# Rotating annulus

Hi ASPECT,

Pretty new to using ASPECT. I’m running a 2D, full cylinder (opening angle = 360). It has a simple viscosity structure of lower mantle = 10^22 Pa s; Top layer (~350km) = 10^19 Pa s, and in between a gradation. Running it for 750 million years results in the entire cylinder rotating, first steadily, then accelerating over the simulation.

It actually looks pretty fun - but definitely an artefact of some sort. I’m sure there’s a way to correct for this rotation - however the point of me calculating this is to estimate true polar wander (so I don’t want to correct all of the rotation signal).

Any thoughts or hints would be appreciated! I’ve attached the parameter file if that’s any help.
original.prm (3.8 KB)

Thanks

Best,

Harriet

Hi Harriet,

Quick reply, but since there are two free slip boundaries in a full (2D) spherical geometry I think the effect you are seeing is likely the net rotation.

This can be removed by adding the following to the PRM file:

subsection Nullspace removal
set Remove nullspace = net rotation
end

Documentation on this parameter and related effects can be found here.

Let us know if this fixes the issue!

Cheers,
John

Hi John,

Thanks for the quick response and help!
Will give it a whirl… (and hope for less whirling…). Will let you know.

Best,

Harriet

Hi Harriet,

That’s tricky!
Since your top and bottom boundary are both free slip, there’s no traction that would prevent this rotation (as John already pointed out). If you wanted to remove all of the rotation, you could use

subsection Nullspace removal
set Remove nullspace = net rotation or net surface rotation (depending on what you want)
end

I am not an expert on True polar wander, but I thought usually one would compute the geoid to compute the inertia tensor, and then use the result to compute TPW as a postprocessing step (and then you can apply the resulting rotation to the model results as a rotation afterwards if you want, because the rotation itself does not affect the solution of the equations in ASPECT since it’s just a solid body rotation). In that case, you could remove the net surface rotation, which would then also allow you to infer the net rotation of the mantle with respect to the surface (in case that’s something you want as well). Is that approximately what you had in mind, or were you thinking of something else?

In any case, let me know how this goes, I’d be quite interested in computing TPW for 3d global models!

Cheers,
Juliane

Hi Juliane,

Thanks for the input. Yea - I totally see your point.

It would be great to output the geoid - but because I’m doing a 2D run, it’s not possible. So currently, outside ASPECT, I post process to find the 2D inertia tensor (from the evolving density file) - and then infer the effective TPW with some handwaving (really looking at how TPW changes with different viscosity profiles).

So interesting - I think I might run both (net rotation and net surface rotation) and see how this goes - this could definitely be useful.

Thanks for your thoughts… I’ll get back to you all on this thread!

Harriet