tutorial 2.0rc2 ccm2

7/27/2019 Tutorial 2.0rc2 Ccm2

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MSc TPFE/4th Year Mechanical and Aerospace Advanced CFD Laboratory

Laminar Flow in tube bundles

using Star CCM+

1 Introduction

Flows through tubes bundles are encountered in many heat exchanger applications. One

example is in nuclear power plants where the nuclear fuel rods are cooled by passing

cold water over them, but similar arrangements are also employed in more conventional

exchangers. The objective of this type of heat exchanger is, of course, to extract as much

heat as possible from the tubes. In a computational study, in order to calculate the heat

transfer reliably, it is necessary to resolve the flow around the tubes correctly.

A typical staggered tube bundle configuration, which is to be studied in this exercise,

can be seen in Figure 1. It is assumed to consist of a large number of tubes, so the flowaround any one is considered to be identical to that around the others. The computa-

tional domain employed is highlighted, which takes advantage of some of the symmetries

present in the flow. Symmetry conditions are applied on the top and bottom boundaries,

whilst periodic conditions (with a prescribed pressure drop) are applied between the east

and west boundaries.

Figure 1: Flow geometry

The flow is to be computed as laminar, and three meshes are provided. The first

one consists of block structured quadrilateral cells with a resolution of 540 cells in total

1


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Laminar flow in tube bundles 2

with 72 points around one cylinder. The second mesh is an unstructured mesh made of

quadrilateral cells (PAVE). The total number of cells is 433 with 64 points around one

cylinder. The third mesh (TETRA) is made of triagular elements, with a total number of

490 cells with 64 points around one cylinder. The meshes can be seen in Figure 2.

Figure 2: Three types of meshes

The use of a structured mesh means that there are rather highly skewed cells at the

interface between the curved edge of the cylinder and the straight symmetry edge. The

unstructured approach deals with this by removing the constraint of the cell edges having

to lie along the same lines as the block boundaries.

By using these relatively coarse meshes, the influence of the skewness of the grid can

be seen. Although the error introduced can be reduced by refining the mesh, the number

of cells required to reduce the error significantly is different for each meshing technique.This means that before doing a mesh refinement study, it is often worth spending some

effort in achieving a grid structure with the least distortion possible.

A converged solution on a very fine grid is also available, allowing comparisons to

be drawn between these grid-independent results and those obtained on the three coarser

grids described above.

2 Objectives

One of the objectives of this exercise is to compare the three different types of mesh-

ing techniques described above for a complex curved geometry, and assess their relative


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strengths and weaknesses in terms of solution accuracy.

A second objective is to gain more experience in using a wider range of CFD and

other software tools for simulations and post-processing; in particular here using the Star

CCM+ program to perform the flow simulations.

3 Investigations

First, follow the instructions given below to run the case using the block-structured grid

(BLOCK), and post-process the results. Then, follow the same steps to compute and

examine the solution for the other two grids.

Comparisons should be drawn between the computed solutions on the different grids,

and the grid-independent one from the provided fine grid solution. Qualitative compar-

isons can be made by examining velocity vector plots, contour plots of velocity or pres-

sure, and streamlines. For quantitative output, useful comparisons include

U-velocity component profiles along the periodic boundary (x = 0) and symmetryline (y = 0).

The flow recirculation length, deduced from the velocity profile along y = 0.

The computed mass flow rate, conveniently expressed non-dimensionally as the

Reynolds number based on tube diameter and bulk velocity (Re = Ubd/).

Compute the pressure drop coefficient Kp as in:

P = Kp 12U2b (1)

The pressure drop P/L = 105 has to be prescribed as a forcing term F = P/Lwhere L is the distance between the periodic faces.

OPTIONAL: You could also investigate how the pressure drop coefficientKp changeswith Re number. By systematically increasing the viscosity (multiply by 2, 4, 8 ...) youcan decrease Re while keeping the same P. Calculate the quantity KpRe and see if itremains constant. How does the recirculation region change with Re?

4 Reporting Requirements

Students should submit a short report within 2 weeks of the laboratory. The report should

contain a brief introductory section, outlining the nature of the investigations conducted,

and should then describe and discuss (with the aid of a selection of figures) the results

obtained and comparisons between the various solutions. Excluding tables and figures

this is unlikely to require more than 45 pages of text.


