CREATING AND MESHING BASIC
GEOMETRY :
This
tutorial illustrates geometry creation and mesh generation for a simple
geometry using GAMBIT.
In this
tutorial you will learn how to:
- Start GAMBIT
- Use the Operation tool pad
- Create a brick and an
elliptical cylinder
- Unite two volumes
- Manipulate the display of
your model
- Mesh a volume
- Examine the quality of the
mesh
- Save the session and exit
GAMBIT
This
tutorial assumes you have no prior experience of working with GAMBIT. You
should, however, read Chapter 0, "Using This Tutorial Guide," to
familiarize yourself with the GAMBIT interface and with conventions used in the
tutorial instructions.
The model
consists of an intersecting brick and elliptical cylinder. The basic geometry
is shown schematically in Figure 1-1.
Figure 1-1: Problem specification
This
first tutorial illustrates some of the basic operations for generating a mesh
using GAMBIT. In particular, it demonstrates:
- How to build the geometry
easily using the "top-down" solid modeling approach
- How to create a hexahedral
mesh automatically
The
"top-down" approach means that you will construct the geometry by
creating volumes (bricks, cylinders, etc.) and then manipulating them through
Boolean operations (unite, subtract, etc.). In this way, you can quickly build
complicated shapes without first creating the underlying vertices, edges, and
faces.
Once you
have built a valid geometry model, you can directly and (in many cases)
automatically create the mesh. In this example, the Cooper meshing algorithm is
used to automatically create an unstructured, hexahedral mesh. More complicated
geometries may require some manual decomposition before you can create the
mesh; this is demonstrated in subsequent tutorials.
The steps
you will follow in this tutorial are listed below:
- Create two volumes (a brick
and an elliptical cylinder).
- Unite the two volumes.
- Automatically generate the
mesh.
- Examine the quality of the
resulting mesh.
To keep
this introductory tutorial short and simple, certain steps that you would
normally follow have been omitted:
- Adjusting the distribution
of nodes on individual edges of the geometry
- Setting continuum types (for
example, identifying which mesh zones are fluid and which are solid) and
boundary types
These
details, as well as others, are covered in subsequent tutorials.
Procedure:
Type
gambit -id mechanicalmines
to start
GAMBIT.
This command opens the GAMBIT
graphical user interface (GUI). (See Figure 1-2.) GAMBIT uses the name you
specify (in this example, mechanicalmines) as a prefix to all files it creates: for
example,mechanicalmines.jou.
Figure 1-2: The GAMBIT graphical user interface
(GUI)
Step 1: Create a Brick:
1. Create a brick by doing the following:
a)
In the Operation tool pad (located in the top right
corner of the GAMBIT GUI), select the GEOMETRY 
command button by
clicking on it with the left mouse button. If the Geometry
sub pad does not appear when you select the GEOMETRY
command button, click it again.
The
name of a command button is displayed in the Description
window at the bottom of the GAMBIT GUI when you hold the mouse cursor over the
command button. The GEOMETRY command button will appear
depressed when it is selected. Selecting the GEOMETRY
command button opens the Geometry sub pad. Note that
when you first start GAMBIT, the GEOMETRY command button
is selected by default.
b) Use the left
mouse button to select the VOLUME
command
button in the Geometry sub pad.
Again,
this command button will be depressed when selected. Selecting this command
button opens the Geometry/Volume sub pad.
c) Use the left
mouse button to select the CREATE VOLUME
command
button in the Geometry/Volume sub pad.
This
command sequence opens the Create Real Brick form.
The above
description of selecting command buttons can be shortened to the following:
GEOMETRY—> VOLUME
—>
CREATE VOLUME
The
selection of the command buttons will be represented using this method for the
remainder of this tutorial, and in all subsequent tutorials.
d) Left-click in
the text entry box to the right of Width in the Create Real Brick form, and enter a value of 10
for the Width of the brick.
e) Use the Tab key
on the keyboard to move to the Depth text entry box, and
enter 6 for the Depth
of the brick.
The text entry box for Height can be left blank; GAMBIT will set this value to be the
same value as the Width by default.
f) Select Centered from the option menu to the right of Direction.
