Step 2 Mesh Geometry GAMBIT

Step 2: Mesh Geometry in GAMBIT

Launch GAMBIT

Create a folder called pipe2 at a convenient location to use as your working folder. Copy your pipe.dbs file containing the 100x5 mesh from the Laminar Pipe Flow tutorial to this folder. If you don't have this file, here's a copy (right-click and select Save As...). Rename this file as pipe100x30.dbs. We'll modify this file to obtain the mesh for the turbulent pipe flow simulation.

Start GAMBIT and load pipe100x30. (Refer to step 1 of the Laminar Pipe Flow tutorial if you've forgotten how to do this.) Recall that GAMBIT will use the id pipe100x30 as the default prefix for all files created during this session.

To make best use of screen real estate, resize the GAMBIT and browser windows so that you approximate this screen arrangement. This way you can read instructions in the browser window and implement them in GAMBIT.

The mesh from the previous tutorial should be displayed. To fit the mesh to the size of the window, select:

Global Control > Fit to Window 

Delete Previous Face Mesh

The first step we have to do is remove the old face mesh. Recall that the face mesh is built on top of meshed edges, thereby forming the grid. In this case, we don't want to remove the underlying edge meshes. So to delete only the face mesh, select:

Operation Toolpad > Mesh Command Button > Face Command Button > Delete Face Meshes  

Since we only have one face, shift-click any edge of the bounding rectangle to select the face mesh we want to delete. The face you have selected should become red and the name of the face listed in the Delete Face Meshes window in the drop down box.

Now, because we don't want to delete the edge meshes, uncheck the Remove unused lower mesh box.

 
 Click Apply.

Check that the face mesh has been removed in the GAMBIT Graphics Window.

Remesh Edges

Since we are still going to use 100 divisions for the horizontal edges, we only need to remesh the vertical edges.

To resolve the much higher gradient near the wall for a turbulent flow, we will use smaller grid spacing near the wall by employing grid stretching.

For each vertical edge, we will specify the division length next to the wall to be 0.001 and the total number of divisions to be 30. In GAMBIT, each edge has a direction associated with it as shown by an arrow. We will set this arrow to point away from the wall. Then the division next to the wall becomes the "First Length" and the division next to the axis becomes the "Last Length". We'll specify the "First Length" to be 0.001 and the total number of divisions to be 30 for the edge; GAMBIT will automatically calculate the appropriate value for the "Last Length".

Operation Toolpad > Mesh Command Button > Edge Command Button

Select the vertical edges by shift-clicking on each of them. Notice the red arrow that appears on the edge when it is selected. Make sure these arrows are pointing down (towards the axis and away from the wall). If both of these arrows are pointing in the wrong direction, you can reverse them by clicking Reverse next to Pick with links. However, if only one of the edges needs to be reversed, you can do that by shift-middle clicking on that edge. You'll have to zoom in to be able to do this. (Recall that you can zoom in by holding down the Ctrl key and then dragging a box with your left mouse button. Double-click with the middle mouse button to go back to the last view.)

For Type in the Mesh Edges menu, select First Length from the drop down box. Next to Length, type in 0.001.

We want 30 divisions on each of the vertical edges; so select Interval Count from the drop down box under Spacing and enter 30 in the text box to its left.

Click Apply.

If you zoom in on the right edge, you should see the following:

 
 
(Click picture for larger image)
Note that the mesh spacing is smaller near the wall as indicated by the blue circles on the edge.

Recreate Face Mesh

The next step is to recreate the face mesh on top of these edge meshes. This is the same procedure as in the previous tutorial:

Operation Toolpad > Mesh Command Button > Face Command Button  > Mesh Faces

Shift left-click on the face and click Apply. The meshed area should look like this after zooming in:

(Click picture for larger image)

Go to Step 3: Specify Boundary Types in GAMBIT