# CFL condition: can a mass balance problem arise if violated?

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 August 21, 2011, 10:52 CFL condition: can a mass balance problem arise if violated? #1 New Member   Join Date: Jan 2010 Posts: 28 Rep Power: 16 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?

 August 22, 2011, 11:37 your question is not easy to answer #2 Member   Wu Jian Join Date: Jun 2009 Location: Poitiers Posts: 33 Rep Power: 17 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

 August 23, 2011, 03:12 #3 New Member   Join Date: Jan 2010 Posts: 28 Rep Power: 16 Thanks a lot.

August 24, 2011, 15:12
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Quote:
 Originally Posted by bzz77 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!

 Tags advection-diffusion, cfl condition, cfl number