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.

Figure above shows the geometry of such case.

Strategy for Creating Geometry

We know that we need a cylinder and farfield boundaries. We also know that there will be active regions around cylinder (call this radius of influence). Therefore we would want the area around cylinder to be of finer mesh. We also know 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 mesh.

We set the geometry upstream to be shorter because we have less activity around the region. 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 cylinder.

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.

Start GAMBIT

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.

Create Cylinder

Create the cylinder using arc

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 > Edge Command Button > Split Edge

Select the cylinder edges. Then change the coordinate Type to Cylindrical and create a vertex by entering the angle, t as -90.

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

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.

Create Downstream Boundaries

In creating downstream boundaries, we will use bottom up approach where we first create vertices and join the vertices to form edges.

Create vertices with following coordinates:

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 three edges.

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

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

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

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

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

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:

Create Faces

We can now join all the edges to form faces.

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.

We are now ready to mesh the geometry.

Go to Step 2: Mesh Geometry in GAMBIT

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