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-   -   [ICEM] Effect of globle mesh size and mesh independency (http://www.cfd-online.com/Forums/ansys-meshing/113772-effect-globle-mesh-size-mesh-independency.html)

sujay February 26, 2013 08:57

Effect of globle mesh size and mesh independency
 
Dear All,

My geometry is similar to water tank with inlet at top and outlet at bottom (both at off center) along the central axis.

For Meshing i am using smaller size at inlet and outlet (Max size 4 mm) through part mesh setting and for rest region through Global mesh size (Max Size).

With fixed max mesh size for inlet and outlet (Max 4 mm) , I am setting global mesh size. The change in total elements and nodes are as below,

Global Mesh Size (Max) Total Element Nodes
64 mm 414383 72421
32 mm 461344 80527
16 mm 841709 145118


How total element/nodes depend on Global mesh size? Please guide to select mesh size for mesh in-dependency.

Thanks,

Sujay Patil

energy382 February 26, 2013 12:38

Quote:

Originally Posted by sujay (Post 410195)
Dear All,

My geometry is similar to water tank with inlet at top and outlet at bottom (both at off center) along the central axis.

For Meshing i am using smaller size at inlet and outlet (Max size 4 mm) through part mesh setting and for rest region through Global mesh size (Max Size).

With fixed max mesh size for inlet and outlet (Max 4 mm) , I am setting global mesh size. The change in total elements and nodes are as below,

Global Mesh Size (Max) Total Element Nodes
64 mm 414383 72421
32 mm 461344 80527
16 mm 841709 145118


How total element/nodes depend on Global mesh size? Please guide to select mesh size for mesh in-dependency.

Thanks,

Sujay Patil


global scale factor means grow ratio in x,y,z direction.

e.g, you have a mesh with 100.000 elements and want to achieve a 300.000 elements mesh. You need an total grow ratio of three, which simply means cube root of three and you have approx. your global scale factor (not absolutely exact, but very close).
you can also scale spacing, but be aware: it's only enlarged up to 33%

speaking for turbomachinery, I normally goes in my example for 100k, 300k and 900k

energy382 February 26, 2013 12:46

Quote:

Originally Posted by energy382 (Post 410230)
global scale factor means grow ratio in x,y,z direction.

e.g, you have a mesh with 100.000 elements and want to achieve a 300.000 elements mesh. You need an total grow ratio of three, which simply means cube root of three and you have approx. your global scale factor (not absolutely exact, but very close).
you can also scale spacing, but be aware: it's only enlarged up to 33%

speaking for turbomachinery, I normally goes in my example for 100k, 300k and 900k

btw: i was speaking about hexa

sujay February 27, 2013 01:55

Quote:

Originally Posted by energy382 (Post 410235)
btw: i was speaking about hexa

Dear Chirstoph,

Thanks.

Meshing Details :

Shell Meshing: Patch dependent with Quad dominant Mesh
Volume Meshing : Tetra/mixed with Robust(Octree)

Can you please guide for tetra mesh.

Sujay Patil

energy382 February 27, 2013 04:38

Hey,

it's better to ask someone more experienced (with tetra), as I usually use hex mesh for my simulation, sorry.

PSYMN February 28, 2013 15:01

I would guess that this has to do with "powers of 2" and the octree process.

Do a search and you will find lots of other posts with details about how this works.

It appears to be seeing your 64 and 32 mm as the same basic size. Then when you go down to 16, it subdivides again. The Octree algorithms uses logic to decide if it should subdivide down to the next level or not. The solution is in your setup.

If you can post your tetin file with the 32 bit setting, I could take a quick look.

sujay March 1, 2013 01:56

http://www.cfd-online.com/Forums/C:%...ormal%2032.tin

sujay March 1, 2013 02:05

tin file
 
1 Attachment(s)
Dear Sir,

Please find the tin file.

Inlet, outlet, pipe submerged and pipe top assigned with smaller size while rest with 32 mm.

During the meshing i am not at all used the shell meshing, directly volume meshed.

Please clarify on the mesh quality also. Regarding the single edges, multiple edges and delauncy violation for the geometry.

Thanks,

Sujay Patil

sujay March 1, 2013 02:06

Quote:

Originally Posted by PSYMN (Post 410724)
I would guess that this has to do with "powers of 2" and the octree process.

Do a search and you will find lots of other posts with details about how this works.

It appears to be seeing your 64 and 32 mm as the same basic size. Then when you go down to 16, it subdivides again. The Octree algorithms uses logic to decide if it should subdivide down to the next level or not. The solution is in your setup.

If you can post your tetin file with the 32 bit setting, I could take a quick look.

tin file added

ashish agrahari March 1, 2013 02:42

Meshing of IC engine
 
hello frnds
I am working on cfd simulation of in-cylinder flow in SI engine, for which i am using dynamic mesh(layering and smoothing method) for movement of piston and valves. I have done full tetra meshing which is giving negative volume during compression. therefore it is required to have hybrid meshing i.e. tetra along with hex. Hex is required mainly at regions which is deforming eg engine cylinder. But, i am not able to mesh both hex and tetra on same geometry for some region. So can anyone help me in this case? how to mesh tetra and hex both in one geometry?

