Posted Wed, 23 Apr 2014 08:23:21 GMT by Zeno Farina
Hi folks!
Just to make sure: if a want to create a completely dry model, assuming the absence of groundwater, is it enough for me to set BC hydraulic head lower than the depth of the model? If my model is 10 m deep, do I just need to assign BC hydraulic head to every node at say -11 m?
Or is there a more proper way to assign a condition of complete absence of water?
Thank you in advance for help,
best regards
Posted Wed, 23 Apr 2014 15:44:22 GMT by adacovsk
I would recommend against putting many constant head BCs across your model. Since the default setup for a model contains all no flow boundaries around the domain, you can get away with just setting 1 head BC to a value of 0 (doesn't matter where) and set the initial condition for hydraulic head to be 0. With more BCs, you might run into convergence/mass balance issues.

I'm under the assumption you currently have no flow BCs in your model.

Posted Thu, 24 Apr 2014 08:50:54 GMT by Zeno Farina
Thank you Adam for the reply,
however, considering that my model is 10 m deep, I guess I have to assign something like -11 m as Hydraulic head BC, not 0, if I want to create a completely dry model. Also I assign -11 m as hydraulic head initial condition to the whole model.. why should I assign 0?
Correct there are no flow BC in the model.
Again thank you for your reply,
kind regards!
Posted Tue, 29 Apr 2014 17:42:44 GMT by adacovsk
Any arbitrary number equal or lower than your lowest elevation should work I think. Setting everything to -11m would give you weird suction pressure -- it would be best steady-state with just one BC and let it set inital conditions and hopefully it would result in a mass balance of 0? My mind doesn't work well with negative elevation heads, sorry!
Posted Wed, 30 Apr 2014 09:42:28 GMT by Zeno Farina
I think I works simply without imposing any hydraulic head BC but only hydraulic head initial conditions (-11m). My model (which is a cube 10x10x10 m) has saturation around 0.004 which shows basically dryness.. I guess that's as much as I can get. Do you think a BC is necessary anyway? When you say "just one BC" do you mean one BC on one node only? I cannot run it steady state as I have to use Richard's equation for unsaturated models.
Let me know what you think, thank you in advance!
Posted Wed, 30 Apr 2014 17:32:25 GMT by adacovsk
To run a flow steady-state simulation, you need at least one flow boundary condition. You can run a flow steady-state for unsaturated models, but the solution is dependent on the initial conditions (if you have a bad initial condition, you'll likely have a bad steady-state solution). I'm unsure what you meant by completely dry; either no groundwater or no water in the pore space (i.e. column of sand)? If you set the residual saturation to 0, you'll have a completely dry model once you run a steady-state.

Posted Sat, 10 May 2014 08:07:31 GMT by Carlos Andres Rivera Villarreyes Global Product Specialist - FEFLOW
Hi all,
Another alternative is to set up a free drainage BC by means of a Fluid-Flux BC (Gradient) equal to the material conductivity. If you assign the correct unsaturated model and properties and model is already dried, gravity-driven flow should not occur or only at a minimum level.
If the residual water content is decreased to zero (not realistic for soils), this does not mean that model will be completely dry. It is only reflects that void space is larger to contain water. In my opinion, you should mainly focused on initial saturation values.

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