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-   -   Size of 3D in 2D problem? (https://www.cfd-online.com/Forums/fluent/27543-size-3d-2d-problem.html)

 Arthur Valais March 6, 2000 20:49

Size of 3D in 2D problem?

Hi!

Does anyone know what the third dimension size is in a two-dimensional problem? I need to know as I'm doing some hand calculations to check fluent. Specifically, density = mass / volume, but for the volume I only have x + y.

Bye,

Arthur

 Detlef Schulze March 7, 2000 10:04

Re: Size of 3D in 2D problem?

Hi Arthur, in a 2d flow the z-coordinate in any quantity such as coefficients etc. is usually "per unit depth". Therefore, you should take 1 m (or whichever physical unit you use).

Greetings Detlef

 Arthur Valais March 7, 2000 20:15

Re: Size of 3D in 2D problem?

Thanks.

So looking at the case file it says... grid was created in m. This indicates that for one cell 1cmx1cm, its depth is 100x the other dimension. This is a serious problem. What's worse is that I found no explicit documentation about this.

So, if I want a very thin layer (depth) eg. 1cm do I need to recreate the grid or can I scale it? Or do I need to move to full 3d?

Many thanks,

Arthur Valais

 Greg Perkins March 7, 2000 22:08

Re: Size of 3D in 2D problem?

I'm not sure if I have understood the problem, but I believe, as stated before, Fluent just assumes a unit depth when computing areas, volumes etc. in 2D. This is normal.

Ie. think of these quantities as area or volume per unit depth.

If you are using 2D presumably you're problem can be approximated as 2D - ie. changes/gradients in the third dimension are small compared to those in the other two dimensions which you simulate on a plane. In this case, I don't see any reason to simulate in 3D to get around the conceptual ideas above. On the other hand if its a 3D problem it needs to be solved in 3D....

Let me know what you think!

 Arthur Valais March 7, 2000 22:41

Re: Size of 3D in 2D problem?

Yes, changes in 3d are not significant.

But I wanted to know how to calculate density of a gas I had in a cell, and then it struck me that I didn't know all the dimensions.

So I needed to find out exactly what fluent did to the 3rd dimension (I couldn't make the assumption that it placed 1m into depth because the grid was made in metres).

So if it is 1m depth then the volume of my 1cmx1cm cell is 1e-4 m3.

Thanks,

Arthur Valais

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