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Author Topic: Change grid size gradually  (Read 7025 times)

Martin

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Change grid size gradually
« on: September 25, 2012, 04:11:29 am »

Hello to everyone,

does anyone know if it is possible with PyroSim to change the grid size gradually. For example I would like to have 0,1  m cells in the centre of the room and 1,0 m cells at the boundary. In some other fluid dynamics programms like CFX or FLUENT you can adjust that the mesh changes continously from a smaler grid size to a bigger one.So you tell the program that you want to have a bigger grid size on the boundary and a smaler in the centre and the rest, the transition, is calculated by the program itself.
If there is no way to do that in PyroSim can I import a mesh from ANSYS CFX maybe that has these properties?
In the User Manual 2012 I read something about refine mesh, but it is not going into detail very much. Is there a way the programm does refining by itself?

Greeting

Martin Rudert
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Andreas

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Re: Change grid size gradually
« Reply #1 on: September 25, 2012, 09:17:19 am »

Hey Martin,

are we talking about a single mesh or do you have several meshes?
FDS doesn't allow different grid sizes in a single mesh. What you can do is to split a mesh in several meshes. These meshes can have a different grid size each (please note the rules for mesh alignments). Anyhow as far as I know, you have to set the grid size for each mesh by hand.

Regards
Andreas
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Martin

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Re: Change grid size gradually
« Reply #2 on: September 26, 2012, 12:45:35 am »

Thanks for the reply Andreas! So basically it is not possible to use one grid that has different grid sizes like you can make it in ANSYS CFX or FLUENT? I guess that means also that I can not import meshes that I designed with CFX/FLUENT?
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Andreas

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Re: Change grid size gradually
« Reply #3 on: September 26, 2012, 03:50:07 am »

So basically it is not possible to use one grid that has different grid sizes
Yes thats right!

Code: [Select]
I guess that means also that I can not import meshes that I designed with CFX/FLUENT?Sorry, don't know about that. But I don't think so.

You could define several meshes and use a coarser grid size for the meshs which are far away from the fire or high fluid flows.
I think that a multi mesh geometry should work one a singel cpu, too. Please have a look at the FDS User Guide. There's a whole chapter with some examples.
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Charlie Thornton

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Re: Change grid size gradually
« Reply #4 on: September 28, 2012, 12:53:46 pm »

It turns out that there is a seldom-used feature in FDS that might do what you are after. You can apply a stretching transformation to meshes via the TRNX, TRNY, and TRNZ keywords. You can learn more in the FDS User Guide, Section 6.3.5 Mesh Stretching. It's likely possible to use those parameters to match your existing meshes.

PyroSim supports the piece-wise linear version of mesh stretching via the non-uniform mesh option in the Create MESH dialog. PyroSim does not support the polynomial variant - where you specify IDERIV.

Practically speaking, I don't know how well transformed meshes work.  The FDS Users Guide warns that applying transformations to the axes has a performance penalty - slower the more axes you transform. I'm also told by one of the FDS devs that the larger the aspect ratio, the greater your risk of numerical instability and that the better approach is usually a multi-mesh solution.
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