FLUENT - Steady Flow Past a Cylinder Step 1

Step 1: Create Geometry in GAMBIT

In an external flow such as the flow past a cylinder, we have to define farfield boundaries and mesh the region between the cylinder geometry and the boundaries. Farfield boundaries should be placed well away from the cylinder so that they are not under the influence of the flow around cylinder.

(Add Figure)
Figure above shows the geometry of such mesh.

Strategy for Creating Geometry

There are different methods in creating geometry. The most fundamental approach in creating geometry is the bottom-up vertical hierarchy approach. We will start with first creating vertices, then connecting the vertices to form edges, and finally connect the edges to form faces. Note that in 3D problems, you'll have to form a "volume" from faces. So the hierarchy of geometric objects in GAMBIT is vertices -> edges -> faces -> volumes.

Create a Working Directory

Create a folder called cylinder in a convenient location. We'll use this as the working folder in which files created during the session will be stored.

Note for ACCEL computer lab users: Each user gets his/her own 100 MB of disk space under S: at ACCEL. You can put your files in S: and it'll be accessible from any computer. This is where you should put files that you want to keep and access later on.

Start GAMBIT

Start your command prompt.

Start > Programs > Lab Apps > Fluent Inc Products > Gambit 2.3.16 > Gambit 2.3.16

This brings up the GAMBIT startup window. Click Browse and select the folder that you just created. Enter -id pipe in the options box to tell GAMBIT to use pipe as the default file prefix, then click Run.

In Windows, the Exceed X-server starts up before the GAMBIT interface comes up. Exceed is a third-party application needed to render the interface in Windows (GAMBIT was originally developed under Unix). To make best use of screen real estate, move the windows and resize them so that you approximate this screen arrangement. This way you can read instructions in the browser window and implement them in GAMBIT.

You can resize the text in the browser window to your taste and comfort:

In Internet Explorer: Menubar > View > Text Size, then choose the appropriate font size.

In Netscape: Menubar > View > Increase Font or Menubar > View > Decrease Font.

The GAMBIT Interface consists of the following:

• Main Menu Bar:

Note that the job name pipe appears after ID: in the title bar of the Utility Menu.

• Operation Toolpad:

We'll more or less work our way across the Operation Toolpad as
we go through the solution steps. Notice that as each of the top buttons
is selected, a different _"sub-pad"_ appears.
The Geometry sub-pad is shown in the above snaphot.

• Global Control Toolpad:

The Global Control Toolpad has options such as Fit to Screen and Undo that are very handy during the course of geometry and mesh creation.

• GAMBIT Graphics:

This is the window where the graphical results of operations are displayed.

• GAMBIT Description Panel:

The Description Panel contains descriptions of buttons or objects that the mouse is pointing to. Move your mouse over some buttons and notice the corresponding text in the Description Panel.

• GAMBIT Transcript Window:

This is the window to which output from GAMBIT commands is written and which provides feedback on the actions taken by GAMBIT as you perform operations. If, at some point, you are not sure you clicked the right button or entered a value correctly, this is where to look to figure out what you just did. You can click on the arrow button in the upper right hand corner to make the Transcript window full-sized. You can click on the arrow again to return the window to its original size. Go ahead, give this a try.

Select Solver

If the window titlebar does not say the solver is FLUENT 5/6, then you need to specify:

Main Menu > Solver > FLUENT 5/6

Verify this has been done by looking in the Transcript Window where you should see:

We can assume that the flow is axisymmetric. The problem domain is:

where r and x are the radial and axial coordinates, respectively.

Strategy for creating geometry

We will put the origin of the coordinate system at the lower left corner of the rectangle. The coordinates of the corners are shown in the figure below:

We will first create four vertices at the four corners and join adjacent vertices to get the edges of the rectangle. We will then form a face that covers the area of the rectangle.

Create Vertices

Find the buttons described below by pointing the mouse at each of the buttons and reading the Description Window.

Operation Toolpad > Geometry Command Button > Vertex Command Button > Create Vertex

Create the vertex at the lower-left corner of the rectangle:
Next to x:, enter value 0. Next to y:, enter value 0. Next to z:, enter value 0 (these values should be defaults). Click Apply. This creates the vertex (0,0,0) which is displayed in the graphics window.

Repeat this process to create three more vertices:

Vertex 2: (0,0.1,0)
Vertex 3: (8,0.1,0)
Vertex 4: (8,0,0)

Note that for a 2D problem, the z-coordinate can always be left to the default value of 0.

Operation Toolpad > Global Control > Fit to Window Button

This fits the four vertices of the rectangle we have created to the size of the Graphics Window.

(Click here for larger image)

Create Edges

We'll now connect appropriate pairs of vertices to form edges. To select any entity in GAMBIT, hold down the Shift key and click on the entity.

Operation Toolpad > Geometry Command Button > Edge Command Button > Create Edge

Select two vertices that make up an edge of this rectangle by holding down the Shift button and clicking on the corresponding vertices. As each vertex is picked, it will appear red in the Graphics Window. Then let go of the Shift button. We can check the selected vertices by clicking on the up-arrow next to Vertices:.

This will bring up a window containing the vertices that have been selected. Vertices can be moved from the Available and Picked lists by selecting them and then pressing the left or right arrow buttons.

After the correct vertices have been selected, click Close, then click Apply in the Create Straight Edge window.

Repeat this process to create a rectangle.

(Click here for larger image)

Create Face

Operation Toolpad > Geometry Command Button > Face Command Button > Form Face

To form a face out of the area enclosed by the four lines, we need to select the four ledges that enclose this area. This can be done by holding down the Shift key, clicking on each line (notice that the currently selected line appears red), and then releasing the Shift key after all four lines have been selected.

Alternatively, an easier way to do this would be to click on the up arrow next to edges:

This will bring up the Edge List window. Click on All-> to select all of the edges at once. Click Close.

Click Apply to create the face.

Go to Step 2: Mesh Geometry in GAMBIT

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