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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 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.

After that, create a vertex at the bottom part of the cylinder by using split function.

Operation Toolpad > Geometry Command Button Image Removed > Edge Command Button Image Removed > Split Edge Image Removed

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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.

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 boundaryNow we can proceed to create the geometry for radius of influence. Since both the cylinder and radius of influence is of same shape. We can create the radius of influence by copying and scaling up the cylinder.

Operation Toolpad > Geometry Command Button > Edge Command Button > Move/Copy Edges

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Select the front cylinder edgesedge. Make sure that the Copy is checked. Under Operation, select Scale. Next to Factor, enter a value of 410. This mean means that the radius of influence we create will be 4 10 times the size of the cylinder. Click Apply.

The current geometry should look like this:

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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 Image Removed > Edge Command Button Image Removed > Create Edge Image Removed > Arc Image Removed > Image Removed

Input the following data and click apply.

Create Downstream Boundaries

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(Click here for animated steps)

Create Back Outer Boundary

For this part, we will start with In creating downstream boundaries, we will use bottom up approach where we first create vertices and join then using the vertices to form create edges. Create vertices with following coordinates:the following vertices.

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 AB, BC and CD to form the vertices to create three edges.

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

After this operation, you should see this geometry.

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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:

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First split the straight edges

Following animated steps show another method in creating vertices.

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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. 

Operation Toolpad > Geometry Command Button > Edge Command Button > Split Edge Image Removed

Then select the respective edges to split to for point E,H,I and J.

After that, split the curved edges

Create Edge Image Added > Arc Image Added > Image Added

Finally split the edge at 90 and -90 deg. 

Operation Toolpad > Geometry Command Button > Edge Command Button > Split Edge Image Modified

Change Remember to change the coordinate Type to Cylindrical and create vertex point F and G according to the table. .

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 Image Added > Vertex Command Button Image Added > Project Button Image Added 

Select the vertex and associated edge. Make sure to select Split edge. At the end of this, you should have 4 new vertices.

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(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.

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Animated steps

We are now ready to mesh the geometry.

Go to Step 2: Mesh Geometry in GAMBIT

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