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Instable interface: capillary force driven imbibition of water in a porous domain

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Old   November 15, 2014, 05:03
Default Instable interface: capillary force driven imbibition of water in a porous domain
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I'm trying to model the imbibition of water in a porous domain. Therefore I set up a simple "1D" model.

My geometry has the following dimensions: 5x5x25 mm, the mesh consists of 1000 stacked cells with symmertries to left+right, front+back, an inlet at the bottom and an opening on the top.

The pressure drop in the porous domain is modelled by setting the appropriate permeabilty. In addition to that I configured a subdomain to model the capillary and the gravity force. Both are take into account as a bulk source by the following term in z-direction:

g*density -C [N/m^2]*WATER.vfs.grad z
C is a material constant (=f(porosity, surfacetension,.....))

The simulation works fine for a few thousand iterations. After that I get a verry blurred asymmetric interface (before the interface gets blurry the pressure field becomes asymmetric):

Detailed view with visible mesh.

I already tried various settings without success:
  • smaller timesteps
  • higher mesh resolution
  • switching inferface compression (0, 1, 2)
  • a different mesh (2x2x1000 cells, same geometry) with a free slip wall instead of a symmetrie boundary
  • serial run (CFX documentation mentions possbile instabilities if the interface crosses the partitition boundary)
The simulations ends with no errors if I remove the capillary source term and set the same pressure at the inlet.

How is it even possible to get this kind of asymmetry in a "1D" mesh?

I would appreciate any kind of help. Thanks!
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