## CartGrid3D

Date

Abstract

CartGrid3D allows you to set up coordinates on a 3D Cartesian grid in a ﬂexible manner. You can choose diﬀerent grid domains (eg octant) to allow you to exploit any symmetry in your problem. CartGrid3D also provides routines for registering symmetries of grid functions and applying symmetry conditions across the coordinate axes.

### 1 Specifying the Grid Symmetry

CartGrid3D allows you to specify the grid symmetry (or lack thereof) with the grid::domain parameter:

grid::domain = "full"

There are no symmetries.
grid::domain = "bitant"

The grid includes only the $z\ge 0$ half-space (plus symmetry zones); there is a reﬂection symmetry across the $z=0$ plane.

The grid includes only the $\left\{x\ge 0,y\ge 0\right\}$ quadrant (plus symmetry zones); there is a reﬂection symmetry across both the $x=0$ plane and the $y=0$ plane.
grid::domain = "octant"

The grid includes only the $\left\{x\ge 0,y\ge 0,z\ge 0\right\}$ octant (plus symmetry zones); there is a reﬂection symmetry across each of the $x=0$ plane, the $y=0$ plane, and the $z=0$ plane.

Note that the implementation of symmetries in CartGrid3D is deprecated, and the SymBase infrastructure should be used instead. The above symmetries can be implemented using SymBase and the ReﬂectionSymmetry thorn. Additionally, there are rotational symmetries provided by the RotatingSymmetry180 and RotatingSymmetry90 thorns.

In each case except grid::domain = "full", symmetry zones are introduced just on the “other side” of each symmetry grid boundary. Each symmetry zone has a width (perpendicular to the boundary) of driver::ghost_size extra grid points. For centered 2nd order ﬁnite diﬀerencing, a width of driver::ghost_size = 1 should be suﬃcient, but for (centered) 4th order ﬁnite diﬀerencing, or for upwinded 2nd order, a width of driver::ghost_size = 2 is needed. Making driver::ghost_size too large is fairly harmless (it just slightly reduces performance), but making it too small will almost certainly result in horribly wrong ﬁnite diﬀerencing near the symmetry boundaries, and may also result in core dumps from out-of-range array accessing.

Note that the symmetry zones must be explicitly included in driver::global_nx, driver::global_ny, and driver::global_nz, but should not be included in any of the grid::type = "byrange" parameters grid::xmin, grid::xmax, grid::ymin, grid::ymax, grid::zmin, grid::zmax, grid::xyzmin, and/or grid::xyzmax described in the next section.

Note also that driver::global_nx, driver::global_ny, and driver::global_nz do not include any ghost zones introduced for multiprocessor synchronization. (For more information on ghost zones, see the section “Ghost Size” in the “Cactus Variables” chapter of the Cactus Users’ Guide.)

### 2 Specifying the Grid Size, Range, and Spacing

CartGrid3D provides several diﬀerent methods for setting up the integer grid size (eg 128), ﬂoating-point grid spacing (eg 0.1), and ﬂoating-point grid range (eg 12.8).1 You specify which method to use, with the grid::type parameter:

grid::type = "byrange"

You specify the $x$, $y$, and $z$ grid ranges, either with separate grid::xmin, grid::xmax, grid::ymin, grid::ymax, grid::zmin, and grid::zmax parameters, or with the grid::xyzmin and grid::xyzmax parameters. The grid spacings are then determined automagically from this information and the driver::global_nx, driver::global_ny, and driver::global_nz grid-size parameters. You should also choose the grid::domain parameter consistent with all these other parameters. (It’s not clear whether or not the code ever explicitly checks this.)
grid::type = "box"

This is a special case of grid::type = "byrange" with the grid ranges hard-wired to grid::xyzmin = -0.5 and grid::xyzmax = +0.5.
grid::type = "byspacing"

You specify the $x$, $y$, and $z$ grid spacings, either with separate grid::dx, grid::dy, and grid::dz parameters, or with the grid::dxyz parameter. You also specify the grid symmetry with the grid::domain parameter. The $x$, $y$, and $z$ grid ranges are then determined automagically from this information and the driver::global_nx, driver::global_ny, and driver::global_nz grid-size parameters: Each coordinate’s range is chosen to be either symmetric about zero, or to extend from 0 up to a maximum value.

There are also a number of optional parameters which can be used to specify whether or not it’s ok to have a grid point with an $x$, $y$, and/or $z$ coordinate exactly equal to 0:

grid::no_originx, grid::no_originy, grid::no_originz, grid::no_origin

These parameters are all deprecated — don’t use them!
grid::avoid_originx

This is a Boolean parameter; if set to true (grid::avoid_originx = "true" or grid::avoid_originx = "yes" or grid::avoid_originx = 1) then the grid will be “half-centered” across $x=0$, ie there will be grid points at …, $x=-\frac{3}{2}\Delta x$, $x=-\frac{1}{2}\Delta x$, $x=+\frac{1}{2}\Delta x$, $x=+\frac{3}{2}\Delta x$, …, but not at $x=0$.
grid::avoid_originy

Same thing for $y$.
grid::avoid_originz

Same thing for $z$.
grid::avoid_origin

Same thing for all 3 axes $x$ and $y$ and $z$, ie no grid point will have $x=0$ or $y=0$ or $z=0$.

