TUTORIAL 1

1. CREATING AND MESHING BASIC GEOMETRY (PRE-PROCESSING)

This tutorial illustrates geometry creation and mesh generation for a simple geometry using

GAMBIT.

In this tutorial you will learn how to:

Start GAMBIT

Use the Operation toolpad

Create a vertices, edges and face

Mesh a face

Examine the quality of mesh

Set Boundary Types

Save the session and exit GAMBIT

1.1 Problem Description

The model consists of a T-junction cylindrical pipe. The basic geometry is shown

schematically in Figure 1.

Figure 1: Problem specification

Consider fluid flowing through a T-junction pipe of constant cross-section. The pipe

diameter d = 0.1 m and length L=1 m. The velocity for both inlet V=1 m/ s. Consider

the velocity to be constant over the inlet cross-section. The fluid exhausts into the

ambient atmosphere which is at a pressure of 101325 Pa (1 atm). The material of fluid

is water-liquid with density = 998.2 kg/m3 and coefficient of viscosity μ = 1.003 x 10-

3 kg/ms.

This is a simple flow problem and we will use FLUENT to determine the flow

behavior inside the pipe. Graphical display the pressure distribution in pipe, and

velocity distribution in pipe. Plot the centerline velocity.

L

L

1.2 Strategy for Creating and Meshing Geometry

This first tutorial illustrates some of the basic operations for generating a mesh using

GAMBIT. In particular, it demonstrates:

How to build 2-D geometry

How to create a hexahedral mesh automatically

The steps you will follow in this tutorial are listed below:

Create vertices, edge and face.

Automatically generate the mesh.

Examine the quality of the resulting mesh.

Setting continuum types (for example, identifying which mesh zones are fluid and

which are solid) and boundary types

These details, as well as others, are covered in subsequent tutorials.

1.3 Procedure to create the geometry

Type

gambit -id basgeom

to start GAMBIT.

This command opens the GAMBIT graphical user interface (GUI). (See Figure 2.)

GAMBIT uses the name you specify (in this example, basgeom) as a prefix to all files

it creates: for example, basgeom.jou.

Figure 2: The GAMBIT graphical user interface (GUI)

Step 1: Run the software GAMBIT to create the geometry and to mesh it.

Now Gambit is launched. Click on Solver menu at the top of the Gambit window and choose

FLUENT 5/6.

Figure 3: The solver change to FLUENT 5/6

Step 2: Create a vertices

In order to create the T shapes geometry, first put the coordinate system on the

geometry and label it.

Figure 4: The coordinate system on the geometry

Create Vertex-From Coordinates. In Create Real Vertex-Global, enter 0, 0, and 0 for

x, y, and z coordinates respectively. Type an appropriate label for this vertex, e.g.,

“A”, and Apply. A vertex is created, as indicated by a small white plus sign at this

location. Do the same operation until I.

Figure 5: Steps to create a vertex

Click Fit to Window Button to fits the geometry into the available screen space.

Figure 6: Global control toolpad

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Undo button Fit to Window

Button

Figure 7: All 9 vertices on screen

Step 3: Create an edges

Join vertex A and B by straight lines to form the "edge" and name as “AB”. Figure 8

show that all edges name.

Figure 8: Namely all edges

Click left mouse on Geometry Command Button and Edge Command Button. Click

on “down arrow button” and selects A and B in vertex list. Then click on button

and put AB as a label. Click Apply and Close. Follow in Figure 9. Do the same

operation for BC, CD, DE, EF, FG, HI and IA.

Figure 9: Steps to join vertex to create a straight line

Figure 10: All vertices joint to create T shape geometry

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Step 4: Create a face

Lastly, create a "face" corresponding to the area enclosed by the edges. Under

Geometry, Face Command Button-R-Form Face-Wireframe. It is important to select

the edges in order when creating a face from existing edges. (I like to select them in

mathematically positive clockwise order). Select the “AB” edge first, followed by the

“BC” edge, the “CD” edge, the “DE” edge, and until “IA” edge or just click ALL.

Type in a label for this face “PIPE CROSS SECTION”, Apply and Close.

Figure 11: Steps to generate face from edges

If all went well, a pretty blue outline of the face should appear on the screen; this is a

face, which is now ready to be meshed.

