On 19 Jul 2012, at 13:02, Erik Schnetter wrote:
On Thu, Jul 19, 2012 at 11:21 AM, Ian Hinder ian.hinder@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?