The Spherical Shell NSinker benchmark
=====================================
[This folder](https://github.com/geodynamics/aspect/tree/main/benchmarks/nsinker_spherical_shell)
contains the "Spherical Shell NSinker" benchmark, which is
loosely based on the NSinker benchmark defined in
Weighted BFBT Preconditioner for Stokes Flow Problems with Highly
Heterogeneous Viscosity
Johann Rudi, Georg Stadler, Omar Ghattas
https://doi.org/10.1137/16M108450X
but adapted to a spherical shell geometry. See the "nsinker" benchmark
in this repository for the version in a unit box. The motivation for
creating this benchmark was the desire to test linear solver
performance with a spherical geometry, which is often used in practice
and that turns out to be more challenging.
We consider a spherical shell with inner radius 0.54 and outer radius
1.0 with N spherical, high-viscosity, high-density sinking spheres at
random (but predetermined in the code) locations in the
domain. Additionally, a transition zone with distance 0.89583 from the
origin (or about 660km depth if this is assumed to be a
non-dimensionalized Earth-like computation) with a sharp viscosity
jump can be enabled.