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 such that the boundary conditions will not affect the flow near cylinder.

Figure above shows the geometry of such case.

Strategy for Creating Geometry

To model this flow, we need a cylinder and farfield boundaries. We need finer meshes around the cylinder to capture the active region (call this radius of influence). Downstream of the flow, there will be wake generated by the cylinder, which requires finer mesh to better capture this phenomena. To be able to specify such regions, we split the model into different faces as shown below.

We set the geometry upstream to be shorter because we have less activity before flow through cylinder. We set the geometry downstream of the cylinder to be relatively longer because we want to make sure that the downstream farfield boundary is not affected by the wake generated.

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 two arcs. The cylinder is created with two arc because they are going to be meshed differently. First arc span from -90 to 90 deg. Second arc span from 90 to -90 deg. Both arc with radius 1.

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

(Click here for animated steps)

Create 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 edge. Make sure that the Copy is checked. Under Operation, select Scale. Next to Factor, enter a value of 10. This means that the radius of influence we create will be 10 times the size of the cylinder.

(Click here for animated steps)

Create Back Outer Boundary

For this part, we will start with bottom up approach where we first create vertices and then using the vertices to create edges. Create 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 the vertices to create three edges

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

 You should see the following figure after you finish this step.
 
 Following animated steps show another method in creating vertices.

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 means 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-circle 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  

You should see have this geometry up till this step.

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

Select appropriate edge one by one. For example, to create vertex H, we have to split the bottom edge.

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

And then select the bottom edge. Enter the coordinate value for vertex H. Do this for the rest of vertices.

Now, we will 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.

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.

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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