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5 Guide to Running Star CCM+

First download the meshs required from the website:

http://cfd.mace.manchester.ac.uk/twiki/bin/view/Main/StarCCMTutorialLaminarFlow (this

web address is case sensitive). Star CCM+ can be found on the computers in the computercluster. It is installed under EPS, MACE as shown in Figure 3.

Figure 3: How to launch Star CCM+ 5.04.006

Once the Star CCM+ window has opened go to file and select new simulation as in

Figure 4.

Figure 4: Setting up a simulation in Star CCM+


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A new window labelled Create a New Simulation is going to open, leave the

settings as default and click on OK. After the case has been set up, click on file again and

then select Import as in Figure 5.


This is going to open another window. Navigate to the mesh Bundle_block.ccm

and open it. This is going to give an import mesh option menu, keep the default settings

and select ok. If every thing has been done correctly then a fluid region should show upas shown in Figure 6.


Now this fluid domain has to be split into different regions, so that boundary condi-


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tions can be assigned. Expand the Regions tab as shown in Figure 7


Select Split by Angle . Another window with several options on splitting the

boundaries by angle is going to appear as in Figure 8. Add the boundary to the Selected

list and leave the setting as default and click on Apply. Note that new boundaries are

going to be added under the Boundaries tab


Now the Default_Boundary_Region2 and Default_Boundary_Region3

have to be further divided and can be done by splitting the domain by angle in the same

way as done previously. The only difference is that this time the prescribed angle is goingto be 30 as in Figure 9


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The boundary conditions are to be defined now and can be done by selecting the

regions under the Boundaries tab. The Default_Boundary_Region should be

assigned a Symmetry Plane as in Figure 10


Similarly the respective boundary conditions for other boundaries can be setup and


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are shown in Table 1

Default_Boundary_Region Symmetry Plane

Default_Boundary_Region2 Wall

Default_Boundary_Region22 Symmetry PlaneDefault_Boundary_Region23 Symmetry Plane





Default_Boundary_Region35 Symmetry Plane

Default_Boundary_Region4 Symmetry Plane

Table 1: Boundary conditions

Now the physical conditions for the simulation can be set-up by expanding the Continua

tab. Double click on the Models tab as in Figure 11



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A new window for physics model selection would appear. This allows the user to

select the required solution methods and models to be used for the calculation as in Fig-

ure 12


In the Physics Model Selection select Steady under Time calculation,

Gas under the Material, Segregated Flow under Flow , Constant Density

under Equation of State and Laminar under the Viscous Regime. The se-

lection is shown in Figure 13. Once every thing has been selected click on Close.



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Once the Physics Model Selection is complete the material properties i.e.

density and dynamic viscosity have to be changed. Expand the Models tab and then

expand the Gas tab as in Figure 14. Change the Density to = 1.17862kg/m3 and thedynamic viscosity to = 1.83 105Ns/m2.



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After updating the material properties the periodic boundary conditions are to be

defined. This can be done by selecting the Default_Boundary_Region 32 and

Default_Boundary_Region 33 at the same time and then right clicking

Default_Boundary_Region 32, a drop down menu would appear go to create in-

terface and select periodic as in Figure 15. If this is done correctly then a new tab calledInterfaces would appear in the simulation menu.



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Now expand the Interface tab and select the tab labelled Periodic a table

would appear in the properties menu as in Figure 16. Change the Interface type to

Fully-Developed Interface as shown in Figure 16.



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Expand Physics Values tab under the Periodic1 tab and specify the pressure

jump as 1 105Pa as shown in Figure 17.



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After setting up the Periodic boundary conditions expand the Stopping Criteria

tab and set the Maximum Steps to 2000 as shown in Figure 18. This is the maximum

number of iterations to be used by the simulation.


In order to run the simulation initilise the solution by clicking on and then click

on to run the simulation for the prescribed number of iterations. After the simulation

has been started a graph appears on the screen which shows the residuals for the respec-

tive simulation. Once the calculation has finished the following message appears in the

Output window.

Stopping criterion Maximum Steps satisfied.


