[CIG-LONG] Extension - Compressible Layer in Gale

[Walter Landry]

"atyl0789 at uni.sydney.edu.au" <atyl0789 at uni.sydney.edu.au> wrote:
> G'day All,
> 
> Thanks very much for the advice so far, it has been very helpful :)
> 
>>The only real problem you may have is from having such a deformed
>>grid.  For runs with an air layer, I would recommend making the top of
>>the simulation fixed.  Air will flow in and out of the simulation, but
>>the mesh will stay the same.  And, as Louis Moresi said, it does not
>>really matter what the air does, since it really only acts as a
>>stabilization term.
> 
> I apologise if this is a silly question, but how do I make the top
> of the simulation fixed/free slip? I initially thought that I would
> just set vy=0 at the top and change WrapTop=true to false, however
> someone suggested that I instead just set the top boundary to be
> static. I'm actually not clear on what the difference between these
> approaches is, so any explanation would be much appreciated :)

Setting vy=0 will make the material not move.  Generally, the mesh
follows the material.  You want the material to move, but not the
mesh.  For that, you set the top and corners to be static.  wrapTop
should not matter, so you should take it out.

>>In HydrostaticTerm, there is a variable 'height'.  Set that to the top
>>of your material layer, not the top of the simulation.
> 
> That makes a lot of sense, I can't believe I didn't think of that!
> However I wonder whether when the model extends, this height would
> eventually become incorrect and I would end up with the hydrostatic
> term in the air layer? Would this be an issue at all?

In your case, this could be a problem.  One way to fix this is to set
a stress boundary on the bottom.  Then material will naturally flow in
as the material thins.

The other way to fix it is to specify the velocity of the bottom such
that the bottom raises in to counteract the thinning.  So the height
will always remain the same.  This is done in

  input/examples/tibet.xml.

It does not need to be an exact match.  Getting the height wrong means
that you will not get as accurate an answer as you might.  But the
code should converge as you increase resolution.

Cheers,
Walter Landry
walter at geodynamics.org