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How to choose the right courant number?

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Old   January 19, 2013, 16:36
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zaynah K.
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Dear Mads,

Thanks to you analyssi, I have solved all other queries concerning the courant numbers..

I have one thing which is quite bothering me and i hope you can help me out of this tight spot.

I have a case where i am simulating wind flow over a sphere ( 2D), its incompressible, turbulent and stready.

I used pisoFoam and i got the same results which i obtained with Fluent for same mesh.
Now i am using simpleFOam, and solution converged. But in paraview i get completely nonsense results..

Your conclusion would be more than welcome dear Mads..
Thanking you in advance if you can help me out from this Tight spot..

Best Regards
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Old   February 11, 2013, 16:41
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how is it possible to add "adjustableRunTime" into sonicFoam?
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Old   February 11, 2013, 17:25
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Originally Posted by immortality View Post
how is it possible to add "adjustableRunTime" into sonicFoam?

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Old   May 30, 2013, 16:23
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I'm trying to resolve supersonic compressible flow with OpenFOAM and I have some questions about the Courant Number calculation.

I fix my Courant number in the ControlDict file, and the time step try to adapt himself.

I look up on the C/H file of the courant number calcultation and I found that the formulation used looks like:

CFL = 0.5 * (sumFace of phi/rho) / VolumeCell ~ 0.5 * [ u * dt / dx + v * dt / dy + w * dt / dz ]

Thus, I don't understand why they put 0.5 coefficient, but I maybe misreaded the code...

I have also another question. For compressible flow where shock occurs, the CFL needs to be (for resolving acoustic and shock)
CFL_comp = dt * (U + a) / dt (a: speed of sound)

Does anyone know if it's the correct formulation for shocked flow?? or the normal form is better?

If I summarize all the CFL calculation (OpenFOAM + compressible/shock), the equation becomes :

CFL_comp = CFL_wanted (ex=0.5) / ( 2 * ( 1 + max(Mach number) ) )

(2 is for compensating the 0.5 coefficient of OpenFOAM and ( 1 + max(Mach number) ) is for (U + a))


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Old   October 3, 2013, 08:28
Default Courant number for non-newtonian fluids
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Albrecht vBoetticher
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Dear modellers,

I have nice results for an adapted InterMixingFoam solver that works with a pressure dependent viscosity in one phase. I use maxCo 0.25 because that is what I got recommended for Large Eddy Simulations. Here I have no turbulence, but when I change maxCo, say to 0.4, the viscosity of the fluid increases!
Has anyone experience or ideas of explanation for this behavior with pressure dependent viscosities?
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Old   October 14, 2014, 20:07
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Hi, Mads!

I'm quite new here and I'm trying to understand Delf 3D so I can work in a model of a river to see the water level changing with this new construction.
I am sorry for my ignorance, but I can't understand how the velocity can affect Courant number. So far I've just added the grid and bathymetry. Can you please explain me?

Thanks a lot!

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Old   July 21, 2016, 04:19
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HiMads and everyone,
I am trying to save computational costs by increasing the time step unti I get some CFL# of 0.5. When my timestep is 10^-6 my mean Courant number (CFL#) is 0.0032.
How can I define the most appropriate Courant number? We know that CFL# should be less than 1, but how shall I know the lower limit of Courant number? How shall I know if 0.4 is better of 0.1 of 0.0032 ?
Is there another criteria to restrict our choices on time-step to reduce the calculation cost?

Thanks in advance for help and ur time,
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