Figure 12: Blue outline will appear after face created

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1.5 Procedure to mesh a face

Generate the mesh on the face:

If you have zoomed in, you will now want to zoom out. Zoom back out so that the

entire mesh can be clearly seen. This is most easily accomplished by clicking on Fit to

Window in the Graphics/Windows Control (near the bottom right of the screen).

Under Operation, Mesh Command Button-Face Command Button. The default

window that pops up should be Mesh Faces. If not, Mesh Faces.

Select the face by shift clicking on one of its edges. Elements should be Quad by

default; if not, change it. Also change Type to Submap. Change to interval size in

spacing option and put the value 0.005.

Generate the mesh by Apply. If all goes well, a structured mesh should appear. Zoom

in to see how the cells are nicely clustered close to the T-junction of geometry.

You can now close the Mesh Faces window.

Figure 13: Steps to generate face mesh

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(a) (b)

Figure 14: Comparison mesh using different interval size (a) 0.005 (b) 0.010

1.6 Procedure to Examine the quality of the mesh

Check the quality of mesh before write out mesh file to FLUENT format.

Figure 15: Steps to check the quality of mesh

1.7 Procedure to define the boundary zones

Specify Boundary Types

Under the Operations panel, click on Zones

Under the Zones panel, click on Specify Boundary Types

In the Specify Boundary Type window, put in a name as “INLET 1”, right click on

the button below Type, hold and choose VELOCITY INLET from the drop-down list.

Click on arrow button and pick AB and IA. Apply and Close.

Do the same operation for all edges based on Table 1.

Figure 16: Steps to namely boundary zone

Table 1: List of boundary zone

EDGE NAME TYPE

AB & IA INLET 1 VELOCITY

INLET

FG INLET 2 VELOCITY

INLET

CD OUTLET PRESSURE

OUTLET

BC, DE, EF, GH

& HI

WALL WALL

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Specify Continuum Types

Under the Zones panel, click on Specify Continuum Types

In the Specify Continuum Types window, put the name as “INTERIOR” and select

FLUID type, right click on the button below Entity, hold and choose faces from the

drop-down list. Select PIPE CROSS SECTION.

Click on the button Apply and Close.

Figure 17: Steps to namely continuum zone

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1.8 Procedure to write out the mesh in the format used by Fluent

In the main Gambit window, File-Export-Mesh. (The default file name can be

changed at this point if desired.) Check the option to export a 2-D mesh, and Accept.

When the Transcript (at lower left) informs you that the mesh is done, File-Exit-Yes.

The mesh file should now be ready for use by Fluent.

Figure 18: Steps to write out mesh in the fluent format

2. RUN SIMULATION USING FLUENT 6 SOLVER (PROCESSING)

2.1 Procedure to load mesh file

Run the software FLUENT 6. The FLUENT Version tab will be appear, choose 2d

and Full Simulation for Mode. Then, FLUENT 6 interface will be appearing as Figure

20.

Figure 19: FLUENT Version Tab

Figure 20: The FLUENT 6 Text User Interface (TUI)

Click File on Main Menu Bar, select Read and then select Case. Browse to and select

the file “T JUNCTION PIPE.msh”. Make sure change All Files from the drop-down

list.

Figure 21: How to load previous mesh file to FLUENT 6.

Main Menu Bar

2.2 Procedure to Check and Display Mesh in FLUENT solver

FLUENT will report the results of the mesh check in the console. Follow below steps:

Main Menu > Grid > Check

Main Menu > Grid > Info > Size

Main Menu > Display > Grid

Figure 22: Grid Check on FLUENT interface

The mesh check ensures that each

cell is in a correct format and

connected to other cells as expected.

It is recommended to check every

mesh immediately after reading it.

Failure of any check indicates a

badly formed or corrupted mesh

which will need repairs prior to

simulation.

2.3 Procedure to Define Solver Properties

Click Define on Main Menu Bar, then select Models and Solver. Use default setting

of the Solver.

Figure 23: The drop-down list from Define Command on Main Menu Bar

Figure 24: The default setting of the FLUENT Solver Tab

Click Define on Main Menu Bar, then select Models and Viscous. Enable the

Laminar model.

Figure 25: The Viscous Model Tab

2.4 Procedure to Define New Material Properties

Click Define Command on Main Menu Bar, then select Material. Materials Tab will

be appear as Figure 25. Left click on Fluent Database, choose “Fluid” on Material

Type and search Water-Liquid. Click Copy and Close button. Change density = 998.2

kg/m3 and coefficient of viscosity μ = 1.003 x 10-3 kg/ms. Click Change/Create button

and Close.

