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Author: Yong Sheng Khoo & Rajesh Bhaskaran, Cornell University Problem Specification |
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If you are having trouble following written tutorial, detailed video tutorials are available here |
Step 1: Create Geometry in GAMBIT
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If you would prefer to skip the mesh generation steps, you can download the mesh from here and go straight to step 4. |
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 such that the boundary conditions will not affect the flow near cylinder.
Figure above shows the geometry of such a case.
Strategy for Creating Geometry
We know that To model this flow, we need a cylinder and farfield boundaries. We also know that there will be active regions around cylinder need finer meshes around the cylinder to capture the active region (call this radius of influence). Therefore we would want the area around cylinder to be of finer mesh. We also know Downstream of the flow, there will be wake downstream of the flow behind the cylinder. Therefore we also want finer mesh around that region. Figure below shows the number of faces required to make such meshgenerated by the cylinder, which requires finer mesh to better capture this phenomena. To be able to specify such regions, we split the domain into different faces as shown below.
We set the geometry upstream to be shorter because we have less activity around the regionbefore flow through cylinder. We set the geometry downstream of the cylinder to be relatively longer because we would like to capture the activity of the wake induced by the such that the boundary conditions will not affect the flow near cylinder.
Create a Working Directory
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Create a new directory called cylinder and start GAMBIT from that directory by typing gambit -id cylinder at the command prompt.
Under Main Menu, select Solver > FLUENT 5/6 since the mesh to be created is to be used in FLUENT 6.0.
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Create the cylinder using two arcs. The cylinder is created with two arc because they are going to be meshed differently. Back arc spans from -90 to 90 deg. Front arc spans from 90 to -90 deg. Both arc with radius 1.
Operation Toolpad > Geometry Command Button 
> Edge Command Button 
> Create Edge 
> Arc 
> 
Input the following data and click Apply.
Radius | 1 |
Start Angle | 90 |
End Angle | 450 |
After that, create a vertex at the bottom part of the cylinder by using split function.
Operation Toolpad > Geometry Command Button > Edge Command Button > Split Edge 
Next to Radius, enter 1. Next to Start Angle, enter -90. Next to End Angle, enter 90. Click Apply. Do the same for front arc but enter different value for angles.
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(Click here for animated steps)Select the cylinder edges. Then change the coordinate Type to Cylindrical and create a vertex by entering the angle, t as -90.
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Create
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Front Outer Boundary
We will create the outer boundary by creating arc 10 times of the cylinder front arc. We can use copy edges and scale by 10 to create the outer boundary.
Operation Toolpad > Geometry Command Button > Edge Command Button > Move/Copy Edges 
Select the front cylinder
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edge. Make sure that the Copy is checked. Under Operation, select Scale. Next to Factor, enter a value of
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10. This
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means that the radius of influence we create will be
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10 times the size of the cylinder. Click Apply.
The current geometry should look like this:
editor NOTE (duplicate vertices created...)
Create Farfield Boundaries
Create Upstream Boundary
We will first create the semi-cylinder upstream boundary.
Operation Toolpad > Geometry Command Button > Edge Command Button > Create Edge > Arc >
Input the following data and click apply.
Radius | 10 |
Start Angle | 90 |
End Angle | -90 |
Create Downstream Boundaries
(Click here for animated steps)
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Create Back Outer Boundary
For this part, we will start with
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bottom up approach where we first create vertices and
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then using the vertices to
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create edges.
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Create
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Label
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x
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y
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the following vertices.
Vertices | X | Y |
|---|---|---|
1 | 40 | 10 |
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2 | 40 | -10 |
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Operation Toolpad > Geometry Command Button > Vertex Command Button 
> Create Vertex 
Create the vertices by entering the coordinates under Global.
Click the FIT TO WINDOW button to scale the display so that you can see all the vertices.
Connect
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the vertices to create three edges
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Operation Toolpad > Geometry Command Button > Edge Command Button > Create Edge
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After this operation, you should see this geometry.
Split Edges
We would like to have more mesh elements at the downstream of the flow, to the right of the cylinder. To accomplish this, we would have to split some edges.
Split the edge according to the figure shown below:
First split the straight edges
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Label
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x
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y
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z
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E
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2.83
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10
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0
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H
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2.83
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-10
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0
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I
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40
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2.83
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0
Following animated steps show another method in creating vertices.
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Do you know that by clicking and holding right mouse click and move up and down, you can zoom in and out in the graphic window? |
Create Radius of Influence
Now we can proceed to create the geometry for radius of influence. Since both the cylinder and radius of influence is of same shape.
Create arc of radius 4 from -45 to 45 deg. Then create another arc from 45 to 45 deg.
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J
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40
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-2.83
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Operation Toolpad > Geometry Command Button > Edge Command Button > Split Edge
Then select the respective edges to split to for point E,H,I and J.
After that, split the curved edges
Label | r | t | z |
F | 4 | 45 | 0 |
G | 4 | -45 | 0 |
Create Edge > Arc >
Finally split the edge at 90 and -90 deg.
Operation Toolpad > Geometry Command Button > Edge Command Button > Split Edge
Change Remember to change the coordinate Type to Cylindrical and create vertex point F and G according to the table.
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Split Edge at Outer Boundary
For regular mesh, each edge has its opposite edge. Because of this, we can use projection method on the outer boundary to create the edge associated with the Radius of Influence edges.
Operation Toolpad > Geometry Command Button > Vertex Command Button > Project Button 
Select the vertex and associated edge. Make sure to select Split edge. At the end of this, you should have 4 new vertices.
(Animated steps)
Connect all Vertices
Finally, connect all the remaining vertices KL, LM, NO, OP, FI and GJ.
the newly created vertices to form two new edges. The current geometry in Gambit should look like this:
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Operation Toolpad > Geometry Command Button > Face Command Button 
> Form Face 
This brings up the Create Face From Wireframe menu. Recall that we had selected vertices in order to create edges. Similarly, we will select edges in order to form a face.
There will be total of six faces.
(Click picture for larger image) Create all the six faces by connecting appropriate edges.
Animated steps
We are now ready to mesh the geometry.
Go to Step 2: Mesh Geometry in GAMBIT
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