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-   -   how many cells with RANS (http://www.cfd-online.com/Forums/openfoam/106989-how-many-cells-rans.html)

 Tobi September 14, 2012 04:55

how many cells with RANS

Hello all,

I have a new :rolleyes: question. I had a look at a few presentations on the 7th OFW and other stuff. I realized that the meshes they use with RANS are not fine at all. Alberto Cuoci (for an example) uses very small cell meshes with RANS modelling of his catalystic and flamelet solver. In work I see always big and fine meshes used with RANS and so I am a bit confused what is the best way.

I think there is no exact answer but most of the meshes that I see I think you can calculate all fluctuations couse of the fine mesh. In a case a student meshed a geometry of about (highxlenthxwidth) 20cm x 50 cm x 20 cm with round about 4.5 million cells.

Is there any litrature or something like that where I can find some informations. Couse I think that its not worth to build a mesh with 4.5 million cells if you get about the same resolution with 800.000 - 1.000.000 cells.

For any suggestions I ll be very thankful

Tobi

 blacksquirrel September 28, 2012 09:08

The correct way is to estimate you discretization error with a mesh refinement study.
(e.g.: http://www.grc.nasa.gov/WWW/wind/val...spatconv.html; Roache grid refinement studies)

in short:
You do the same simulation on different meshes (coarse and fine) and compare the solution. Your grid is fine enough, when the difference to the result of a simultaion of a finer grid is less than your margin of error.

 colinB September 28, 2012 09:40

Dear Tobi,

the number of cells in your mesh depend highly on what you are actually
calculating.
Unfortunately I cannot comment on the case from Alberto you mentioned
since I don't do such type of calculations, but for ship hull evaluation
I use the following parameters:

- check the resolution of the boundary layer (y+value e.g. with
checkMesh) this value should be below 300

- resolution of the free surface: over the expected wave hight there
should be a layer of at least 12 cells

- in the area where you are expecting the waves the cells cross section
should be less than a square meter

- finally use a smooth transition from a dense grid to a coarse grid.

in the other areas you might use a coarser grid ( for me sometimes cells
with a volume of several cubic meters
(domain size 500 x 200 x 55 [m x m x m])) and the total number of cells
is sometimes up to 6M cells when using a very dense mesh, but usually
much less (approx. a third of it).

So this is how I end up with my number of cells. But I guess for other
problems the required resolution might be different.

I hope I could contribute

regards Colin

PS: Grüße ins Ländle meine alte Heimat!

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