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Hello,
We have an instability problem at the boundary conditions. The boundaries are "open boundary with prescribed H" with H varying with time, sourced by a text file.
With water level increasing or decreasing, the boundary should be acting as an inflow or outflow to obtain the right water level at the limits. We assumed that this flow would be uniformly distributed on all the boundary, but it seems like TELEMAC is concentrating the inflow on a small number of cells at the edge of the boundary.
Does anybody ever encounter this situation and how did you resolve the issue?
We tried different turbulence models, advection models, shortening the boundary and refining the mesh at the boundary, but nothing has worked.

I've link a screenshot of the issue, the .cas file and the boundary files.

Thank you very much for any help this community can provid us.
Pablo

It seems like attachments were not linked properly

Hello Pablo,

What about the boundary perpendicular to the flow. Do you define it totally open or a mix open and close?

What about the bottom elevation along this boundary: is it always below the surface elevation you want to prescribe or not? If not, you may have dry nodes and close to the interface wet/dry, there may be strong velocities.

Anyway, what kind of flow do you have? Subcritical, supercritical? Prescribing 2 times water level or surface elevation (if you have 2 open boundaries) may not be correct.

Hope this helps,

Chi-Tuan