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pls help fluctuating residuals due to courant no

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Old   March 27, 2006, 05:09
Default pls help fluctuating residuals due to courant no
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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 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 it right?in that still solution is can i put confidence in that result?

Thanks all.
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Old   March 27, 2006, 06:21
Default Re: pls help fluctuating residuals due to courant
James Date
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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!
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Old   March 27, 2006, 09:32
Default Re: pls help fluctuating residuals due to courant
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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.
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Old   March 28, 2006, 01:10
Default Re: pls help fluctuating residuals due to courant
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Thanks james and TB for ur suggestions.
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