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-   -   [ICEM] Export mesh statistics for use in Richardson Extrapolation for grid sensitivity study (http://www.cfd-online.com/Forums/ansys-meshing/116265-export-mesh-statistics-use-richardson-extrapolation-grid-sensitivity-study.html)

JuPa April 16, 2013 06:33

Export mesh statistics for use in Richardson Extrapolation for grid sensitivity study
 
Hi,

Following the post from another user in this thread (http://www.cfd-online.com/Forums/cfx...-analysis.html) I want to do a grid sensitivity study using the Richardson extrapolation method.

The first step is to define a representative cell, mesh or grid size, h. For three-dimensional simulations

h=\left[ \frac{1}{N}\sum^{N}_{i=1}{ \Delta V_{i}} \right]^{ \frac{1}{3}}

where \Delta V_{i} is the volume of the i^{th} cell, N is the total number of cells used for the computations.

My problem is I don't know how to extract the values of \Delta V_{i} from ICEM as ideally I'd like to do the Richardson Extrapolation method in Excel, where I can compare different meshes. In ICEM how do I export mesh statistics such as volume of each cell? Can it be exported as a handy .csv file?

I refer you to the paper I am using: http://journaltool.asme.org/Template...umAccuracy.pdf

Thank you

RodriguezFatz April 16, 2013 06:48

Since you just need the ratio of h(fine)/h(coarse) calculation of an actual value of "h" is not necessary.
How do you want to do the refinement in ICEM?

JuPa April 16, 2013 07:01

Quote:

Originally Posted by RodriguezFatz (Post 420883)
Since you just need the ratio of h(fine)/h(coarse) calculation of an actual value of "h" is not necessary.
How do you want to do the refinement in ICEM?

My variable of interest is wall heat transfer coefficient at the various boundaries. I am using the SST turbulence model. For accurate heat transfer predictions I need a Y+ < 1. So far the maximum Y+ I have on any of my boundaries is 1.6.

I will do the refinement in ICEM by applying more nodes in the thermal boundary layer.

RodriguezFatz April 16, 2013 07:03

Do you use blocking / hexa meshing or unstructured?

JuPa April 16, 2013 07:11

Quote:

Originally Posted by RodriguezFatz (Post 420886)
Do you use blocking / hexa meshing or unstructured?

I use blocking (hexa mesh). Before I export the mesh in .cfx5 format I right click on "Pre-Mesh" and click on "Convert to Unstructured Mesh".

RodriguezFatz April 16, 2013 07:13

Great. Then you have all the numbers...
Do you refine the whole grid by using the mesh->refine utility, or do you change the number of nodes of some relevant edges?

Is this 2d? Can you post some picture?

JuPa April 16, 2013 07:24

Quote:

Originally Posted by RodriguezFatz (Post 420892)
Great. Then you have all the numbers...
Do you refine the whole grid by using the mesh->refine utility, or do you change the number of nodes of some relevant edges?

Is this 2d? Can you post some picture?

It is 2D axisymmetric. I change the number of nodes of relevant edges. The geometry is shown below:

http://i.imgur.com/uRL1727.png

Do you want a picture of the mesh?

RodriguezFatz April 16, 2013 07:31

How large is your coarsest mesh?
In 2d (and such a simple geometry) it is often affordable to just refine the whole mesh, which means you will end up with a 4 times larger mesh (2 x in "x" and 2 x in "y" direction). Would that be ok for you?

JuPa April 16, 2013 07:48

Quote:

Originally Posted by RodriguezFatz (Post 420896)
How large is your coarsest mesh?
In 2d (and such a simple geometry) it is often affordable to just refine the whole mesh, which means you will end up with a 4 times larger mesh (2 x in "x" and 2 x in "y" direction). Would that be ok for you?

The coarsest mesh has 6500 elements and 3072 nodes. I do have the computational resources to double, triple and quadruple the number of elements and nodes.

However what I am interested in is the Richardson extrapolation method I mentioned in my original post.

RodriguezFatz April 16, 2013 08:05

It's always the same method:
In your case, save your coarse mesh. Then refine the (unstructured) mesh, by clicking on "Edit Mesh -> Adjust Mesh Density". Here you can use "Refine All Mesh", method "pure refinement", steps "1". ->apply.
You will get a mesh, that is 2 times finer in each dimension.
Save that mesh. Do the same refinement again and you will end up with a even finer mesh.

Now run your simulation on all three meshes. Use the coarse results as interpolation for initializing the other runs.
For each simulation you need one single "important" value, such as you heat transfer coefficient or whatever. Using the nomenclature of your pdf, these values are the phi 3, 2 an 1. Since you doubled the number of points in each dimension the value of r21 and r32 is "2".
Here we go!

JuPa April 16, 2013 08:45

Thank you! That actually does help.

RodriguezFatz April 16, 2013 09:34

If you did that, you could post the values here for others to see how it works...

lentschi January 18, 2014 19:57

Hello,

I have one additional question regarding the Richardson Interpolation: What is about r21 and r32 if only the total number of cells is doubled (by splitting one direction)?

Example:

Coarse: 3 Million cells
Reference: 6 Million cells
Fine: 12 Million cells

I then r21=r32=1.25??

Thanks in advance.

//Markus


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