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Hi everyone,
I have posted the running of Model which is simple embankment overtopping.I have two problems, first, when running said " ILL-POSED PROBLEM, ENTERING FREE VELOCITY
FOR EXAMPLE AT BOUNDARY POINT NUMBER 1129" and second is "DIFFUSION-PROPAGATION STEP
RESCJG (BIEF) : EXCEEDING MAXIMUM ITERATIONS: 100 RELATIVE PRECISION: 0.1454032E-02"

ANY SOLUTION please?

Thanks
AMANJ

Hi

Hard to say without any more information on your model...
But let's try!

Ill-posed problem indicate that you probably have a boundary with a water level imposed and where in the simulation, some water come in the domain (value of water level at boundary is higher than the water level in the domain). In a sub critical flow, this means the problem has an infinity of solution as you could give different values to the velocity at this boundary...
Nevertheless, this case sometimes occur (for tide simulation for example) and this could be solved by the use of Thomson boundary conditions method.

Exceeding maximum iteration shows the computation had some difficulties to converge to the solution. As it seems you have a small time step, this probably means you have a shock sowewhere in your domain (probably on boundaries).

Both problems are probably linked! have a look on the boundary with a water level impose and check the initial value and the impose value.

Hope this helps

Thank you dear C.Coulet,

I will post the steering file. In fact, I have an embankment with an overtopping flow. I tried to model it just like installed in a flume with width 0.30 m and height 0.3m and length 12m. I set the initail elevation as 0.3m while the bed elevation is zero. with a constant elevation. and I have u/s open boundary with prescribed discharge of 150 l/s and d/s open boundary with prescribed elevation 0.1m.

Hello,

It seems that you have copied your parameter file on a hydraulic jump case, which was done to test rather unusual features, so it would be better to start with more common parameters :

1) you use primitive equations (historically the first option in Telemac-2D, so default) but the pseudo-wave equation is much faster, try : TREATMENT OF LINEAR SYSTEM : 2. The primitive equation cannot work with the conjugate residual iterative solver (number 2) and this is why you have this maximum of iterations (in your case GMRES, number 7 was necessary because the linear system is not symmetric, it is symmetric with the pseudo-wave equation).

2) you use quadratic elements. Try first with linear : DISCRETIZATIONS IN SPACE : 11;11. Only if you have wiggles of the free surface try 12;11. Quadratic is very expensive and generally you can avoid it by playing with the keyword FREE SURFACE GRADIENT COMPATIBILITY, reducung it from 1. down to 0.

3) If you do a real case, have some VELOCITY DIFFUSIVITY (try 0.1 or 1. to start with)

4) The MATRIX STORAGE : 1 is also not the best, try 3 instead (same results but faster).

5) Your time step is very small, with the modifications above you should be able to increase it a lot.

Have a nice day,

Jean-Michel Hervouet

Hello dear Jean-Michel Hervouet,

I would like to thank you for your advice and points you explained. I am really new in Telemac and I would like to do some numerical modelling on non-cohesive embankment breaching. First, I stated with just 2D hydrodynamic overtopping flow and see how it works, then I would like to couple it with Sisyphe and see how erosion take place for certain flow rate. I followed the steps you mentioned. the first one didn't work when changing the TREATMENT OF LINEAR SYSTEM from 1 into 2 it gives ERROR. But the rest are working and gives faster result.I would like to ask what is the reason of ILL-POSED PROBLEM in some boundary points?

Kind Regards,
AMANJ

Here I attached geometry and boundary files.

Hello,

We give the message ILL_POSED PROBLEM when we see velocities entering a boundary where velocity is free. In this case you may have an infinity of solutions, or divergence. Generally it happens when an eddy gets out of a domain (case pildepon for instance) and it is not too much of a problem. In maritime situations on the contrary it may give fancy results and the user must be aware of it. The theoretical background is the theory of characteristics saying how many informations must be given by upstream and downstream, it depends on the regime : subcritical or supercritical.

With best regards,

Jean-Michel Hervouet