CFL number
 

Hi all,

I am using "density based implicit " solver with steady state.For Turbulene: RNG k-epsilon model with volumetric chemical reactions.

For this problem what should be the maximum CFL number?

with regards sisir

Re: CFL number
 

for most of the problems, the solution will be stable if its close to 1.. exceptions do exist...if your soln is converging slowly, try increasing d Courant no.

Re: CFL number
 

Thanks Deepak for ur reply. But in my problem, the residuals start oscillating after few iterations. So what shall i do for this? In the mannual it is mentioned i can go for higher CFL number .

with regards sisir

Re: CFL number
 

there are so many factors cencerning oscillation, it dostnt necessary have something to do with CFL;in my understanding usually CFL smaller than 1;for LES it might between 3-1, but at end it will be smaller than 1; for normal turbulence Model, CFL has to be smaller than 1 to converge

Re: CFL number
 

Thanks Jesse for your information. CFL number will be 1 even for "density based implicit " solver with steady state! But FLUENT manual is suggesting for increase of CFL number beyond 1 even more than 5 also. What shall i do ?

with regards sisir

Re: CFL number
 

See, manual suggests to increase the CFL, if your residual plot is going smoothly but slowly. Increasing the CFL no will accelerate the convergence (Correct me if iam wrong).

But to start with, go ahead with 1, if it diverges, fluent automatically decreases to .1,.001 .....if it converges try increasing.

Oscillating residual plots occur either if your bcs are not correct or your mesh is not as per the requirement of the turbulence model.

make sure that ur bcs are as per the models and if thats fine, try a finer mesh.

i hope this is of some use.

Re: CFL number
 

Thanks for the suggestion. I will try with finer mesh.

sisir

Re: CFL number
 

Hi Sisir

For an implicit scheme your courant number do not have to be equal or less than 1. Infact in order to obtain more stable solution you should increase your courant number as recommended in documentation. Osciallating residuals is more complicated issue. Residuals can osciallate due to bad mesh or there can be other reasons. As a rule of thumb you need to understand that oscillating residuals indicate unsteady flow. Try running your case as unsteady and see what happens.