Figure 26: The Materials Tab

Figure 27: Fluent Database Materials Tab

2.5 Procedure to Define Operating Conditions

Click Define Command on Main Menu Bar, then select Operating Conditions. Use default

setting which is the operating pressure = 101325 Pascal (1atm). This simulation ignores the

gravitational effect.

Figure 28: Operating Conditions Tab

2.6 Procedure to Define Boundary Conditions

Click Define Command on Main Menu Bar, then select Boundary Conditions. Select

inlet 1 zone, click Set button and enter value velocity magnitude =1m/s. Click OK. Do

the same operation for inlet 2 zone.

Figure 29: Set the Boundary Condition for Inlet 1

Remain the default setting for outlet zone and wall zone. For this problem the outlet

gauge pressure is 0 and no slip wall will be used.

Figure 30: Pressure Outlet setup

Figure 31: Wall zone setup

Change the material in interior zone from air to water-liquid. Choose interior zone,

then click Set and change water liquid on material name. Click OK and Close.

Figure 32: Material setup in interior zone.

2.7 Procedure to set solution method

This simulation will be use First Order Upwind for the momentum (to get better

result, change to second order upwind that can increase the accuracy). This simulation

also use default number for under-relaxation factors.

Main Menu > Solve > Controls > Solution

Figure 33: The drop-down list from Solve Command on Main Menu Bar

Figure 34: Solution Controls Tab

Discretization schemes define

how the solver calculates

gradients and interpolates

variables to non-stored

locations. First-order schemes

are more stable but less accurate

than higher order schemes. This

case is well defined and will be

stable using second-order

numerics from the start.

Set Initial Guess

Main Menu > Solve > Initialize > Initialize.

Initialize the flow field to the values at all zones. Select “all-zones”, click Init button

and Apply. Press Close button.

Figure 35: Solution Initialization Tab

Set Convergence Criteria

Iterate the solution until the residual for each equation falls below 1x10-3.

Main Menu>Solve > Monitors > Residual.

Figure 36: Click Residual Command

Figure 37: Set the value 0.001 on Absolute Criteria for all residual

2.7 Run the simulation

Iterate Until Convergence

Start the calculation by running 500 iterations:

Main Menu > Solve > Iterate.

Figure 38: Set number of iteration

Wait the simulation complete until solution is converging.

Figure 39: Simulation complete

Save the solution to a data file by follow the step below:

Main Menu > File > Write > Data.

3. DATA ANALYSIS USING FLUENT 6 (POST-PROCESSING)

3.1 Analysis Data

To see the contour of pressure distribution in pipe. Click Display Command on Main

Menu Bar and select contours.

Figure 40: Contours command selected

Tick Filled on Options, the click Display button. The pressure distribution contour

will be display. The minimum and maximum value also will be display.

Figure 41: Contours tab

Figure 42: Contour of static pressure in T junction pipe

To see the vector of velocity magnitude in pipe. Click Display Command on Main

Menu Bar and select vector. Just click Display button. The velocity vector will be

display.

Figure 43: Vectors tab

Figure 44: Vector of velocity magnitude in T junction pipe

Create the line along centerline. Put the origin coordinate for first point, x0 = 0 and y0

= 0. For second point, x1 = 1 and y1 = 0. Type “centre-line) on the new surface name,

then click create button and close.

Main menu > surface > line/rake

Figure 45: Coordinate to create center line

Plot the variation of the axial (x) velocity along the centerline.

Main Menu > Plot > XY Plot.

Figure 46: XY Plot command selected

Select centre-line on Surfaces option, then click Plot button. Axes button is to

customize the graph plot.

Figure 47: Solution XY Plot tab

Figure 48: Graph velocity magnitude against x direction of center line.

Saving the Plot

File > Hardcopy

3.2 Export Data

Export data to excel file and open it in Microsoft Office Excel. Follow steps below:

Main Menu > File > Export

Choose ASCII > centre-line > Velocity Magnitude > Write

Save on Desktop and put the file name as “CENTRE LINE VELOCITY

MAGNITUDE”. Click OK.

Run Microsoft Office Excel Software.

Office button > Open > search file save location

Click Yes on below tab appear.

Click Next >

Tick Tab, Semicolon, and Comma. Click finish button.