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..:p
Thanking you in advance if you can help me out from this Tight spot..

Best Regards
Zaynah.

hi
how is it possible to add "adjustableRunTime" into sonicFoam?

See http://www.cfd-online.com/Forums/ope...tml#post407241

Hi,

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))

right??

thx

Courant number for non-newtonian fluids
 

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?

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!

Joana

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,

Convergence can be obtained for sure, but your information travels accross hundreds of cells so you will lose accuracy. but what was the purpose of your simulations? we can not judge the used time steps of courant number without knowing the purpose.

high courant number but accurate results
 

hi, i have a similar problem. I did several simulations of natural convection in closed spaces in a transitory state in laminar flow. At first I did not take into account the value of the Courant number, however later I determined, now, the courant number in all simulations is greater than 1 but less than 2, does this indicate that my results are not correct? Because when I compare it with experimental studies, the relative error percentage is less than 3%.

What can this mean?

Hi,

https://www.cfd-online.com/Forums/op...treatment.html

The last reply may help. For transient problems, the courant number a little higher than 1 may not be wrong if your result is reasonable. If you use the implicit scheme, there is no fixed limit for the courant number。 just prevent it become too large to diffuse I think.

Regards,