Hi Samuel,
I haven't simulated flow in the unsaturated zone in the last years.
But so far I can remeber, it is hard to find good boundary conditions.
So I have some small comments on the boundary conditions on the top.
If you have a no-flow condition in the middle for water and gas in combination
with a high infiltation rate on both sides, this can lead to a fully saturated
wetting
front where the gas can not escape.
One solution is to check if the infitration rate is higher than the infiltraion
capacity of your soil. If so, it is often better to switch to a Dirichlet
condition with (fully saturated + pressure).
Looking on your inital SW_profile, the saturation above the layer (z=9m) is
high,
so a low capillary pressure looks realistic. In general it is easier to let the
problem
settle for a while to get realistic initial conditions.
Paraview is not the best tool for looking at your results since it interpolates
also
data that is constant inside your scv. You should imangine a scv arround every
node.
If you have a fully saturated clay layer above the coarser sand, the air can be
traped
in the corser material. There are some nice movies "Water moevement in soil" on
youtube.
Kind regards,
Leo
> Samuel Scherrer <[email protected]> hat am 6. März 2019 um
> 14:36 geschrieben:
>
>
> Hi,
>
> I'm currently trying to set up a 2D model for water, air, and heat flow
> in the unsaturated zone with a clay layer between two sand layers.
>
> The upper boundary is partly no-flow, partly fixed air pressure,
> temperature, and water infiltration flux.
> The side boundaries are no-flow boundaries.
> The groundwater table is inside the domain, 1 m above the bottom
> boundary, and the bottom boundary has a constant temperature and gas
> content in water (since only the liquid phase is present) and it is a
> no-flow boundary for air.
>
> The formulation I'm using is air pressure - water saturation.
>
> As a initial condition I'm using hydrostatic air pressure and a water
> saturation calculated from hydrostatic capillary pressure, initial
> temperature is the same as the boundaries.
>
> I'm using the Box discretization.
>
> The clay layer extends from 5m to 9m and the spatial parameters are
> assigned such that vertices at 5m or at 9m still belong to the clay
> layer (function isClay_ in problem.hh and spatialparams.hh).
>
> When I run this setup I get high spikes in capillary pressure at the
> boundaries between clay and sand, even in the initial output file (*-
> 00000.vtu, shown in pc_initial.png). I also use the gnuplot interface
> to plot the initial capillary pressure and saturation vs depth along
> x=0 using the same method as for setting the initial conditions (i.e.
> using the method initialSwitchVariable_(pos) for getting the saturation
> and calculating the capillary pressure using the material law).
> In this plot no capillary pressure discontinuities are visible.
>
> In the first output this is only visible at the upper clay-sand
> interface, where the capillary pressure is much lower than it should
> be, but over time a very high pc develops at to lower clay-sand
> interface(up to 1000 bar, see pc_5min.png and pc_final.png).
>
> During inspection of the vtk-output with Paraview I also saw that the
> porosity of the clay only reaches to slightly below 9m, while the
> initial water saturation has the values it should have in clay up to
> slightly above 9m (see porosity.png and initial_water_saturation.png).
> The porosity is set using porosityAtPos in spatialparams.hh, the
> initial water saturation with initialAtPos.
>
> I figured it might be either due to my poor understanding of the Box
> method, or might be related to the discontinuous water saturation (see
> initial_Sw_profile.png).
>
> Does anyone know what my problem is or what the solution could be?
>
> Kind regards,
> Samuel
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Im Auftrag
Dr.-Ing. Leopold Stadler
--
Referat Numerische Verfahren im Wasserbau
Abteilung Wasserbau im Binnenbereich
Bundesanstalt für Wasserbau
Federal Waterways Engineering and Research Institute
Kußmaulstraße 17 | 76187 Karlsruhe
E-Mail: [email protected]
Tel.: +49 721 9726-3525
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