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-   -   scaling in Gambit. Please help!! (http://www.cfd-online.com/Forums/fluent/34342-scaling-gambit-please-help.html)

Michelle August 6, 2004 00:00

scaling in Gambit. Please help!!
 
Hello, i am constructing my geometry in gambit, but the dimensions of the gasket i am using and my set up are really small, i.e in the order of 0.001mm. if i were to draw such a structure in gambit, it would be too tiny. does it matter , what dimensions i use, or if i scale everything by a factor of 100. what about my inlet flow rate, would i have to scale it too? How would all these affect my results?I would really appreciate any help.

Michelle August 6, 2004 00:01

Re: scaling in Gambit. Please help!!
 

Dan Brzozowski August 6, 2004 00:39

Re: scaling in Gambit. Please help!!
 
You can scale geometry in Fluent by going to the Grid/Scale... menu. So for example you could just take one gambit unit to be 1 micron. Then after you read the mesh into fluent, scale it by 10^6 in all dimensions.

-Dan

Oliver August 6, 2004 02:32

Re: scaling in Gambit. Please help!!
 
That is right Dan but when you scale the grid, only the grid and not the physical value is scaled so if you have 10^-6m3/s as inlet flow rate you have to enter 10^-6. Such value involve convergence difficulties don't you think so?

Dan Brzozowski August 6, 2004 03:11

Re: scaling in Gambit. Please help!!
 
I don't think there should be any convergence difficulties since Fluent uses scaled residuals to judge convergence (see User Guide section 24.16.1 "Monitoring Residuals").

If I understand this right, for an inlet flowrate of 10^-6m/s, the default convergence cirterion of 10^-3 would essentially require that the error not exceed 10^-9m/s. Here "error" is defined as some average of residuals from the continuity equation in each cell.

-Dan

Alex Chirokov August 6, 2004 12:34

Re: scaling in Gambit. Please help!!
 
It is usually good practice to work with non-dimensional variables. But if so some reasons you want to work with dimensions choose units that are appropriate for your problem. Let say in 1 unit is 1um=1e-3mm. Ideally your geometry in gambit would extend from [-1 to 1] in order to minimize round off errors. Put origin in the middle of your model and choose units the give you maximum deviation from the origin less that 10. Note that you can scale your model as you please after you import it in fluent.

Good luck! Alex-

Michelle August 9, 2004 10:31

Thanks to Dan, oliver and Alex.
 
Thanks to Dan, Alex and Oliver. your help is really appreciated.


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