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chris wetton February 16, 2008 22:55

post processing dam break problem
 
Hi, im currently working on a problem of solving the dam break problem using the VOF method on CFX. I have managed to generate the models however when post processing my models apear as blocks instead of smooth surfaces. At first I thought this was due to the way i defined the simulations however if i put a plane through the middle at any point in the model the flows are smooth. Any ideas any one. thanks again Chris

Glenn Horrocks February 17, 2008 06:12

Re: post processing dam break problem
 
Hi,

How are you displaying the surface? Usually you use an isosurface set at a volume fraction of 0.5

Glenn Horrocks

chris wetton February 17, 2008 06:23

Re: post processing dam break problem
 
Hi Glen, yeah I am using an isovolume with a volume fraction of 0.5 but still cant think why the simulation is blocky. Do you think it could be a problem with my settings when creating the model? regards Chris

andy20 February 17, 2008 11:46

Re: post processing dam break problem
 
Use an *isosurface* not an isovolume. Here's the difference:

- The edge of an isovolume is defined by the mesh cell boundaries and is therefore rough even for smooth data. Roughly speaking, a single value is associated with each cell, and so each cell is either completely in or completely out of the isovolume. This makes it look rough.

- The surface of an isosurface is defined by interpolating the results from the mesh and solving to find a smooth surface at the required value. This results in a smooth boundary.

Try, for example, doing both an isovolume and an isosurface for something like the X or Y coordinates in CFXPost, which of course should be smooth, and you'll se the effect[*]

Hope that helps.

andy.
[*] Unless of course you have a structured mesh where all the cells are aligned to share X or Y coordinate boundaries! In that case try another variable. ;-)

chris wetton February 17, 2008 13:35

Re: post processing dam break problem
 
Andy, thanks for that. Just a quick question though, Will the isosurface be a 2D surface and not 3D? cheers Chris

Glenn Horrocks February 17, 2008 17:17

Re: post processing dam break problem
 
Hi,

An isosurface, in general is a surface with curvature in 3 dimensions. If you have done a 2D simulation it will have curvature in 2 dimensions and be flat in the 3rd dimension.

If you have done a 2D simulation and you want contour lines (the 2D version of isosurfaces), first generate a plane or use a symmetry plane to generate a contour object.

Glenn Horrocks

andy20 February 18, 2008 04:58

Re: post processing dam break problem
 
Chris - Yes it will be the 2D surface that encloses the volume described by the condition that defines it...so it will be 'hollow' - but for visualising a free surface after a dam break I think it's what you want.

I think the best thing is for you to just try it and see, rather than me explaining in great detail - it's only a 2 minute job to try it out!!

I don't know of any way of getting a smooth isovolume in CFX-Post. I honestly think there isn't one, but if I'm wrong I hope someone more knowledgable will post the details for both of us...

Regards, Andy

Chris Wetton February 22, 2008 07:33

Re: post processing dam break problem
 
Andy/Glen thank you so much for your help. The isosurface looks great and when combined with an isovolume I have managed to get a complete model of the Dam Break Problem. Will owe both of you a pint. Chris


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