# Use of grid stretching functions in DiveMESH

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 April 23, 2022, 06:53 Use of grid stretching functions in DiveMESH #1 New Member   YN Join Date: Apr 2022 Posts: 19 Rep Power: 2 Hi! When I am using reef3d,I do not understand some commands of divemesh,such as,B 101,B 102,B 103.Could you reply it? Last edited by kamath; April 26, 2022 at 03:15. Reason: edited title for better visibility

April 25, 2022, 03:52
#2
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Arun Kamath
Join Date: Nov 2014
Location: Trondheim, Norway
Posts: 175
Rep Power: 10
The commands you name are relvant if you are using a stretched grid fro your simulations.

For eg.
B 1 0.05
B 10 0.0 10.0 0.0 0.05 0.0 1.0
Will result in a two-dimensional domain that is 10 m long in the x-direction, 1 cell wide (2D) and 1 m high.
In this domain, the grid size is dx=dy=dz= 0.05 m in all directions, with a total of 4000 grids (see fig1)

On the other hand, if you want finer meshes in a part of the domain compared to the rest, you can apply a stretched mesh. For eg.

B 2 200 1 20
B 10 0.0 10.0 0.0 0.05 0.0 1.0

B 101 5
B 111 2.0
B 114 5.0

B 103 5
B 111 2.0
B 116 0.5

will create a mesh with 4000 cells but with a focus point for x-grids at x=5.0 m, and z=0.5 m. The stretching factor is 2.0. This will result in a mesh as seen in Fig 2.
The file DIVEMesh_Log/DIVEMesh-Non-Uniform-Mesh.txt will show you the max and min sizes of the grids and the location of the finest grid.
This example uses a simple sinh function based stretching. Other options can be explored in the B 101/2/3 commands to define different types of grid stretching.

Hope that helps.

PS. stretching factor of 3.0 is used purely for demonstration. In practical cases, this might be too high. You need check the aspect ratios of the grids in the DIVEMesh-Non-Uniform-Mesh.txt file and is recommended to keep the maximum cell ratios to around 1.05 or lower to ensure simulation stability.
Attached Images
 Fig1.jpg (55.1 KB, 17 views) Fig2.jpg (60.5 KB, 19 views)
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Arun
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April 25, 2022, 08:50
#3
New Member

YN
Join Date: Apr 2022
Posts: 19
Rep Power: 2
Quote:
 Originally Posted by kamath The commands you name are relvant if you are using a stretched grid fro your simulations. For eg. B 1 0.05 B 10 0.0 10.0 0.0 0.05 0.0 1.0 Will result in a two-dimensional domain that is 10 m long in the x-direction, 1 cell wide (2D) and 1 m high. In this domain, the grid size is dx=dy=dz= 0.05 m in all directions, with a total of 4000 grids (see fig1) On the other hand, if you want finer meshes in a part of the domain compared to the rest, you can apply a stretched mesh. For eg. B 2 200 1 20 B 10 0.0 10.0 0.0 0.05 0.0 1.0 B 101 5 B 111 2.0 B 114 5.0 B 103 5 B 111 2.0 B 116 0.5 will create a mesh with 4000 cells but with a focus point for x-grids at x=5.0 m, and z=0.5 m. The stretching factor is 2.0. This will result in a mesh as seen in Fig 2. The file DIVEMesh_Log/DIVEMesh-Non-Uniform-Mesh.txt will show you the max and min sizes of the grids and the location of the finest grid. This example uses a simple sinh function based stretching. Other options can be explored in the B 101/2/3 commands to define different types of grid stretching. Hope that helps. PS. stretching factor of 3.0 is used purely for demonstration. In practical cases, this might be too high. You need check the aspect ratios of the grids in the DIVEMesh-Non-Uniform-Mesh.txt file and is recommended to keep the maximum cell ratios to around 1.05 or lower to ensure simulation stability.

