In this tutorial you will learn how to:
- Start GAMBIT
- Use the Operation
toolpad
- 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
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.
- 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
Type
gambit -id basgeom
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, basgeom) as a prefix to all files it creates: for example, basgeom.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 toolpad
(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
subpad does not appear when you select the GEOMETRY
command button, click it again.
command button by clicking on it with the
left mouse button. If the Geometry
subpad 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
subpad. 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 subpad.
command button in the Geometry subpad.
Again, this command button will be depressed
when selected. Selecting this command button opens the Geometry/Volume
subpad.
c) Use the left mouse button to select the CREATE VOLUME 
command button in the Geometry/Volume
subpad.
command button in the Geometry/Volume
subpad.
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
—> 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.
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)
1. Create an elliptical cylinder.
a) Hold down the right mouse
button while the cursor is on the CREATE
VOLUME command button.
command button.
b) Select the CREATE
REAL CYLINDER option 
from the resulting menu.
from the resulting menu.
! CREATE
REAL CYLINDER is the text that is written in the Description window
when you hold the mouse cursor over the menu item.
menu 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
—> VOLUME
—> CREATE
VOLUME R
where R
indicates a toolpad 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
1. Unite the brick and elliptical cylinder into one volume.
GEOMETRY
—> VOLUME
—> BOOLEAN
OPERATIONS 
This command sequence opens the Unite Real Volumes 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
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 center 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
toolpad. 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
center 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
toolpad. 8. Scale the model to fit the graphics window by clicking the FIT TO WINDOW
command button in the Global Control
toolpad. 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
Modeling 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. 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
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 EquiAngle 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 toolpad.
This action opens the Examine Mesh form.
a) 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 
.
.
b) Select EquiAngle
Skew from the Quality
Type option menu.
c) 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 fifth bar from the left on the histogram (representing cells
with a skewness value between 0.4 and 0.5).
Figure
1-8: Elements of the mesh within a specified quality range
d) Move the Upper
and Lower
slider boxes beneath the histogram to redefine the quality range to be
displayed.
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.
This tutorial provided a quick introduction to GAMBIT by demonstrating how to create a simple 3-D geometry using the "top-down" modeling approach. The Cooper scheme was used to automatically generate an unstructured, hexahedral mesh. For more information on the Cooper scheme, consult the GAMBIT Modeling Guide.
© Fluent, Inc. 12/26/01
No comments:
Post a Comment