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-   -   Surface from VoF calculation (http://www.cfd-online.com/Forums/cfx/100016-surface-vof-calculation.html)

Claudia April 18, 2012 10:57

Surface from VoF calculation
 
Hi,

we made a calculation with Vof to get the free surface between air and water. Now we want to use this surface as a new boundary (wall). This surface should be a kind of "cover plate".

How to get this surface from CFD post to Pre for setting the boundary condition to wall?

Thanks,
Claudia

ghorrocks April 19, 2012 08:53

This is a tricky thing to do. You could export the surface to the solid modelling package (as a cloud of points I guess) and loft a surface through them and generate a new solid.

Alternately you can use the model you already have and freeze the VOF solution using expert parameters. Not as clean, but much easier to do (just set an expert parameter and off you go!)

Claudia April 19, 2012 09:03

Hi Glenn,

concering your first suggestion, we tried to do it by this way but the number of points is about 200000 and it is very tricky to get a closed loft surface. We tried it with ICEM I don't konw a better way for it.
The second suggestion is the reason for my question, we did that way, but if we use for the next calculation an additional variable (in this case a tracer) it diverged every time. Even the support has no idea what the problem is.

More ideas? :-)

ghorrocks April 19, 2012 09:19

Quote:

the number of points is about 200000
Then cull them down to a more sensible number. Regardless, high end solid modelling packages (eg unigraphics, Pro-E) should be able to put surfaces through point clouds like this.

Recently I have done something similar. This is what I did:
1) Wrote a program which read the massive points file and generated an STL file. STL is a simple format and is public domain so a google search will find it.
2) Read the STL surface into Solidworks.
3) Used the STL surface to trim a solid body
4) Now I have a Solidworks solid with one face defined by the STL surface. This can be exported to ANSYS for meshing and ultimately CFD simulation.

Quote:

it is very tricky to get a closed loft surface.
Yes, I said it was tricky.

Claudia April 19, 2012 09:30

The number of points results from the model size and an appropiate description of the flow on the surface, so I can't reduce it.

I will check if we can use STL for the surface and if there might be a way to use it further because we have no solidworks... :-(

Thanks,
Claudia

ghorrocks April 19, 2012 18:27

Quote:

The number of points results from the model size and an appropiate description of the flow on the surface, so I can't reduce it.
You can always cull points if they are simply the exported data points. You just need to work out a sensible way of culling or averaging them down. If your surface is so topographically complex that 2M points is required to describe it then you will need a lot of computing power to handle it.

I mention solidworks as that was simply the package I used. All high-end solid modelling packages will be able to load STL files, and hopefully they can use it for solid modelling. So any high end solid modelling package should be OK.

Claudia April 20, 2012 08:07

Sorry, my fault. You are rigth, culling is a good way to reduce the number of points.
After a view trials we exported a vrml file. This contains the surface we need. With this it is possible to use ICEM for the next steps.


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