of the fluid
In the experiment performed, the Nusselt number changes from 10 to over 1000 over the range of velocities
0.038mm/s to 730mms/s. This indicates the change from laminar flow to turbulent flow.
The experiment also sums up the research on flow over cylinders by giving the relationship between the
Reynolds number and the Nusselt number and how they vary. It indicates that they vary proportionally and
their increase shows the effects of both conduction in laminar flow to both conduction and convection of
heat in the turbulent flow.
Fig 10. A graphical representation Reynolds number vs Nusselt number
Fig 11. A graphical representation showing the effect ofhigher Reynolds number on heat transfer
via convection and conduction.

14. 13
When air flows through the channel, heat is transferred by both conduction and convection. The convective
heat transfer coefficient increases with the increase of Reynolds number in an obstruction created between
two parallel plates. In this case, the surface friction coefficient of the length tube decreases when the
Reynolds number is increased[8].
3.4 Validation with available experimental data.
When Re<1, the fluid does not leave the cylinder but flows uniformly along its surface. This state is called
laminar flow. As the Reynolds number gradually increases,the flow separates from the surface behind the
cylindrical surface, and vortices begin to form a wake region.
Furthermore, when Reynolds number gets between 50-500, separated vortices are generated regularly
above and below the cylinder. This is called Karman vortex. Finally, when Re> 1000, the vortex behind the
cylinder is randomly generated,forming a turbulent flow. Turbulence is closely related to the generation of
vortices, especially because their location and size are random, so they need to be treated stochastically
rather than deterministically.
To correctly determine the validity of the experimental data obtained from this analysis, a closer comparison
is made with the available data to compare the results. It is found that the results from the experiment are
correctly in line with the experimental data done by Van Dyke, 1982; JSME,1992[9]
1(b) Re=0.038
Case 1 (a) Re=0.038
Case 2(b) Re=50
Case 2(a) Re=50

15. 14
Casse 3(b) Re=200
Case 3(a) Re=200
Case (4b)Re=10000
Case 4(a)=10000
Fig 12. A representation ofvelocity magnitude at the wake region from case 1a- 4a compared with
experimental data for case 1b-4b

16. 15
4. CONCLUSION
The flow past a circular cylinder placed symmetrically in a channel is simulated using ANSYS fluent. The
cylinder-based Reynolds numbers considered in the simulation are 0.038, 50, 200 and 10000 with the
normalized channel height of 0.8m.
The results obtained for velocity magnitude profiles indicate that they are in line with previous experiments.
The relationship between Nusselt number and Reynolds number was clearly determined and how an
increase in Reynolds number affects the heat transferthrough conduction and convection when air is blown
over a cylinder confined between two plates[10]. It is noted that an increase in Reynolds numbers results
to an increase in Nusselt number
However,in obtaining the relationship between the coefficient of drag with Reynolds number in the flow,
the values obtained are slightly higher than those for the previous experiments. Furthermore, if the Reynolds
number becomeshigher than 200, three-dimensional characteristicsof the von Karmanvortices may appear,
and then flow will be transitioned into a turbulent flow. These phenomena will be addressed in a future
study. Adeeperunderstanding of fluid flow mechanics and heattransferprinciples needs to be incorporated
in future to obtain better results.

17. 16
5. REFERENCES
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/ CFD,” 2012 SIMULIA Community Conf.,pp. 1–15, 2012.
[2] M. Asif and A. Dhiman, “Analysis of laminar flow across a triangular periodic array of heated
cylinders,” J. Brazilian Soc. Mech. Sci. Eng.,vol. 40, no. 7, 2018.
[3] R. Golani and A. Dhiman, “Fluid flow and heat transfer across a circular cylinder in the unsteady
flow regime,” Int. J. Eng. Sci.,vol. 3, no. 3, pp. 8–19, 2014.
[4] H. M. S. Bahaidarah, M. Ijaz, and N. K. Anand, “Numerical study of fluid flow and heat transfer
over a series of in-line noncircular tubes confined in a parallel-plate channel,” Numer. Heat
Transf. Part B Fundam.,vol. 50, no. 2, pp. 97–119, 2006.
[5] R. P. Bharti, R. P. Chhabra,and V. Eswaran,“A numerical study of the steady forced convection
heat transfer from an unconfined circular cylinder,” Heat Mass Transf.und Stoffuebertragung,vol.
43, no. 7, pp. 639–648, 2007.
[6] W. B. R. Dsc and H. B. Design, “Stationary Parallel Plate PracticalGuidelines for CFD Simula-
tion and Analysis,” 2012.
[7] B. Gera, P. K. Sharma, and R. K. Singh, “CFD analysis of 2D unsteady flow around a square
cylinder,” Int. J. Appl. …,vol. 1, no. 3, pp. 602–610, 2010.
[8] V. M. Boiko, A. A. Pivovarov, and S. V. Poplavski, “Measurement of gas velocity in a high-
gradient flow, based on velocity of tracer particles,” Combust. Explos. Shock Waves,vol. 49, no. 5,
pp. 548–554, 2013.
[9] S. Singha and K. P. Sinhamahapatra, “Flow past a circular cylinder between parallel walls at low
Reynolds numbers,” Ocean Eng.,vol. 37, no. 8–9, pp. 757–769, 2010.
[10] X. F. Xin, C. Chen, B. F. Wang, D. J. Ma, and D. J. Sun, “Local Heating Effect of Flow Past a
Circular Cylinder,” Chinese Phys. Lett.,vol. 27, no. 4, 2010.