April 26, 2022, 11:32
#4
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Join Date: Sep 2020
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Rep Power: 3
Quote:
 Originally Posted by kamath The commands you name are relvant if you are using a stretched grid fro your simulations. For eg. B 1 0.05 B 10 0.0 10.0 0.0 0.05 0.0 1.0 Will result in a two-dimensional domain that is 10 m long in the x-direction, 1 cell wide (2D) and 1 m high. In this domain, the grid size is dx=dy=dz= 0.05 m in all directions, with a total of 4000 grids (see fig1) On the other hand, if you want finer meshes in a part of the domain compared to the rest, you can apply a stretched mesh. For eg. B 2 200 1 20 B 10 0.0 10.0 0.0 0.05 0.0 1.0 B 101 5 B 111 2.0 B 114 5.0 B 103 5 B 111 2.0 B 116 0.5 will create a mesh with 4000 cells but with a focus point for x-grids at x=5.0 m, and z=0.5 m. The stretching factor is 2.0. This will result in a mesh as seen in Fig 2. The file DIVEMesh_Log/DIVEMesh-Non-Uniform-Mesh.txt will show you the max and min sizes of the grids and the location of the finest grid. This example uses a simple sinh function based stretching. Other options can be explored in the B 101/2/3 commands to define different types of grid stretching. Hope that helps. PS. stretching factor of 3.0 is used purely for demonstration. In practical cases, this might be too high. You need check the aspect ratios of the grids in the DIVEMesh-Non-Uniform-Mesh.txt file and is recommended to keep the maximum cell ratios to around 1.05 or lower to ensure simulation stability.

Hii,

Is it necessary to use B 1 or B 2 in control.txt ??

I have used the following codes to generate the grid ---

############################
B 10 0.0 5.0 0.0 0.06 0.0 0.3 //domain

B 101 11 // cell based
B 127 0.0025 0.005 3.7725 0.545 1.03
B 102 11
B 128 0.0025 0.0025 0.03 0.01 1.0 // uniform grid
B 103 11
B 129 0.0025 0.005 0.15 0.14 1.03

#######################

In B 127/128/129 - we are giving min/max cell size, the refinement region and cell ratio, so this would work without B1 and B2, correct ?

Paul

 April 27, 2022, 04:24 #5 Super Moderator   Arun Kamath Join Date: Nov 2014 Location: Trondheim, Norway Posts: 175 Rep Power: 10 Yes it is essential to define B 2 in your input. As you can see the inputs for your commands do not have an idea of what the maximum number of cells is. This is obtained from B 2. Without this information, the stretched grids cannot be defined. raul_xusernamex_ likes this. __________________ Arun Team REEF3D www.reef3d.com

April 27, 2022, 04:50
#6
New Member

Join Date: Sep 2020
Posts: 13
Rep Power: 3
Quote:
 Originally Posted by kamath Yes it is essential to define B 2 in your input. As you can see the inputs for your commands do not have an idea of what the maximum number of cells is. This is obtained from B 2. Without this information, the stretched grids cannot be defined.
Thank you for the clarification. I will correct my code.

April 27, 2022, 08:23
#7
New Member

YN
Join Date: Apr 2022
Posts: 19
Rep Power: 2
Quote:
 Originally Posted by kamath The commands you name are relvant if you are using a stretched grid fro your simulations. For eg. B 1 0.05 B 10 0.0 10.0 0.0 0.05 0.0 1.0 Will result in a two-dimensional domain that is 10 m long in the x-direction, 1 cell wide (2D) and 1 m high. In this domain, the grid size is dx=dy=dz= 0.05 m in all directions, with a total of 4000 grids (see fig1) On the other hand, if you want finer meshes in a part of the domain compared to the rest, you can apply a stretched mesh. For eg. B 2 200 1 20 B 10 0.0 10.0 0.0 0.05 0.0 1.0 B 101 5 B 111 2.0 B 114 5.0 B 103 5 B 111 2.0 B 116 0.5 will create a mesh with 4000 cells but with a focus point for x-grids at x=5.0 m, and z=0.5 m. The stretching factor is 2.0. This will result in a mesh as seen in Fig 2. The file DIVEMesh_Log/DIVEMesh-Non-Uniform-Mesh.txt will show you the max and min sizes of the grids and the location of the finest grid. This example uses a simple sinh function based stretching. Other options can be explored in the B 101/2/3 commands to define different types of grid stretching. Hope that helps. PS. stretching factor of 3.0 is used purely for demonstration. In practical cases, this might be too high. You need check the aspect ratios of the grids in the DIVEMesh-Non-Uniform-Mesh.txt file and is recommended to keep the maximum cell ratios to around 1.05 or lower to ensure simulation stability.
Hi! I have one question.How to control the maximum cell ratios?

 April 27, 2022, 08:55 #8 Super Moderator   Arun Kamath Join Date: Nov 2014 Location: Trondheim, Norway Posts: 175 Rep Power: 10 Depending on what you want to control, you need to select the grid generation method. If you have a very particular mesh in mind, then simply use B 101/2/3 10 and provide files x-spacing, y-spacing, z-spacing define the grids in these directions. The contents should range from 0 to 1 in these files. __________________ Arun Team REEF3D www.reef3d.com

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