[CIG-LONG] frictionless surfaces using Gale

[Walter Landry]

Matty Mookerjee <matty.mookerjee at sonoma.edu> wrote:
> Hello all,
> 
> I was wondering if anyone has had success creating frictionless surfaces in
> Gale.  If I squish (technical term) a box against one of the bounding walls it
> behaves frictionless (at least it appears to), but when I have one rigid block
> squishing a weaker block that surface seems to have a lot of friction.  In
> fact, where those to objects are in contact with each other no sliding occurs
> at all.  I have tried setting boundaryCohesion,
> boundaryCohesionAfterSoftening, boundaryFrictionCoefficient and
> boundaryFrictionCoefficientAfterSoftening all to zero and adding <param name=
> ”boundaryBottom”>True</param>.
> 
> I have done this for both objects (although obviously boundaryTop = True for
> the lower block) which are both using the DruckerPrager rheology.
> 
> Even so, the boundaries seem to be stuck together.

Yes, they will be stuck together.  It is pretty fundamental to how the
Stokes equations are solved.  Gale does not track material interfaces
directly, so it can not know to use a special low viscosity at the
interface.

The only way I can think of to get what you want is to add a low
viscosity layer between the two materials.  However, that layer will
thicken and thin, so you are not always guaranteed to have that layer
between them.

Cheers,
Walter Landry