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.
|
