BLMesher

[Category:PHASTA Tutorials]]

How to create the mesh using script

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Start

• To begin, we need a geometry model. It can be created using commercial softwares such as Solid Work and saved as *.x_t
• You need the BLMesher code which is available on svn:

  svn co https://redmine.scorec.rpi.edu/svn/phasta/BLMesher

Files needed

Create a folder and copy all these files into it.

• Geometry model file (geom.xmt_txt or geom.x_t)
• Mesh generation attribution file (Attributes.inp)
• exe file (Run.sh)

File Attributes.inp is the script that defines the mesh generation characteristics for different entities of the model.

Run.sh is an exe file including commands as below:

 PATH=/users/chitak/develop/Meshing/BLMesher
 $PATH/bin/x86_64_linux/BLMesher-0 geom.xmt_txt geom.sms 1 0.1 Attributes.inp

It defines the proper path for mesh generator. It also defines that geom.xmt_txt is the model file and the mesh must be created using the attributes defined in Attributes.inp and the mesh data must be written into geom.sms

In the second line of the Run.sh file, the value before attributes.inp (in this case, 0.1) is the size of the boxes that are created before the the final mesh generation. (In fact, BLMesher tries to divide the whole domain into small boxes and then divide these boxes to create the mesh). Lets call the characteristic size of the domain, d, and the size of the boxes E.

You must choose E as below and use it in Run.sh:

<math> E=\frac{d}{2^{(m+1)}} </math>  or  <math> E=\frac{d}{2^m} </math> 

m is an integer.

Other flags like optimization and smoothing can also be given in this line. Use -h option to view the complete list of the options and their meaning.

Edit the script

• The first line is the version number (for future use). For eg.: version 1
• Use "#" to comment a line or disable it.
• To create attributes for each model entity, two lines in the following format are being used.

 <entity dimension> <entity tag> <size type> <attribute type> <extra> 
 <value or expression for mesh size on this entity>

• Note that a blank line between these two lines is not allowed.

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Create the mesh

• After preparing the script, execute the file Run.sh. It creates the mesh and write the data to geom.sms (as you defined in Run.sh).
• Note that after executing the Run.sh, you must use command "top" to check the memory usage. If you find out that memory usage is huge and is increasing, you must kill the job before the machine crashes

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Visualize the mesh

To see the generated mesh, you can use simapps:

  /usr/local/simapps/<simapps version>/simapps ThreeDViewer &

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Define the mesh characteristics in attribute file

The parameters in the first line, respectively are:

- Entity dimension:          2 for 2-D cases and 3 for 3-D cases
- Entity tag:                You can find the entity tag using simapps
- Size type on entity:       1 for absolute mesh size and 2 for relative mesh size
- Attribute type on entity:  
                             0 just imposes the size on entity. <extra> field is NULL
                             1 for entity with boundary layers. <extra> field is explained below.
                             2 for periodic entity: In this case, <extra> field is followed with slave entity tag
                             3 for refinement source. Specify entity dimension as -1 in this case. <extra> field is explained below.

<extra> field:

For Boundary layer: 
   is followed with (a) faceSide/useDir (BL orientation), (b) numEndEnts (==0, layers with fixed heigths), (c) firstLayerHeight, (d) totalHeight, (e) nLayers, (f) blMixed, (g) blBlends, (h) blPropagate and (i) edgeBLFaceTag (only if 2D BL on edge)   
   is followed with (a) faceSide/useDir (BL orientation), (b) numEndEnts (==2, to vary height linearly between two end/bounding entities), (c) varyDirection, (d) endEntTag1 , (e) firstLayerHeight1, (f) totalHeight1, (g) endEntTag2, (h) firstLayerHeight2, (i) totalHeight2, (j) nLayers, (k) blMixed, (l) blBlends, (m) blPropagate and (n) edgeBLFaceTag (only if 2D BL on edge)

For refinement: 
   is followed with (a) ref. source type (1-cube, 2-cylinder etc.), (b) source info. as Simmetrix APIs (like radius, length, center and normal in case of cylinder, and center, wdir, hdir and ddir in case of cube/parallelepiped)

• Note if entity dimension is -1, (in case of refinement) do not specify the entity tag and size type.

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Example 1: Uniform mesh

Simple uniform mesh on region with tag 205

3 205 1 0
2e-4

The above lines can be divided as below:

(3) (205) (1) (0)
(2e-4)

(3): Entity dimension. It's a 3-D region.
(205): Entity tag
(1): Size type on entity. 1 for absolute mesh size
(0): Attribute type on entity. 0 just imposes the size on entity. <extra> field is NULL
(2e-4): Mesh size (This is in the second line) 

Note: The second line (Mesh size) can be a value or an expression such as:

0.0125+0.1*(1.0-exp(-3.0*sqrt(($x-0.0)^2+($y-0.0)^2+($z-0.5)^2)))

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Example 2: Refinement in a cylindrical region

-1 3 1 1.5e-3 1 2 0.3 0 0 0 1
2.5e-4

The above lines can be divided as below:

