[Users] coordinate systems

[Ian Hinder]

On 19 Jul 2012, at 13:02, Erik Schnetter wrote:

> On Thu, Jul 19, 2012 at 11:21 AM, Ian Hinder <ian.hinder at aei.mpg.de> wrote:
>> 
>> On 19 Jul 2012, at 11:19, Erik Schnetter wrote:
>> 
>>> Comer
>>> 
>>> Do you intend to use existing thorns to evolve e.g. the Einstein
>>> equations, or do you plan to write your own thorns?
>>> 
>>> If you use existing thorns: As written, they will not be able to
>>> handle the coordinate singularities on the axes. Instead, we use
>>> multi-block methods.
>>> 
>>> If you write your own thorns: Since you will be implementing your own
>>> differencing methods, the Cactus coordinates are basically only needed
>>> to label grid point. CartGrid3D can handle this; e.g. spherical or
>>> cylindrical coordinate systems are orthogonal. You will only need to
>>> correctly specify the respective coordinate ranges (e.g. using
>>> [0,rmax] for r and [0,2pi] for phi) in your parameter file. The
>>> unification of f(phi=0) and f(phi=2pi) would be handled via a
>>> periodicity condition, which is provided e.g. by thorn Periodic.
>>> 
>>> What remains is that Cactus labels the coordinates x,y,z instead of
>>> r,z,phi... but this is mainly a small inconvenience. Altogether, it is
>>> straightforward to use spherical or cylindrical coordinates with
>>> existing thorns, and we therefore never bothered to implement any new
>>> coordinate systems for this.
>> 
>> I think that CoordBase does not support anything other than CartGrid3D, so you won't be able to use any thorn which uses CoordBase.
> 
> What I describe above uses CartGrid3D...

Aha. Yes.  So what does CartGrid3D actually do when you ask for spherical coordinates?  x, y and z are mapped to r, theta and phi, but since you set the ranges yourself anyway, what more is needed?

-- 
Ian Hinder
http://numrel.aei.mpg.de/people/hinder