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-   -   pls help fluctuating residuals due to courant no (http://www.cfd-online.com/Forums/cfx/22355-pls-help-fluctuating-residuals-due-courant-no.html)

sagar March 27, 2006 05:09

pls help fluctuating residuals due to courant no
 
Hi to all, i have some query about courant number.because i am getting fluctuating residuals in my problem. (1) what should be the limit of courant number (Max courant no or rms courant no) for feasible solution. I have done several tries for one prblem by changing mesh sizes.and mesh type.(tet and hex mesh).In tet mesh i have got higher courant number than hex mesh.so i have query that what shoud be the limit of courant no. to get feasible solution. (2) Suppose residuals are getting fluctuating then is it problem with meshing or wrong turbulence model?or is it due to courant number very high.? (3) Does courant no. depend on mesh? (4) I have also got courant no. upto 245 (max courant no.)in one problem.is it right?in that still solution is converging.so can i put confidence in that result?

Thanks all.

James Date March 27, 2006 06:21

Re: pls help fluctuating residuals due to courant
 
You must reduce the time step so the courant number is less than 1 (MAX Courant) if you really want to capture the true transient behaviour in the flow, anything else will give you the wrong answer! Unless you are time stepping to a steady state solution that is!

TB March 27, 2006 09:32

Re: pls help fluctuating residuals due to courant
 
There is a discussion about significance of CFL in community forum. CFL < 1 is a condition for explicit transient scheme. CFX used implicit approach for calculation of transient term. It's possible to get an accurate solution with CFL higher than 1.

CFL of 254 seems to be very high. Try to reduce your timestep and get CFL within 10-20.

sagar. March 28, 2006 01:10

Re: pls help fluctuating residuals due to courant
 
Thanks james and TB for ur suggestions.


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