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geodynamics / aspect

A parallel, extensible finite element code to simulate convection in both 2D and 3D models.
https://aspect.geodynamics.org/
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Update Stokes preconditioner with free surface? #5071

Closed tjhei closed 1 year ago

tjhei commented 1 year ago

This might be a bug: if we have a free surface, shouldn't we always recompute the preconditioner? We currently have no logic to recompute the preconditioner even though the mesh gets deformed in every step.

While the preconditioner is likely similar (if deformations are small), there is no guarantee that it is still good. Note that tests like tests/remove_angular_momentum only update the preconditioner if the mesh changes (and would never update with a fixed mesh).

While it seems efficient to maybe reuse the preconditioner for a while, I would still suggest to always rebuild it to be consistent. Thoughts?

_Originally posted by @tjhei in https://github.com/geodynamics/aspect/pull/5070#discussion_r1106108216_

tjhei commented 1 year ago

This change would only affect computations with a free surface with "simple" material models where the matrix does not depend on the solution (constant viscosity etc). In that case we would spend more time setting up the AMG preconditioner (and maybe get fewer iterations).

bangerth commented 1 year ago

We already have logic to update the preconditioner, for example if the viscosity changes, right? Can't we hook into that flag?

Or are you talking about the preconditioner for anything else than the Stokes problem?