NOTE:
When you first open the Create Real Brick form, the Centered option is selected by default.
i) Hold down the left
mouse button on the option button to the right of Direction
until the option menu appears.
ii) Select Centered from the list.
g) Click Apply.
A message appears in
the Transcript window at the bottom left of the GAMBIT
GUI to indicate that a volume, called volume.1, was
created. The volume will be visible in the graphics window, as shown in Figure
1-3.
If
you make a mistake at any point in the geometry creation process, you can use
the UNDO command button
to
undo multiple levels of geometry creation. At this point, you have only
performed one operation, so you can only undo one operation.
Figure
1-3: Rectangular brick volume (side view)
Step 2: Create an Elliptical Cylinder:
1. Create an elliptical cylinder.
a) Hold down the right
mouse button while the cursor is on the CREATE VOLUME 
command button.
b) Select the CREATE REAL CYLINDER option
from
the resulting menu.
! CREATE REAL CYLINDER is the
text that is written in the Description window when you
hold the mouse cursor over themenu item.
This action opens
the Create Real Cylinder form.
The above method
of selecting command buttons can be shortened to the following:
GEOMETRY—> VOLUME—> CREATE VOLUME
R
where R indicates a tool pad choice using the right mouse button.
c) Enter a Height of 10.
d) Enter a value
of 3 for Radius 1.
e) Enter a value
of 6 for Radius 2.
f) Retain the
default Axis Location of Positive Z.
g) Click Apply.
The brick and
elliptical cylinder are shown in Figure 1-4.
Figure
1-4: Brick and elliptical cylinder
Step 3: Unite the Two Volumes :
1. Unite the brick and elliptical cylinder
into one volume.
GEOMETRY—> VOLUME—> BOOLEAN OPERATIONS 
This command sequence
opens the Unite Real Volumes form.
Notice that the top Volumes list box is yellow in the Unite Real
Volumes form at this point. The yellow colour indicates that this field
is active, and any volume selected will be entered into this box on the form.
a) Hold down the Shift
key on the keyboard and select the brick by clicking on one of its edges in the
graphics window using the left mouse button.
! The Shift key must
always be held down when selecting entities in the graphics window using the
left mouse button. This operation will be referred to as Shift-left-click in
all further steps.
The brick will
appear red in the graphics window and its name (volume.1)
will appear in the Volumes list box in the Unite Real Volumes form.
b) Shift-left-click
the elliptical cylinder in the graphics window.
c) Click Apply to accept the selection and unite the elliptical
cylinder and brick.
! 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 volumes. This method allows
you to rapidly accept selections and apply operations with minimal movement of
the mouse.
! The Shift key
must always be held down when clicking the right-mouse button to accept the
selection of entities in the graphics window. This operation is referred to as
Shift-right-click.
The volume is
shown in Figure 1-5. You can rotate the display (as shown in Figure 1-5) by
holding down the left mouse button in the graphics window and moving the mouse
to the left. More information on manipulating the graphics display is given in
the next step.
Figure
1-5: Brick and elliptical cylinder united into one volume
Step
4: Manipulate the Display:
1. Zoom out from the current view by holding
down the right mouse button in the graphics window and pushing the mouse away
from you.
2. Rotate the view around the screen centre
by holding down the right mouse button and moving the mouse from side to side.
3. Rotate the view in free-form mode by
holding down the left mouse button and moving the mouse.
4. Translate the display by holding down the
middle mouse button and moving the mouse.
5. Divide the graphics
window into four quadrants by clicking the SELECT PRESET
CONFIGURATION
command
button in the Global Control tool pad.
GAMBIT divides the
graphics window into four quadrants and applies a different orientation to the
model in each of the four quadrants. Each view of the graphics window can be
manipulated independently. All changes to the model appear in all portions of
the graphics window, unless you disable one or more quadrants.
Figure
1-6: GAMBIT GUI-four graphics-window quadrants
6. Restore a single display of the model.
a) Use the left mouse button
to select the graphics-window "sash anchor"—the small gray box in the
centre of the graphics window.
b) Use the mouse
to drag the sash anchor to the bottom right corner of the graphics window.
