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Head transient bcs seem don't influence the flow in FEMWATER


mgtanda

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Hi,

I'm working with a 3D model for flow only simulation with FEMWATER. I have fixed head bc on a side and transient head bc on the opposite side; saturated conditions in all the field.

The change in head of the boundary condition doesn't affect the flow in the rest of the model; it seems that the boundary is not connected to the rest of the model!

The compressibility of the water is put to zero, then the change in head have to be instantaneously transmitted to the model.

The steady flow simulation works well.

Did this problem occurred to anyone?

Thank you for your attention

 

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First thing I would look at is convergence on each time step. Is the model iterating and converging? It sounds like it is not. This could be because your convergence settings are too loose or too tight, or because your discretization is too coarse.

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Thank you very much for your answer.

I have checked and the numerical process is converging! I tried to change the Run option and the iteration parameters, I also converted the heterogeneous materials in homogeneous ones: the heads remain as in the initial condition with the exception of the side where the transient boundary conditions are applied.

Any ideas?

Regards

Maria

 

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Could be that you have accidentally applied constant head bcs also at the transient boundary?

Also with transient simulations, it is often a good idea to start with small time steps, and increase them.

You are welcome to send me the project and I can try it out here. Although not sure if my license is up to date.

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I solved the problem adopting the Van Genuchten Curves for the unsaturated soil properties instead the linear front mode I had initially inserted (thinking to simplify my simulation!). 

Thank you Woodward for your help!

Regards to all

Maria

 

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Yes the unsaturated curves can cause problems. I also recommend NOT using the spline interpolation, as sometimes this results in a non-monotonic curve. Be careful about the curve spacing and how far away from the water table you specify it (as the last value will be used for all higher tensions as well).

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