open TELEMAC-MASCARET The mathematically superior suite of solvers

Hello,

please consider reading my post in the Telemac-2D thread with the same subject line. It concerns Sisyphe even more then Telemac.

Best regards,
jaj

Thank you Jacek.

Note that our nightly validation system is about to be applied to this type of comparisons. Hopefully in time, we will resolve all problem.

Kind regards,
Sébastien.

Dear Sébastien,

unfortunately after a more thorough examination it occures that wetting and drying algorithms (or their treatment in parallel) cannot be blamed for the differences along the shoreline between runs with different number of mesh partitions. The differences these "tidal flats" algorithms produce are secondary. The real error cause seem to be the instabilities of the free surface which are, unfortunately, intrinsic/inherent to finite element type applied in Telemac-2D. Google for "spurious instabilities in P1-P1 elements". In the eighties of the twentieth century they were very popular and so easy to treat, especially in pre- and postprocessing, but nowadays no-one would seriously develop a program for the shallow water equations using these elements. The main cause of *our* parallel problem is that the P1-P1 element instabilities itself tend to differ in differently partitoned meshes, due to their erratic character.

The bad news is that presently probably no-one would do the titanic work of rewriting the fundamental BIEF library for another computationally efficient, but stable element. The good news is that the instabilities intrinsic to the method can be damped by changing the method the gradients of the free surface are computed, by reducing the value of the parameter FREE SURFACE GRADIENT COMPATIBILITY from the default 1 to something between <0,1>, even downto zero (gradients computed totally piecewise).

In our examples we can try reduce the number and "steer" the place of occurance of the differences effectively only with this parameter. But because of the erratic character of these instabilities, some problems might still remain. This can be treated by changing the mesh structure locally, if you have an idea how the wetting/drying methods would work in these places.

Well, of course, this is a method from the shadowy backyard of the numerical methods, just a pain-easing tablet removing most of the symptoms, but not the cause of the disease. Sorry, the users of the parallel Telemac have to learn to live with it, this is typical for most ageing programs nowadays (and not only for programs, unfortunately...).

The other thinkable causes (and there quite a few possibilities in the very shallow shore areas) have been presently excluded.

Best regards,
jaj