(-1) (3) (1) (1.5e-3) (1) (2 0.3 0) (0 0 1)
(2.5e-4)

(-1): Entity dimension, for refinement cases
(3): Attribute type. in this case, refinement.
(1): For refinement in a cylindrical region
(1.5e-3 ): Radius 
(1): Length of the Cylinder (Half)
(2 0.3 0): The center of the circle 
(0 0 1): Axis
(2.5e-4): mesh size (This is in the second line)

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Example 3: Refinement in a box region

-1 3 2 6e-3 5e-3 0.0 1.5e-3 0.0 0.0 0.0 5.0e-4 0.0 0.0 0.0 5.0e-4
2.5e-4

The above lines can be divided as below:

(-1) (3) (2) (6e-3 5e-3 0.0) (1.5e-3 0.0 0.0) (0.0 5.0e-4 0.0) (0.0 0.0 5.0e-4)
(2.5e-4)

(-1): Entity dimension. -1 only for refinement cases.
(3): Attribute type. in this case, refinement.
(2): For refinement in a box region
(6e-3 5e-3 0.0): The center of the box 
(1.5e-3 0.0 0.0): x-Half length  
(0.0 5.0e-4 0.0): y-Half length  
(0.0 0.0 5.0e-4): z-Half length  
(2.5e-4): mesh size (This is in the second line)

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Example 4: Refine the mesh along 3 lines on face with tag 5

3 5 1 0
(0.5e-03) +0.1*(1.0-exp(-2.0*sqrt(($x-1.9)^2+($y-0.36)^2)))*(1.0-exp(-10.0*sqrt(($x-1.77)^2+($y-0.42)^2)))*(1.0-exp(-6.0*sqrt(($x-1.8)^2+($y-0.43)^2)))

The first line can be divided as below:

(3) (5) (1) (0)

(3): Entity dimension, 3-D
(5): Entity tag
(1): Size type on entity. 1 for absolute mesh size
(0): Attribute type on entity. 0 just imposes the size on entity. <extra> field is NULL

This is an example of an expression that will make the mesh fine in three lines. e.g., x=1.9 y=0.36 on model entity number 5.

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Example 5: Periodic mesh

If periodic BC is used at two faces, the mesh must be identical in both of them.

In this case, faces 1 and 81 are periodic

2 1 1 2 81
2.1e-4

The above lines can be divided as below:

(2) (1) (1) (2) (81)
(2.1e-4)

(2): Entity dimension, 2-D. it's a face.
(1): Entity tag
(1): Size type on entity. 1 for absolute mesh size
(2): Attribute type on entity. 2 for periodic entity. <extra> field is followed with a slave entity tag
(81): Slave entity tag
(2.1e-4):mesh size (This is in the second line)

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Example 6: Boundary layer mesh , constant thickness

On face with tag 173

2 173   1 1 0 0 0.4e-4 3.e-4 5 0 0 0
1.61e-4

The above lines can be divided as below:

(2) (173) (1) (1) (0) (0) (0.4e-4) (3.e-4) (5) (0) (0) (0)
(1.61e-4)

(2): Entity dimension, 2-D. it's a face.
(173): Face tag
(1): Size type on entity. 1 for absolute mesh size
(1): Attribute type on entity. 1 for entity with boundary layers. 
(0): Boundary layer orientation (FaceSide/useDir)
(0): NumEndEnts. 0 for layers with fixed heights 
(0.4e-4): First layer height
(3.e-4): Total height of boundary layer
(5): Number of layers
(0): Boundary layer Mixed topology (1 for triangles and quads)
(0): Boundary layer blending.  0 for no BL blending 
(0): Boundary Layer propagation
(1.61e-4): mesh size on the surface (This value is in the second line)

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Example 7: Boundary layer mesh, ramped thickness

On face with tag 27

2 27 1 1 0 2 0 22 1.e-3 2.e-2 20 2.e-3 10.e-2 5 1 0 0
0.1

The above lines can be divided as below:

(2) (27) (1) (1) (0) (2) (0) (22) (1.e-3) (2.e-2) (20) (2.e-3) (10.e-2) (5) (1) (0) (0)
(0.1)

(2): Entity dimension, 2-D. it's a face.
(27): Face tag
(1): Size type on entity. 1 for absolute mesh size
(1): Attribute type on entity. 1 for entity with boundary layers. 
(0): Boundary layer orientation (FaceSide/useDir)
(2): For Layers with linearly varying height (NumEndEnts)
(0): Direction of variation (pos. x-dir.)
(22): Edge tag at pos. 1
(1.e-3): First layer height at pos. 1
(2.e-2): Total height of all layers at pos. 1
(20): Edge tag at pos. 2
(2.e-3): First layer height at pos. 2
(10.e-2): Total height of all layers at pos. 2
(5): Number of layers
(1): Boundary layer Mixed topology (1 for triangles and quads)
(0): Boundary layer blending.  0 for no BL blending 
(0): Boundary Layer propagation. 0 for no propogation
(0.1): mesh size on the surface (This value is in the second line)