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VGRID Source Specification

  VGRID supports the use of Nodal, Linear, and Surface sources which provide a varying level of control over the mesh characteristics.  The source type is selected from the options menu under the VGRID tab.  Each source type is described below.

1) A Nodal source enables the control of mesh characteristics about a point in 3D space with or without a directional bias.

2) A Linear source enables the control of mesh characteristics about a finite linear segment with or without a directional bias.

3) A Surface source enables the control of mesh characteristics across a surface entity.

  Each source is characterized by its type as well as additional properties which are described below:
 

Source Property
Description
s
Primary spacing value
S
Secondary spacing for stretched sources or max surface spacing value
an
Radial intensity factor
bn
Bias intensity factor
Alpha
Directional bias toggle
P
Stretching vector
U
Bias vector

The effects of each of the source properties is described below.

The Nodal Source

  The Nodal source is the simplest source and most useful when a desired cell size is required at a point within the domain.  To demonstrate the use of this type of source, a surface mesh is to be generated in a square region (10 units x 10 units) in the z-plane as shown below.

  A nodal source has been placed at each corner and at the center of the surface.  Source properties as specified in the table below have produced the mesh shown above.  Note that the desired physical spacing for the Nodal source positioned at the origin has been prescribed to be smaller than the others with the remaining source properties unchanged from their default values.
 
 
Source Number
Coordinates
s
1
(-5,-5,0)
1.5
2
(-5,5,0)
1.5
3
(5,-5,0)
1.5
4
(5,5,0)
1.5
5
(0,0,0)
0.15



  By increasing an for a particular source, it is possible to further extend its influence into the surrounding mesh.  It is important to note that it is not the magnitude of this term but the relative magnitude between sources which controls the relative influence.  That is to say if we had assigned a value of 2 to each of the intensity factors in the example above, an identical mesh would have resulted.  For the mesh shown to the left, the intensity for the Nodal source positioned at the origin has been increased to 2 as summarized in the table below with the remaining source properties unchanged from their default values.  Note the extended influence of this source radially from the origin.
 
 
Source 
Number
Coordinates
s
an
1
(-5,-5,0)
1.5
1
2
(-5,5,0)
1.5
1
3
(5,-5,0)
1.5
1
4
(5,5,0)
1.5
1
5
(0,0,0)
0.15
2



  The ability to directionally bias the influence of the nodal source is present through the bias intensity, bn, the directional bias flag, Alpha, and U, the bias vector.  In this example the nodal source centered at the origin is assigned source properties as indicated in the table below with the resulting mesh shown to the left.
Source Number
Coordinates
Spacing (s)
Intensity (an)
Bias Intensity (bn)
Alpha
Bias Vector (U)
1
(-5,-5,0)
1.5
1
0
-
-
2
(-5,5,0)
1.5
1
0
-
-
3
(5,-5,0)
1.5
1
0
-
-
4
(5,5,0)
1.5
1
0
-
-
5
(0,0,0)
0.15
2
0.25
0
(1,1,0)

Note that the effect of an is unchanged in the direction perpendicular to the bias vector though the extent of the influence along the bias vector is increased due to a non-zero value prescribed for bn.  Note also that in this instance the components for U were entered directly into the adjacent text fields which allows vector lengths greater than unity to be prescribed; the nature of the directional bias is sensitive to the magnitude of this vector as well.

  Lastly, a non-zero value for Alpha toggles the bias in a direction colinear with the bias vector.   In the mesh shown at the left, Alpha has been set to unity which then limits the effect of bn along the positive direction of U.  A value of -1 for Alpha would redirect the bias along the negative direction of U.  As shown earlier, an Alpha value of zero institutes no bias along the bias vector.

The Linear Source

  The Linear source extends the concept of the nodal source as two nodal sources now become linked by a line segment which exerts a continuous influence along its length upon the mesh.  Though the source properties continue to apply to the source as a single entity, the spacing constraints need not be identical at the termination points, thus the behavior of the source can be tailored to meet a variety of requirements.

  To demonstrate the use of a Linear source, the previous example has been revisited with the Nodal source at the origin replaced with a Linear source which has been positioned diagonally between (2,2,0) to (-2,-2,0).  At each termination point, the spacing constraint has been set to 0.015 units, with the specifications for the 4 corner sources remaining unchanged.  The radial intensity has been set to 3 with the resulting mesh shown to the left.



  A bias for this source is introduced by increasing bn to 7, Alpha to unity, and the bias vector to (-0.7071, 0.7071, 0.0).  In this instance the components were entered as (-1,1,0) in the Node sub-panel with the components of the resulting unit vector displayed in the U text fields.  The resulting mesh is shown at the left.  Clearly there is some iteration required to "fine-tune" the source parameters, but the concept permits a wide variety of meshing situations to be addressed.


The Surface Source

  The Surface source enables the user to discretize one or more surfaces using a fixed spacing as entered in the S text field.


Responsible NASA Official: William T. Jones
Site Curator: William T. Jones
Comments and Questions
Last Updated: Dec 18, 2001