# Gambit 1: Mixing Elbow (2D)

In this tutorial, you will use GAMBIT to create the geometry for a mixing elbow and then generate a mesh. The mixing elbow configuration is encountered in piping systems in power plants and process industries.

• Create vertices using a grid system
• Create arcs by selecting the center of curvature and the endpoints of the arc
• Create straight edges between vertices
• Split an arc using a vertex point
• Create faces from edges
• Specify the distribution of nodes on an edge
• Create structured meshes on faces
• Set boundary types
• Prepare the mesh to be read into FLUENT
• Export a mesh

In this tutorial, a 2-D mesh build using a “bottom-up” approach . The “bottom-up” approach means that you will first create some vertices, connect the vertices to create edges, and connect the edges to make faces (in 3-D, you would stitch the faces together to create volumes). While this process by its very nature requires more steps, the result is, just as in Tutorial 1, a valid geometry that can be used to generate the mesh.

It must be a single block, structured mesh. However, this mesh can also be used in any of the other Fluent solvers. This type of mesh is sometimes called a mapped mesh, because each grid point has a unique I, J, K index. In order to meet this criterion, certain additional steps must be performed in GAMBIT and are illustrated in this tutorial

Step 1: Choose the solver to run your CFD calculation by selecting the following from the main menu bar

Step 2 : Create the Initial Vertices

a) Create vertices to define the outline of the large pipe of the mixing elbow.

b) Check that Visibility is selected.

This ensures that the background grid will be visible when it is created.

• Select X (the default) to the right of Axis.
• Enter a Minimum value of –64, a Maximum value of 64, and an Increment of 16.
• Click the Update list button.

This creates a background grid with four cells in the x direction and enters the x coordinates in the XY_plane X Values list.

1. Select Y to the right of Axis.
2. Enter a Minimum value of –64, a Maximum value of 64, and an Increment of 16.
3. Click the Update list button.

This creates a background grid with four cells in the y direction and enters the y coordinates in the XY_plane Y Values list.

• Check that Snap is selected under Options.

The vertices you create later in this step will be “snapped” to points on the grid where the grid lines intersect.

• Select Lines (the default) to the right of Grid.

The grid will be displayed using lines rather than points.

• Click Apply.

GAMBIT creates a four-by-four grid in the graphics window. To see the whole grid, you must zoom out the display . You can zoom out the display by pressing and holding down the right mouse button while moving the cursor vertically upward in the graphics window.

Ctrl-right-click the nine grid points.

Ctrl-right-click” indicates that you should hold down the Ctrl key on the keyboard and click on the point at which the vertex is to be created using the right mouse button.

Unselect the Visibility check box in the Display Grid form and click Apply.

The grid will be removed from the graphics window and you will be able to clearly see the nine vertices created.

Step 3: Create Arcs for the Bend of the Mixing Elbow

GEOMETRY > EDGE > CREATE EDGE ARC

Retain the default Method.

Notice that the Center list box is yellow in the Create Real Circular Arc form at this point. The yellow color indicates that this is the active field in the form, and any vertex selected will be entered into this box on the form.

Shift-left-click the vertex in the center of the graphics window

The selected vertex will appear red in the graphics window and its name will appear in the Center list box under Vertices in the form.

Left-click in the list box to the right of End-Points to accept the selection of vertex E and make the End-Points list box active.

Alternatively, you could continue to hold down the Shift key and click the right mouse button in the graphics window to accept the selection of the vertex and move the focus to the End-Points list box.

Note that the End-Points list box is now yellow—that is, this is now the active list box, and any vertex selected will be entered in this box.

Shift-left-click the vertex to the right of the center vertex in the graphics window

The vertex will turn red.Select the vertex directly below the one in the center of the graphics window

Step 4: Create Straight Edges

Step 5: Create the Small Pipe for the Mixing Elbow

Step 6: Create Faces From Edges

. Create a face for the large pipe.

GEOMETRY>FACE>FORM FACE

Step 7: Specify the Node Distribution
to define the grid density on the edges of the geometry

1. Specify the node density on the inlet and outlet of the large pipe.

MESH  EDGE   MESH EDGES

This command sequence opens the Mesh Edges form.

Step 8: Create Structured Meshes on Faces

Create a structured mesh for the large pipe.

1. Shift-left-click the large pipe in the graphics window.

Note that four of the vertices on this face are marked with an “E” in the graphics window; they are End vertices. Therefore, GAMBIT will select the Map Type of Scheme in the Mesh Faces form. See the GAMBIT Modeling Guide for more information on Map meshing.

2. Click the Apply button at the bottom of the form.

GAMBIT will ignore the Interval size of 1 under Spacing, because the mapped meshing scheme is being used and the existing edge meshing fully determines the mesh on all edges.

Step 9: Set Boundary Types

1. Remove the mesh from the display before you set the boundary types.

This makes it easier to see the edges and faces of the geometry. The mesh is not deleted, just removed from the graphics window.

1. Click the SPECIFY DISPLAY ATTRIBUTES command button  at the bottom of the Global Control toolpad.
2. Select the Off button to the right of Mesh near the bottom of the form.
3. Click Apply and close the form.

Step 10: Export the Mesh and Save the Session

1. Export a mesh file for the mixing elbow.

File Export Mesh

This command sequence opens the Export Mesh File form. Note that the File Type is Structured FLUENT Grid.

1. Enter the File Name for the file to be exported (2-DELBOW.GRD).
2. Click Accept.

The file will be written to your working directory.