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-   -   CFL condition: can a mass balance problem arise if violated? (http://www.cfd-online.com/Forums/main/91740-cfl-condition-can-mass-balance-problem-arise-if-violated.html)

bzz77 August 21, 2011 10:52

CFL condition: can a mass balance problem arise if violated?
 
I'm a newbie to fluid dynamics. I'm trying to figure out the source of some strange results in code that I wrote for tracking concentrations during advection/diffusion.

If there is a problem with the time step I use (i.e., if the time step violates the CFL condition), could that lead to mass balance problems?

harbinyg August 22, 2011 11:37

your question is not easy to answer
 
according to my experimence,

if you violate the CFL condition , you could not get the convergent solution

however, it is not easy to define accurately the CFL condition for most numerical methods

and even if you know the CFL condition, people would use a safety factor;

best wishes

bzz77 August 23, 2011 03:12

Thanks a lot.

cfdnewbie August 24, 2011 15:12

Quote:

Originally Posted by bzz77 (Post 320904)
I'm a newbie to fluid dynamics. I'm trying to figure out the source of some strange results in code that I wrote for tracking concentrations during advection/diffusion.

If there is a problem with the time step I use (i.e., if the time step violates the CFL condition), could that lead to mass balance problems?


Hm, typically, if you violate the CFL condition by a considerable amount, your code blows up.... it might however be that you are dancing around the permittable CFL number, and your code is somewhat semi-stable....that might indeed lead to an mass balance problem....however, that's just a theory....

try the following: take half of your CFL an rerun your code. if the problem persists, you can rule out the time step. What kind of scheme are you using? FV?

Post back here if you need any more help!

Cheers!


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