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I was using version v8p2r1 and all my models ran perfectly well.
Almost always, I prescribe open boundaries with free depth and velocity (4 4 2).
I have now switched to version v8p4r0 and am getting ILL-POSED boundary errors.

The boundary inflow is positive, though it should be negative because the only source is rain.
The mass balance printed at the end is therefore incorrect.

I understand there are plenty of discussions on ILL-POSED boundary problems on this forum, but I would like to know why v8p2r1 runs smoothly and v8p4r0 does not. Every single model I try to run now displays the error above. Has there been any major changes?

I have attached a small example along with the .cfg and .cas files for those willing to verify this issue. Thanks in advance.

PS: I also tried v8p4r0_breach version, to no avail.

I tried it again with v8p5 and the issue remains.

However, I was able to get reasonable results without any errors by setting the boundary conditions to (5 6 6).
With:
Prescribed depth HBOR=0.0
Prescribed velocity LIUBOR=0.0, LIVBOR=0.0

This setup ran well without having to set thompson boundary conditions. Does anyone have any thoughts on this approach? Is it correct and is there room for improvement?

The main issue I wanted to raise is that we have A LOT of models using (4 4 4) BCs which are now broken for versions upward of v8p2r1. In a sense, there is no backward compatibility of boundary condition settings. (This might be a large pitfall for users)
I feel like using (4 4 4) should be usable out-of-the box as it was before without causing these issues.

I am providing an updated test case for those willing to verify. The one I provided before seems to have some encoding errors in the mesh.

Hello,

Sorry for the delay to answer. I needed some discussion with colleagues who better know what is implemented in Finite Volumes part.

Since release v8p3, there is a splitting of VOLFIN subroutines to give the right time step to source terms and Thompson boundary conditions. This may explain that results have changed.

Hard to say if results with v8p2 were really good but the change was mandatory (otherwise the time step was not good one).

Hope this helps,

Chi-Tuan