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Hi,

In case of unequal triangular sides, which length is used to calculate Courant number in Telemac2d(I mean if a triangular has 3 sides of lengths 4 , 5, 8 m which one is used in calculating courant number).

Also, I noticed that telemac2d gives usually lower Courant number, can some one tell me what is the formula used for calculating it.

Hello Mohamed

Happy to read from you.
The CFL number is used and computed for explicit schemes of both Finite element and Finite volumes. This is done both for hydrodynamics and for tracers.
For Finite volumes, this computation is done in routine caldt.f, and the used formula depends on the used scheme. The caracteristic length used to compute the Courant number is computed in routine hloc.f (in bief) and it corresponds to the radius of the greatest circle included in the basis polygone (whose vertices are the centers of mass of surrounding triangles).
For finite elements, the cfl is computed in routine cflpsi.f for N and psi schemes. Courant number is computed at each node of the mesh with routines cflp11.f or cflp12.f.
For tracer, Courant number is computed in routine cflfv.f
For all thes routines, the criteria of stability uses the area of the basis around the node to compute DT. This could be interpreted in the same way as the criterion used in the finite volume part of Telemac2D. In fact, this is equivalent to the use of the radius of the greatest circle included in the basis polygone.

I hope that this helps

With my best regards

Riadh ATA

Hallo Riadh,

It is nice to hear from you too. Thank you very much for your reply, I am very gratefull.

Kind regards,
Mohamed

Hi,

I have a variable mesh size where I have 0.25, 1 and 4 m mesh sizes. Foe calculating courant number, which mesh size should I use? As the velocity is 0.6 m/s and model works better with time step 0.01, then if I use 0.25 mesh size, courant number becomes 0.02 which is a good value. But if I use 4 mesh size, the the courant number becomes 0.0015, which is very low. Is there a minimum threshold for courant number?

Hi,

First of all don't double post!

There is no lower limit on courant number, you could increase the time step when courant number is low. This is very basic hydraulic modelling knowledge.

Cheers,
Patrick

Hi PMV,

yes, this is a basic question and I am a learner.

My question was when I have variable mesh sizes, 0.25, 1, and 4 m, which mesh should I use for the courant number calculation? Thanks!

Do you mean your mesh has element size going from 0.25m to 4m or you have 3 different mesh for the same area?

Anyway, courant number is related mesh when using explicit scheme, it should respect <1 (often <0.9 is used). Calculation can help to determine the timestep to use. If you already simulating, you can have it outputted by TELEMAC as a control.

If variable element size within your mesh you use the smallest element size as this is the most restictive.

If you have different meshes (models) you should calculate for each and you can use larger tiemsteps for larger elements.

Hope that helps,
Patrick