SimScale CAE Forum

CFL number

Hi there,

Could I ask a question?
I am using Simscale to research water flow in a river by diverting a part of the water.
The model is a section of the river, constructed like a closed box which contains elements to redirect the water.
Just a part of the box is filled with water, the rest would be air (like in reality the air above the river…)
So the water should flow in a certain speed in one direction and the result I´m searching for is… how is the water flow impacted by the elements in the river…
So I prepared everythink… I hope right but then this pops up when I start the Run: “The Courant number (CFL) exceeded the limit of 1. You may experience either instability or bad temporal accuracy. It is recommended to keep the CFL number below 0.7. In order to achieve this you need to decrease the time step.”

I just don´t understand whats wrong…

Thanks for help!!! (-:
(Project name is F1 Multi)


The Courant number is the ratio of the distance traveled by the flow during 1 iteration, to the mesh element length. A Courant number higher than 1 means the flow is travelling a distance greater than a mesh element during a single iteration, which is not good because CFD is an iterative process.

To reduce the Courant number you need to increase mesh density or reduce time step

1 Like

Thank you for your quick answer!!

Could you just tell me how to increase the mesh density or reduce the time step… I´m a little lost…

Thank you!!!

Your mesh is already fine and your time step (under simulation control) is small. You could try reducing your delta t from 0.005 to 0.001. You’d also be better off reducing your end time as I doubt you need 10 seconds of simulations time.

Are you trying to model open channel flow? You should set the top surface to slip wall not to outlet. That might be causing you a problem. Also this looks like a turbulent flow so in your solver (near the top of the menu) select turbulent , not laminar (though first get your simulation working)

I think you could learn a lot from the following tutorial;

A open channel flow…Yes thats right!
I saw the tutorial… and tried to follow it while i was preparing the simulation… I never used Simscale or another another Program compareable…
I changed what you told me… so I will try it again…

Thanks a lot!!
I´ll let you know if it works (-:

Hi Roy,

same result as before… “The Courant number (CFL) exceeded the limit of 1“
Any other ideas? Thanks for the help again!

hi @gmijic,

your last Run (8,9,11) doesn’t content this error.
and simulation results also quite stable.

what kind of flow you are expecting here? menas you have any validation results for comparison?


1 Like

in Addition,
there is no velocity was define.


yes it did work, because i changed the algorithm from Hex-dominant to Hex-dominant parametric.
Right now I´m not sure if my Boundary Condition for the outlet is right. The model should be a simulation of a river that means the water is flowing canstantly… how can I manage that?

thanks for help

And what do you mean with there is no velocity defined?

You have assigned the velocity inlet B.C to the face but did not assign velocity magnitude(Uy in your case).

1 Like

hi @gmijic,

so, in this case, apply river velocity for your inlet face (very small value for your case).
JUST check my past project to understand simulation set up for anything you need. (very similar project )


1 Like

Hi Rohit,

do you know why my simulations are getting blurred, no matter what I change and do the are blurred?
Any Idea?

Hey @gmijic!

Could you post a screenshot of that issue? We will investigate this for you then!



hi @jousefm
I think she was talking about this.
I will also try to investigate your problem.


1 Like