ELEMENT-SIZE ASSIGNMENT METHODS
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GiD automatically corrects element sizes according to the shape of the entity to
be meshed and its surrounding entities. This default option may be deactivated
and reactivated by going to the Mesh menu, selecting Preferences ,
and then Automatic correct sizes .
Footnote: The Preferences option can also be found in the Utilities
menu.
Footnote: Automatic correct sizes automatically executes
the options Assign sizes
By geometry
and Assign sizes
Correct sizes .
Sometimes, however, this type of correction is not sufficient and it is
necessary to indicate where on the mesh greater accuracy is needed. In these
cases, GiD offers various options and methods allowing sizes to be assigned to
elements.
Five examples are shown to illustrate the default method and the four other
methods.
Assignment using default options
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Select Mesh
Generate Mesh .
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A window appears showing the maximum element size. Leave this default size
unaltered and click OK .
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A meshing process window opens. Then another window appears with information
about the mesh generated. Click OK to visualize the mesh (see Figure 2).
Figure 2. Meshing
by default.
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Note that in the zone highlighted in Figure 3, elements are smaller than in the
rest of the model. This is because of the shape of the surface placed there.
When all meshing preferences are set to their default levels, as for this
example, the RFAST surface mesher is used. In this way, geometrical entities
are meshed hierarchically: first of all lines are meshed, then the surfaces,
and finally the volumes. The line elements size depends on the shape of
surfaces (as can be seen in this example). Later on we will see an example
using RJUMP mesher, where element sizes are distributed differently.
Figure 3. Meshing
by default. Zone where elements are smaller because of the surface shape.
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Select Mesh
Unstructured
Assign
size on points . A window appears in which to enter the element size
around the points to be chosen. Enter 0.1 and click OK .
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Select the point indicated in Figure 4. Press ESC to indicate that the
selection of points is finished.
Footnote: Instead of pressing the ESC key, the center mouse
button or the mouse wheel can also be used.
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Select Mesh
Generate Mesh .
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A window opens asking whether the previous mesh should be eliminated. Click Ok
.
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GiD then asks you to enter the maximum element size. Leave the default value
unaltered and click OK .
Figure 4.
Geometry of the model. The point around which the mesh will be concentrated.
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Figure 5. The mesh
with a concentration of elements around the point.
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A concentration of elements appears around the chosen point, given the selected
size (0.1) of these elements (see Figure 5).
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Go to Utilities and open Preferences. Click Meshing . In
the window that appears there is the option Unstructured Size Transitions
. This option defines the transition gradient of element sizes (size gradient),
whose values are between 0 and 1. The greater the size gradient, the greater
the change in space. To test this, enter the value 0.4 and click Accept .
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Again, select Mesh
Generate Mesh
.
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A window opens asking whether the previous mesh should be eliminated. Click Ok
.
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GiD then asks you to enter the maximum element size. Leave the default value
unaltered and click OK .
Figure 6. Mesh
with the elements concentrated around a point, with a size gradient of 0.8.
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The size gradient (0.4) results in a higher density around the point (see
Figure 6).
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Now go back and enter 0.6 in Unstructured Size Transitions . This will
result in a mesh more suitable for our objectives. Click Accept .
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Select Mesh
Unstructured
Assign
size on lines . In the window that appears, enter the size of the
elements around the lines that will be chosen. Enter 0.5 and click OK .
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Select the lines defining the base of the prism (i.e. lines 1, 2, 3, 4 and 40).
To see entity numbers select Label from the mouse menu or from the View
menu. If you wish geometrical entity labels to be displayed, the view mode has
to be set to Geometry using View
Mode
Geometry
(this option may also be found in the GiD Toolbox), and the render mode must be
set to Normal . Press ESC .
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Select Mesh
Generate Mesh .
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A window opens asking if the previous mesh should be eliminated. Click Yes
.
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Another window appears in which you may enter a maximum element size. Leave the
default value unaltered and click OK . This results in a high
concentration of elements around the chosen lines, given that the selected
element size (0.5) is much smaller than that of the rest of the elements in the
model (see Figure 7).
Figure 7. Mesh
with a concentration of elements around lines.
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Select Mesh
Unstructured
Assign
size on surfaces . In the window that appears, enter the size of the
elements to be assigned on the surfaces that will be chosen. Enter 0.5 and
click OK .
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Select the triangular surface resulting from the section of one of the vertexes
of the prism (surface number 1). Press ESC .
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Select Mesh
Generate Mesh .
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A window opens asking if the previous mesh should be eliminated. Click Yes
.
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Another window appears in which you can enter the maximum element size. Leave
the default value unaltered and click OK . This results in a high
concentration of elements on the chosen surface; due to the value selected
(0.5) (see Figure 8).
Figure 8. Mesh
with a concentration of elements on a surface.
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Assignment with Maximum Chordal Error
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Select Mesh
Unstructured
Sizes
by chordal error… .
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GiD asks for the minimum meshing size. Enter 0.1.
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GiD asks for the maximum meshing size. Enter 10.
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Enter the chordal error. This error is the maximum distance between the element
generated and the real object (geometry). Enter 0.05 and press OK .
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Select Mesh
Generate Mesh .
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A window opens asking if the previous mesh should be eliminated. Click Yes
.
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Another window appears in which you can enter a maximum element size. Leave the
default value unaltered and click OK . This results in a high
concentration of elements in curved areas. Now our approximation is
significantly improved (see Figure 9).
Figure 9. Mesh
using sizes assignment by chordal error. Here, there is a greater concentration
of elements in the curved zones.
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