CFD

1. CFD PROJECT REPORT
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
ABSTRACT
Forced convection heat transfer from a confined circular cylinder between two parallel plates has
been studied using Ansys Fluent for CFD. The results are used to develop simple correlations for
Nusselt number and the effects of Reynolds number, the formation of turbulence in the wake region
and thermal boundary conditions on the temperature field near the cylinder and on the local Nusselt
number distributions have also been presented to provide further physical insights into the nature
of the flow. The rate of heat transfer increases with an increase in the Reynolds number.
1.INTRODUCTION
The flow past a circular cylinder has been of great interest to engineers for many years because of
the interesting and yet complex flow structures developed in it. Depending on the flow conditions,
a number of different flow patterns may exist in the wake of the cylinder, such as steady attached
flows, steady recirculated flows, vortex shedding and turbulence. The effect of Nusselt number in
convection and conduction of heat has been a key area of study in fluid mechanics, however little
research has been made to unlock its potential.
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 <50, the onset of laminar vortex shedding at Re
≈ 49, three-dimensional (3D) wake-transition at Re ≈ 200, and shear-layer transition at Re ≈103
and
105
[1]–[6].
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[7].

3. 2
2.MODEL SET UP AND ANALYSIS
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 2. A mesh representation of the 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

4. 3
2.2 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.
Fig 1; Average velocity at different grid sizes; 30704 cells for series Flu, 8892 cells for series
Flu 1 and 3648 cells for series Flu 2
2.3 Problem set up and boundary conditions
Fig A 2D representation of the experiment set up

5. 4
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 k epsilon with standard wall functions. This
turbulence model was chosen mainly because of its suitability to solve simple 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 1.789x10-6
.
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.
𝑹𝒆 =
𝝆𝒗𝑫
𝝁
(1)
Where 𝜌=density of air
𝑣= velocity of air
𝐷 =hydraulic diameter of the plates
𝑢=kinematic viscosity of air

6. 5
3. RESULTS AND ANALYSIS
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.

7. 6
Fig 10. A graphical representation Reynolds number vs Nusselt number

8. 7
Fig 11. A graphical representation showing the effect of higher Reynolds number on heat
transfer via convection and conduction.
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.
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. 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.

9. 8
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