### 3 An Example

Here is an example of setting up a grid using the PUGH unigrid driver. The relevant parts of the parameter ﬁle are as follows:

# PUGH
driver::ghost_size = 2
driver::global_nx = 61
driver::global_ny = 61
driver::global_nz = 33

# CartGrid3D
grid::avoid_origin = "no"
grid::domain = "bitant"
grid::type = "byrange"
grid::xmin = -3.0
grid::xmax = +3.0
grid::ymin = -3.0
grid::ymax = +3.0
grid::zmin =  0.0
grid::zmax = +3.0

The resulting Cactus output (describing the grid) is as follows:

INFO (CartGrid3D): Grid Spacings:
INFO (CartGrid3D):  dx=>1.0000000e-01  dy=>1.0000000e-01  dz=>1.0000000e-01
INFO (CartGrid3D): Computational Coordinates:
INFO (CartGrid3D):  x=>[-3.000, 3.000]  y=>[-3.000, 3.000]  z=>[-0.200, 3.000]
INFO (CartGrid3D): Indices of Physical Coordinates:
INFO (CartGrid3D):  x=>[0,60]  y=>[0,60]  z=>[2,32]
INFO (PUGH): Single processor evolution
INFO (PUGH): 3-dimensional grid functions
INFO (PUGH):   Size: 61 61 33

Since there’s no symmetry in the $x$ and $y$ directions, the grid is set up just as speciﬁed, with ﬂoating-point coordinates running from $-3.0$ to $3.0$ inclusive, and 61 grid points with integer grid indices $\left[0,60\right]$ (C) or $\left[1,61\right]$ (Fortran).

However, in the $z$ direction there’s a reﬂection symmetry across the $z=0$ plane, so the speciﬁed range of the grid, $z\in \left[0.0,3.0\right]$, is automagically widened to include the symmetry zone of driver::ghost_size = 2 grid points. The grid thus actually includes the range of ﬂoating-point coordinates $z\in \left[-0.2,3.0\right]$. The original speciﬁcation of 33 grid points is left alone, however, so the grid points have integer array indices $\left[0,32\right]$ (C) or $\left[1,33\right]$ (Fortran). The “physical” (ie non-symmetry-zone) part of the grid is precisely the originally-speciﬁed range, $z\in \left[0.0,3.0\right]$, and has the integer array indices $\left[2,32\right]$ (C) or $\left[3,33\right]$ (Fortran).

### 4 Coordinates

CartGrid3D deﬁnes (registers) four coordinate systems: cart3d, cart2d, cart1d, and spher3d.

The Cartesian coordinates supplied by this thorn are grid functions with the standard names x, y, and z. To use these coordinates you need to inherit from grid, ie you need to have an

inherits: grid

line in your interface.ccl ﬁle. In addition a grid function r is provided, containing the radial coordinate from the origin where

$r=\sqrt{{x}^{2}+{y}^{2}+{z}^{2}}$

CartGrid3D registers the lower and upper range of each coordinate with the ﬂesh.

### 5 Symmetries for Grid Functions

If your problem and initial data allow it, CartGrid3D allows you to enforce even or odd parity for any grid function at (across) each coordinate axis. For a function $\varphi \left(x,y,z\right)$, even parity symmetry on the $x$-axis means

$\varphi \left(-x,y,z\right)=\varphi \left(x,y,z\right)$

while odd parity symmetry means

$\varphi \left(-x,y,z\right)=-\varphi \left(x,y,z\right)$

Note that the symmetries will only be enforced if a symmetry domain is chosen (that is, if grid::domain is set to something other than grid::domain = "full".

#### 5.1 Registering Symmetry Behaviour

Each grid function can register how it behaves under a coordinate change using function calls in CartGrid3D. These symmetry properties can then be used by other thorns, for example CactusBase/Boundary uses them to enforce symmetry boundary conditions across coordinate axes. Symmetries should obviously be registered before they are used, but since they can be diﬀerent for diﬀerent grids, they must be registered after the CCTK_STARTUP timebin. The usual place to register symmetries is in the CCTK_BASEGRID timebin.

For example, to register the symmetries of the xy component of the metric tensor from C, you ﬁrst need to get access to the include ﬁle by putting the line

uses include: Symmetry.h

in your interface.ccl ﬁle. Then in your thorn you can write (C)

#include "Symmetry.h"
static int one=1;
int sym[3];
sym[0] = -one;
sym[1] = -one;
sym[2] =  one;