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-   -   non positive volumes (http://www.cfd-online.com/Forums/fluent/27398-non-positive-volumes.html)

pankaj shukla December 3, 1999 09:03

non positive volumes
 
Dear Friends,

I have made an axisymmetric geometry with the help of gambit.But when i read this mesh file in Fluent5.o it gives warning the non positive volumes exist.What is meant by that .How it can be solved.Please response soon.

pankaj

Jonas Larsson December 3, 1999 10:06

Re: non positive volumes
 
It probably means that your grid is bad - go back to gambit and check the quality (click on the "mirror-glas" icon down to the right). Identify where you have bad cells (skewness>=1) and try adjust the gradings etc. to fix it.

pankaj shukla December 3, 1999 11:10

Re: non positive volumes
 
Dear friends and Jonas,

Thank you for your fast response.However i got the actual reason for the negative volumes.I was told that whenever imake axisymmetric grid,my centreline or axis of symmetry should be X axis and Whole of geometry should be above the y axis .when i did this i Didn't got any warning.

I also want to know that is it required that when an axisymmetric geometry is made ,you have to connect the axis of symmetry on either side to the domain where i want to do my analysis.Please response as soon as possible.

pankaj

Sandeep Abhyankar December 3, 1999 14:48

Re: non positive volumes
 
I would like to add a few comments on pankaj' last message. I had the same problem about non positive volumes which vanished after I made the problem axisymmetric about the X-axis. However even after doing that sometimes it stills gives an error while reading in the grid. This can be fixed by translating the entire grid by a very small extent in the " positive y-direction ". (Its wierd since when you check the fluent domain extents it doesnt give the y-extent as negative..!..but never the less it works.)


Sanat Shetty December 6, 1999 10:42

Re: non positive volumes
 
I agree with Jonas that negative volumes usually show up due to a bad grid, i.e. inappropriate meshing density..

You can however try to fix this problem inside of the solver by doing a repair-facehandedness(rfh)..This is under :

grid > modify-zone > rfh

Once you do this, check the quality of the grid to see if any negative volumes still show up..If they do, you will have to go back to the pre-processor and adjust the mesh density.


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