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arunsmec November 13, 2012 02:37

High Courant number
 
I am performing LES of a roundjet using OpenFOAM. SGS model is smagorinsky.
Upto around 12500 time steps my courant number was well below 1 (0.5-0.7). After that it has been increasing and, now at 13750 iterations, it is ~14. Is the solution right? I am using a time step 5e-5. Why is this sudden increase in courant number happening?

FMDenaro November 13, 2012 05:02

Quote:

Originally Posted by arunsmec (Post 391762)
I am performing LES of a roundjet using OpenFOAM. SGS model is smagorinsky.
Upto around 12500 time steps my courant number was well below 1 (0.5-0.7). After that it has been increasing and, now at 13750 iterations, it is ~14. Is the solution right? I am using a time step 5e-5. Why is this sudden increase in courant number happening?

if your time-step is fixed then you have an increasing of the velocities, that can indicate a numerical instability

arunsmec November 13, 2012 11:38

I am surprised why this instability has come up after these many timesteps

FMDenaro November 13, 2012 12:10

Quote:

Originally Posted by arunsmec (Post 391888)
I am surprised why this instability has come up after these many timesteps

why not? numerical instability can be a slow process ...

Tushar@cfd March 28, 2013 07:04

Quote:

Originally Posted by arunsmec (Post 391888)
I am surprised why this instability has come up after these many timesteps

I agree with "FMDenaro",

@Arun...

The error usually builds up during iterations, try to add under-relaxation factor to the variables. Also, re-check your mesh.


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