[Users] questions about working elliptic solver thorns

[Ian Hinder]

On 27 May 2015, at 20:50, Comer Duncan <comer.duncan at gmail.com> wrote:

> Hi Erik,
> 
> Frank suggested you might be an appropriate person to ask about elliptic solver thorns that work in the current release of ET. I have, as you know, built Hilbert and have built the development version.  

Hi Comer,

There is a page (https://docs.einsteintoolkit.org/et-docs/Generic_elliptic_solver) on the wiki describing our attempts to round up all the elliptic solvers in November 2011. I don't think it's linked from https://docs.einsteintoolkit.org, but I found it with a search.  

Since this list was written, a multigrid solver has been developed by Eloisa Bentivegna, and has been proposed for inclusion in the ET.  The corresponding ticket is https://trac.einsteintoolkit.org/ticket/1676.  It is a part of the Cosmo Toolkit (https://bitbucket.org/eloisa/cosmology), a collection of Cactus thorns focused on cosmology.

I include below the email Eloisa sent to the ET list on 28-Aug-2013.  We are going to push to get this included in the toolkit as soon as possible, but definitely before the next release.  According to the ticket, we need to provide some examples for the gallery page before it can be included, and this is what is holding it up!

Regarding the last part of your question,

> Can you thus please bring me up to the current moment on just what elliptic solvers are in use for initial data construction in Hilbert?  I know many people have been focussing on the multiple BH problem, so there's not much traffic anymore about Brill.  I do want to run Brill though and also do the "geon" problem.

I think you have characterised the situation quite well; most focus has been on TwoPuncture for binary black holes, and LORENE for binary neutron stars.

As far as I can tell, the IDBrillData thorn can use either BAM_elliptic, SOR or petsc, as you can see from its param.ccl.  I have never used this thorn, but my guess is that it can be made to work.  In fact, in the par directory is an example using SOR, from the CactusElliptic arrangement, which IS in the ET:

	IDBrillData/par/brilldata.par

You could also give that a try, if you are willing to lower yourself to using SOR :) I just ran it, and it spits out some warnings about symmetry boundary conditions, but it generates a metric which isn't 0 or NaN, so there's a good chance it works!

sim create-run arrangements/EinsteinInitialData/IDBrillData/par/brilldata.par



Here is Eloisa's email about the multigrid solver:

On 28 Aug 2013, at 17:56, Eloisa Bentivegna <bentivegna at cct.lsu.edu> wrote:

> Dear Cactus and Einstein Toolkit users,
> 
> I have been working on a solver for elliptic PDEs based on the multigrid paradigm, and developed as a Cactus thorn using Carpet. The solver is now able to tackle some basic problems and could be applied to other cases of interest to this community. You can obtain a copy by checking out the Cosmology arrangement from its repository:
> 
> git clone git at bitbucket.org:eloisa/cosmology.git
> 
> There you will also find documentation, license information, and a test suite. There is no tutorial, but I think that what is under docs/ and par/ should suffice to get most people started. For more information, you can also peruse <http://arxiv.org/abs/arXiv:1305.5576>, especially if you are interested in elliptic problems on spaces without boundaries.
> 
> Please notice that the spirit of the solver is not to replace existing state-of-the-art tools, but to provide a way of solving elliptic PDEs with is both easy to learn and to extend, and gives reasonable answers in a reasonable amount of time. In order to keep this flexibility, the solver is simply unable to compete with other software tailored to specific equations or geometries. In areas where these alternatives exist, they should be used instead.
> 
> Finally, beware of the lack of explicit support for some of the latest Carpet features, such as multipatch and cell-centered grids. Being unfamiliar with these features, I cannot estimate how much work would be necessary to include them. 
> 
> I hope many of you will find this tool useful, and I look forward to hear your feedback.
> 
> Eloisa
> 
> 
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-- 
Ian Hinder
http://members.aei.mpg.de/ianhin