# pls help fluctuating residuals due to courant no

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 March 27, 2006, 05:09 pls help fluctuating residuals due to courant no #1 sagar Guest   Posts: n/a 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.

 March 27, 2006, 06:21 Re: pls help fluctuating residuals due to courant #2 James Date Guest   Posts: n/a 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!

 March 27, 2006, 09:32 Re: pls help fluctuating residuals due to courant #3 TB Guest   Posts: n/a 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.

 March 28, 2006, 01:10 Re: pls help fluctuating residuals due to courant #4 sagar. Guest   Posts: n/a Thanks james and TB for ur suggestions.