7. Restore the front view of
the model by left-clicking the ORIENT MODEL
command
button in the Global Control tool pad. 8. Scale the
model to fit the graphics window by clicking the FIT TO WINDOW
command
button in the Global Control tool pad.
Step
5: Mesh the Volume:
1. Create a mesh for the volume.
MESH
—> VOLUME—> MESH VOLUMES 
This command sequence
opens the Mesh Volumes form.
a) Shift-left-click the
volume in the graphics window.
GAMBIT will
automatically choose the Cooper Scheme
Type as the meshing tool to be used, and will use an Interval
size of 1 (the default) under Spacing. See the GAMBIT Modelling Guide, Chapter 3 for details
about the Cooper meshing tool.
b) Click Apply at the bottom of the Mesh Volumes
form.
This accepts the volume
you selected as the one to be meshed. It also accepts the source faces (the
faces whose surface meshes are to be swept through the volume to form volume
elements) that GAMBIT has chosen for the Cooper meshing
scheme and starts the meshing operation based on the current Mesh
Volumes form specifications. A status bar appears at the top of the GAMBIT
GUI to indicate how much of the meshing is complete.
The volume will be
meshed as shown in Figure 1-7.
Figure
1-7: Meshed volume
Step
6: Examine the Mesh:
It is important that you check the quality of
the resulting mesh, because properties such as skewness can greatly affect the
accuracy and robustness of the CFD solution. GAMBIT provides several quality
measures (sometimes called "metrics") with which you can assess the
quality of your mesh. In the case of skewness measures such as Equi
Angle Skew and EquiSize Skew, for example, smaller
values are more desirable. It is also important to verify that all of the
elements in your mesh have positive area/volume. You should consult the
documentation for the target CFD solver for additional mesh quality guidelines.
1. Select the EXAMINE MESH
command
button at the bottom right of the Global Control tool pad.
This action opens the Examine Mesh form.
a) Click Update at the bottom of the Examine Mesh
form.
GAMBIT does not
automatically update the graphics display when you open the Examine
Mesh form or modify its specifications, such as Display
Type or Quality Type. To update the graphics
display, you must click the Update pushbutton located at
the bottom of the form. GAMBIT displays the Update
pushbutton label in red lettering whenever the display needs to be updated to
reflect the current Examine Mesh specifications.
Some Examine Mesh operations automatically update the graphics
display. For example, if you select the Display Type: Range
option and click one of the histogram bars (see below), GAMBIT automatically
updates the display.
b) Select Range under Display Type at the top of
the Examine Mesh form.
A histogram appears at
the bottom of the form. The histogram consists of a bar chart representing the
statistical distribution of mesh elements with respect to the specified Quality Type. Each vertical bar on the histogram corresponds
to a unique set of upper and lower quality limits.
The 3D Element type selected by default at the top of the form is a
brick
.
c) Select or retain EquiSize Skew
from the Quality Type option menu.
d) Click on one of
the green vertical bars in the histogram to view elements within a certain
quality range.
Each element has a
value of skewness between 0 and 1, where 0 represents an ideal element. The
histogram is divided into 10 bars; each bar represents a 0.1 increment in the
skewness value. For a good mesh, the bars on the left of the histogram will be
large and those on the right will be small.
Figure 1-8 shows
the view in the graphics window if you click on the fourth bar from the left on
the histogram (representing cells with a skewness value between 0.3 and 0.4).
Figure 1-8: Elements of the mesh within a specified
quality range
e) Move the Upper and Lower slider boxes beneath
the histogram to redefine the quality range to be displayed.
Step 7: Save the Session and Exit GAMBIT:
1. Save
the GAMBIT session and exit GAMBIT.
File
—> Exit
GAMBIT will ask you whether you
wish to save the current session before you exit.
a) Click Yes to save the current
session and exit GAMBIT.
Summary:
This
tutorial provided a quick introduction to GAMBIT by demonstrating how to create
a simple 3-D geometry using the "top-down" modelling approach. The
Cooper scheme was used to automatically generate an unstructured, hexahedral
mesh. For more information on the Cooper scheme, consult the GAMBIT Modelling
Guide.