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6 Post-Processing in Star CCM+

In order to analyse the results from the simulation the integrated post-processing tools

available in StarCCM+ are used. In order to make plots from the simulation, nevigate

to the Scenes tab and right click on it, a drop down menu would appear. Then go toNew Scene and select Scalar as in Figure 19. If this step is followed correctly a new

tab Scalar Scene 1 should appear under the Scenes tab.

Figure 19: Post-Processing in Star CCM+


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Now expand Scalar Scene 1 Displayers Scalar 1 and right click

on the Parts tab under the Scalar 1 tab then select Edit as in Figure 20.


A new window is going to open which gives the option to select the regions to be

displayed in the plot. Check the box next to Regions as in Figure 21 and clickOK



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After selecting the parts select the Scalar Field tab and then select the Function

in the Scalar Field properties window as in Figure 22. Note that in Figure 22 the

i,j and k represent the respective X,Y and Z components of velocity. Select the i

component of the velocity.


If every thing has been done correctly a plot shown in Figure 23 should show up in

the visualisation window.


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In order to make the plots smooth select Scalar 1 tab under the Scalar Scene 1

tab, then in the properties window change the Contour Style to Smooth Filled

as in Figure 24



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The default pressure option in Star CCM+ is given as absolute pressure rather than

the relative presure. In this study relative pressure is required so in order to rectify

this a field function is used to post-process the pressure field in the simulation. A new

field function can be introduced by expanding the Tools tab and then right clicking the

Field Functions tab and selecting New as in Figure 25


A new field function labelled as User Field Function1 is going to appear un-

der the Field Functions tab. Select the User Field Function1 and change

the Definition in the properties window to :

$Pressure+(((1e-5)*($${Position}[0]))/2)


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The definition for the field function is shown in Figure 26. Now in order to plot

pressure follow the same procedure as done for velocity and make a new scalar scene, but

this time select the User Field Function1 instead of velocity when specifying the

scalar.


A quantitative way of comparing the results is to plot velocity profiles along particular

lines. For this exercise the bottom symmetry line at y = 0 and the periodic line at x = 0can be used. Before creating the line make sure that a Scalar Scene is open in the

visualisation window. Then right click on the Derived Parts tab go to New Parts

then probe and then select Line as in Figure 27. An input menu is going to appear

in the tabs menu as in Figure 28 and the line is going to be visible in the visualisation

window. To create the line at the symmetry plane y = 0 enter the points as (0.5, 0, 0.2)

and (1.5, 0, 0.2) and set the resolution to 20 points. Click onCreate

and then onClose


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To create another probe line along the periodic plane x = 0 follow the same procedureas done for the y = 0 plane, but now use the points as (0, 0.5, 0.2) and (0, 1, 0.2) and setthe resolution to 10 points.

To plot the velocity or pressure profiles along the lines created earlier right click on the

Plots tab go to New Plot and then select XY Plot as in Figure 29


A new tab labelled XY Plot1 is going to appear under the Plots tab. Select the

XY Plot1 tab and select the Parts in the properties window then select line-probe

as in Figure 30



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Now expand the Y Types tab under XY Plot1 tab and select the Scalar in the

properties window as in Figure 31. Select i component of the velocity.


In order to reset the scale in the XY plot the expand the XY Plot1 YType 1 and

select line-probe. Then in the properties window change the X Offset to 0.5 asin Figure 32

If every thing has been done correctly then an XY plot should be obtained in thevisualisation window as in Figure 33


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Now repeat these steps to obtain a new XY plot along the x = 0 plane. The differencein this plot is that the position has a different direction to the position in the previous plot

and the value of the X Off set is 1.0. To change the position for this plot expand the

respective XY Plot tab and then expand the X Type tab. In the properties window


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change the direction to [0.0,-1.0,0.0] as in Figure 34


Note that all the figures can be exported by right clicking on the respective scene

and selecting export. In case of XY plots a spread sheet is exported and can be used byMicrosoft Excel or MATLAB.

7 Fine Grid Solutions

After post processing the results for the Block mesh, the same procedure can be applied

to the other two meshes, i.e. PAVE and TETRA. Create a new case for all the mesh and

follow the same instructions.

After the results for the three meshes have been obtained, they can be compared with the

reference data available on the website. These results were obtained with a mesh of 30180cells using the same version of Star CCM+, and can be used to compare the coarse mesh

results obtained during the exercise.

tutorial 2.0rc2 ccm2

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