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GridTool: A Tool for Structured and Unstructured Grid Generation
Surface Representation and Grid Projection
References [4]-[6]
contain detailed descriptions of Surface Representation and Grid
Projection. For completeness sake, a short summary will be provided
here. In CAD systems, curves and surfaces are represented typically
by NonUniform Rational B-Splines (NURBS) which is the most general
mathematical representation for curves and surfaces. Most parametric
curves and surfaces can be converted to an equivalent NURBS [7]-[8] representation without any loss of
accuracy.
The surface-grid can be generated either in a parameter space or on
approximated/simplified NURBS surfaces. Grids generated in a parameter
space have two serious restrictions. The first restriction is that the
choice of surface parameterization affects the CFD surface-grid. As
shown in Refs. Ref. 6 and
Ref. 9, a poor parameterization may cause the
CFD
surface-grid to be highly skewed. The second limitation is that a CFD
surface-grid can not be generated over several overlapping NURBS
surfaces which is the most serious restriction.
In the second method , the NURBS surfaces are approximated by a few
bi-linear patches, then, the surface grids are generated based on these
bi-linear patches. The resulting surface grids are close but they
are not on the original NURBS surfaces. This problem can be alleviated
by projecting the resulting grid points onto the original NURBS
surfaces. This method is easy to implement, and it avoids the problems
associated with surface parameterization. This method of grid
generation will require a very robust and fast grid point projection.
This method is implemented for
structured and
unstructured grid.
Surface Attributes
This panel is designed to allow the user to manipulate the surface
properties. In order to change the background color of the display
window, the "Background" button in the "Attributes" panel
should be pressed. Then, the color can be changed by moving three
sliders for the colors or by inputing the RGB color number (Red, Green,
Blue) in the input-fields.
The surface properties such as color, direction and number of display
paths can be changed in this panel. First, the "Surfaces" button
in this panel should be pressed, and then "On/Off" browser will
appear. In order to change the properties of some of the surfaces,
first they must be turned on. This can be done either from the display
window or from the "On/Off" browser by placing the cursor over the
surface number and clicking with the left mouse. In order to turn a
surface on/off from the display window, first the surface must become
an active surface by placing the cursor over the surface and hitting
the hot key "s". Then, the active surface can be
turned on/off by hitting the hot key "w". Once
the appropriate surfaces are turned on, then the properties can be
changed. Surface colors can be changed by using the three sliders for
colors or by inputing the RGB color number in the input-fields. The
direction of surfaces can be changed by pressing the "Directions"
button. The number of display paths can be changed by inputing the
desired numbers of display paths in u and v coordinates in the
input-field for "number of display path" and then pressing
"Change DP" button.
Projection Properties
This panel is designed for modifying the projection parameters, and it
displays the projection parameters for the active surface. For detail
description of these properties, readers are referred to
Refs. [4]-[6]. In this panel, it is also
possible to change the parameters such that part of a surface is
displayed and projected to. This can be done by changing the limits of
the surface parameters, us (minimum u), ue (maximum u), vs (minimum v),
and ve (maximum v). The "Itmax" is the number of iteration for
the projection to converge. The "EPS" is the residual limit for
the projection to converge. The "du" and "dv" are the
step size in u and v directions, respectively. The "magn" are
parameters used to pull grid points to boundaries, and their values
range from 0-1.
- "mag1" for minimum v
- "mag2" for maximum u
- "mag3" for maximum v
- "mag4" for minimum u
FAQ
- What is a display path?
- In GridTool a surface is defined everywhere.
In order to display it, lines are drawn on the surface.
- How do I turn on/off surfaces?
- Activate the Surface Panel from the Main panel.
To turn off a single surface,
- set selection type to single,
- select the display type,
- place the cursor over the
surface number and click with the right mouse.
To turn off a family or all surfaces,
- set selection type to family or all,
- select the display type.
- How do I make a surface active?
- Either place the cursor over the desired surface in the display window,
and then hit the "s" key. Or Activate the Surface Panel from the Main panel.
- set selection type to single,
- select "Active Surface" from the display type,
- place the cursor over the
surface number and click with the right mouse.
- How do I group surfaces together?
-
- Turn the surfaces that belong to the family on,
- Select a family name,
- push the "Change Family" button.
- How do I change the directions on the surfaces?
-
- Turn the desired surfaces on,
- Push the "Direction" button.
- How do I change the background color?
-
- Push the "Background" button,
- Then, the color can be changed by moving three
sliders for the colors or by inputing the RGB color number
(Red, Green, Blue) in the input-fields.
- How do I change the surface color?
- Turn the desired surfaces on.
Surface colors can be changed by using the three sliders for
colors or by inputing the RGB color number in the input-fields.
- How do I change the number of display path?
- Turn the desired surfaces on.
Select the number of display path
inputing the desired numbers of display paths in u and v coordinates
in the input-field for "number of display path" and then
pressing "Change DP" button.
- How do I convert the existing display path to a NURBS surface?
-
- Turn the desired surfaces on,
- push the "DP to NURBS" button.
This will not delete the original surfaces.
- How do I delete surfaces?
-
- Turn the desired surfaces on,
- push the "Delete Surfaces" button.
What are the projection properties for?
Some surfaces may have a bad parametrization and may be very complex.
In order to project to these surfaces, the projection properties may
need to be fine-turned.
How do I change the projection properties of a surface?
- Activate the projection property panel from surface panel.
- Make the surface active
- Change the property
How do I change the projection properties of a displayed surfaces?
- Activate the projection property panel from surface panel.
- Turn the surfaces on
- Change the property for one of them
- Push Set Displayed Surfaces button
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