Far March 3, 2013 15:52

Played with global scale factor (kept every thing else same, surface mesh from octree and volume mesh from denauly)

a) 1 >>>> fine mesh (500,000 nodes)
b) 2 >>>> Coarse mesh (250,000 nodes)


Coarse mesh:

http://imageshack.us/a/img441/267/coarse001.jpg

http://imageshack.us/a/img812/8050/coarse002.jpg

Fine mesh:

http://imageshack.us/a/img534/2175/a001zh.jpg

http://imageshack.us/a/img687/1513/a002u.jpg

http://imageshack.us/a/img189/6182/a003lo.jpg

http://imageshack.us/a/img201/2826/a004wk.jpg

http://imageshack.us/a/img268/6768/a005d.jpg

PSYMN March 3, 2013 18:28

Hey Far, Did you figure out what he did wrong, or did you just leave the size alone and play with the scale factor?

Far March 3, 2013 23:38

I just played with scale factor.

I did steps in following manner:

1. Surface sizing through part mesh setup.

2. Density box at few locations

3. Gave tetra width and ratio

Once above parameters were set (though were random, may be he requires some different settings), then I played with scale factor : 1 and 2.

For scale factor 1, I had the fine mesh (double size) ~500,000
and for scale factor 2, mesh was coarse ~ 250,000

sujay March 4, 2013 03:07

Quote:

Originally Posted by Far (Post 411249)
I just played with scale factor.

I did steps in following manner:

1. Surface sizing through part mesh setup.

2. Density box at few locations

3. Gave tetra width and ratio

Once above parameters were set (though were random, may be he requires some different settings), then I played with scale factor : 1 and 2.

For scale factor 1, I had the fine mesh (double size) ~500,000
and for scale factor 2, mesh was coarse ~ 250,000

Dear Sir,

Thanks.

can you show these steps with snaps in ICEM

1. Surface sizing through part mesh setup.

2. Density box at few locations

3. Gave tetra width and ratio

Sujay Patil

Alia Amer April 3, 2016 05:47

Hi Everyone,
i am using ansys flotran for studying airflow inside buildings... i want to ask a question about generating the mesh ...after meshing the whole model....( i dont mesh the entire volume at once, i mesh it volume by volume ) .... how can i know the number of elements used in the mesh ????? i want to display it in the same way that you did:
Global Mesh Size (Max) Total Element Nodes
64 mm 414383 72421
32 mm 461344 80527
16 mm 841709 145118


is the element size measured in milimeters???

and i want to ask another question... is the scaling factor in ansys the same like grid refinement ratio???? and how should i use it????

Far April 3, 2016 07:12

Quote:

Originally Posted by Alia Amer (Post 593102)
Hi Everyone,
i am using ansys flotran for studying airflow inside buildings... i want to ask a question about generating the mesh ...after meshing the whole model....( i dont mesh the entire volume at once, i mesh it volume by volume ) .... how can i know the number of elements used in the mesh ????? i want to display it in the same way that you did:
Global Mesh Size (Max) Total Element Nodes
64 mm 414383 72421
32 mm 461344 80527
16 mm 841709 145118


Element Size
is the element size measured in milimeters???

and i want to ask another question... is the scaling factor in ansys the same like grid refinement ratio???? and how should i use it????

Scaling Factor vs Grid Refinement:

Roughly speaking yes. but technically by reducing the scale factor, mesh sizes on the surfaces become half. But this may or may not be translated into double the mesh from previous one. As it largely depends on volume mesh and it is difficult to predict it.

Units of Element size
Element size in measured in the units you are using for your model.

No nodes vs element size
64 mm 414383 72421
32 mm 461344 80527
16 mm 841709 145118


As you can see , decreasing the element size is increasing the no of nodes, but it is not linear. And you can also see that same size (64 * 0.5 of scale factor = 32 and 64*0.25 of scale factor = 16) can also be obtained with scale factor. So roughly speaking chaning element size through scale factor or manually does change the mesh density, but you cannot be sure what is gonna happen precisely. Hit and trail is the way to go :)

Alia Amer April 3, 2016 09:03

thank you so much Far for your informative reply :)
but what i want to know about the element number is how can i know it (i mean the way by which i can display it or the command that shows me the ni of elements used ) i use ansys flotran..can you help me to know from which command or main menu can i know the no of elements in my mesh?
thank you

Far April 3, 2016 09:57

Quote:

Originally Posted by Alia Amer (Post 593123)
thank you so much Far for your informative reply :)
but what i want to know about the element number is how can i know it (i mean the way by which i can display it or the command that shows me the ni of elements used ) i use ansys flotran..can you help me to know from which command or main menu can i know the no of elements in my mesh?
thank you

If you are talking about flotran, I do not know it.


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