[aspect-devel] free surface with non-zero surface pressure

Wolfgang Bangerth bangerth at tamu.edu
Thu Dec 8 18:59:22 PST 2016

Carolyn & John,
great question indeed.

> Currently, it is not possible to mix a free surface and traction boundary
> condition.  However, as you pointed out this feature would be very useful for
> a number of applications and I do not see any reason why it can't be implemented.
> I think this kind of feature would likely be implemented as an added option to
> the free surface portions of the code or as a new boundary condition
> model/option.  Others will have a much better idea of exactly how/where this
> should be done.

There are basically two places where you have to do something. The first is 
that you still have to add the traction boundary terms even though it's a free 
boundary. This should not be very difficult: you could define a new type of 
boundary, say "free surface with traction", and then the place where we add 
the terms for tractions that currently reads
   if (boundary is a traction boundary)
would simply have to be extended to say
   if (boundary is a traction boundary
       boundary is a free boundary with traction)

The second place is where we currently compute the response of the boundary to 
the internal stresses. Right now, it uses the entire normal component of the 
stress to move the boundary. In your case, it needs to use
   "normal component of stress"   minus   "prescribed traction"
to move the boundary.

Neither of these two places (plus the addition of a new kind of boundary) are 
going to be very difficult. You just need to find the right places to add the 
necessary code. I suspect this can be done with not much more than 100 lines 
of additional code.

If you want to go this route, feel free to ask the mailing list for concrete 
questions on how each of the steps above can be implemented and where! Ask 
early and often, as it's easier to first discuss a design then implement it, 
than it is to implement something that later needs to be rewritten in response 
to comments from the developer community :-)


Wolfgang Bangerth          email:                 bangerth at colostate.edu
                            www: http://www.math.colostate.edu/~bangerth/

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