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-   -   Question about ANSYS Mesher and Global Min Size Setting (http://www.cfd-online.com/Forums/ansys-meshing/86928-question-about-ansys-mesher-global-min-size-setting.html)

jonny_b April 6, 2011 09:14

Question about ANSYS Mesher and Global Min Size Setting
 
Does anyone have any good advice in terms of how to adjust this value for particular situations. To put it another way, how often should this value be changed from the default. I read in the ANSYS documentation that "One of the most important parameters related to the advanced size function is minimum size". I feel that the default value is sometimes too big for certain CFD simulations that require a fine mesh.

I am trying to gain a better understanding of how the mesher uses min size when generating a mesh. In particular:

1) how often is this value considered during the meshing process and for what stages in the meshing (surface mesh, inflation mesh, volume mesh)?

2) when is it a good idea to change this value from the default, or when is it a bad idea?

Sorry about the lack of concreteness about my question but any insight will be greatly appreciated.

Jonny B

PSYMN April 8, 2011 10:55

Min Size Limit
 
This is a Min Size Limit... It is used during Octree tetra refinement...

For Curvature and Proximity Refinement, There are goals, such as Cells in Gap and Curvature refinement (cells in 360 degrees). The mesher will try to refine the mesh to meet these goals. But you don't want this to run wild and give you billions of cells. Imagine if your model had a small unintentional gap or ripple, you don't want it refining to put 3 cells across that gap or 12 elements around that tiny ripple that you didn't even notice... You want to limit it some how.

You do that globally with the Min Size Limit.

Set it to the smallest size you want to capture...

If you want it smaller in a certain location, you can set a smaller value on parts or surfaces or curves and still use the larger global value elsewhere.

Remember in ICEM CFD, the smallest size wins over a larger size.

PSYMN April 8, 2011 10:58

Limit, not a goal...
 
One other thing... There are posts about "why didn't it refine down to my Min Size"... Keep in mind that this is a Limit, not a Goal... You don't want it to reach your min size, you just want to capture your model with sufficient curvature and proximity based refinement (the goals) to resolve your problem with the minimum number of cells.

The min size is just a safety catch to prevent it from running away unexpectedly.

jonny_b April 11, 2011 09:12

Thanks for the clarification PSYMN. It always helps to know more about the inner workings of the tool. Especially when it comes to CFD. This is what I half expected and I realize that the global settings are more goals than hard fast rules.

PSYMN April 11, 2011 10:05

Not just goals... Some are limits. There is also the behind the scenes goal of capturing your features with as few cells as possible.


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