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underworldcode / underworld2

underworld2: A parallel, particle-in-cell, finite element code for Geodynamics.
http://www.underworldcode.org/
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How to keep isostasy balance at the bottome correctly #463

Closed alexstruc closed 4 years ago

alexstruc commented 4 years ago

Hi, all I am trying to establish a model allowing materials to escape from the bottom. In order to keep isostasy balance, I employed Neumann condition on the bottom wall like the following codes. image

The isostasyField is a mesh variable and is calculated like that: image

Then I tried two experiments. The first one had Neumann condition on the bottom (the first picture). The second one had a open bottom (the second picture). However, the results of the two tests are nearly the same, which makes me so confused. image image

Moreover, both results seem to be wrong because the velocity field of the mantle will show very creasy convection after running for hundreds of steps(~2Myr). image

I guess that is might because I did not set the isostasy balance correctly. I don't know what is wrong in my code and I hope you can point out my mistakes. If you know anything about this issue, please give me your suggestions. Just let me know if you need to check more details. Thanks in advance Alex

tingyang2004 commented 2 months ago

I need to apply a similar boundary condition to one of my models. I believe the reason of the phenomenon you observed lies in the fact that the Stokes equation involves the pressure gradient, rather than the pressure itself. Consequently, if a constant boundary force is applied exclusively at the top or bottom boundary, it has no impact on the ultimate solution.