The Multipole thorn performs spherical harmonic mode decomposition of Cactus grid functions on coordinate spheres.
This thorn allows the user to compute the coefficients of the spherical harmonic expansion of a field stored in a Cactus grid function on coordinate spheres of given radii. A set of radii for these spheres, as well as the number of angular points to use, can be specified. Complex fields can be used, but they must be stored in pairs of real Cactus grid functions (the CCTK_COMPLEX type is not supported).
The angular dependence of a field \(u(t, r, \theta , \varphi )\) can be expanded in spin-weight \(s\) spherical harmonics [1]:
where the coefficients \(C^{lm}(t,r)\) are given by
At any given time, \(t\), this thorn can compute \(C^{lm}(t,r)\) for a number of grid functions on several coordinate spheres with radii \(r_i\). The coordinate system of the Cactus grids must be Cartesian and the coordinates \(r\), \(\theta \), \(\varphi \) are related to \(x\), \(y\) and \(z\) by the usual transformation between Cartesian and spherical polar coordinates (\(\theta \) is the polar angle and \(\varphi \) is the azimuthal angle).
The spin-weighted spherical harmonics are computed using Eq. 3.1 in Ref. [1].
The coordinate sphere on which the multipolar decomposition is performed is represented internally as a 2-dimensional grid evenly spaced in \(\theta \) and \(\varphi \) with coordinates
so \(n_\theta \) and \(n_\varphi \) count the number of cells (not the number of points). The gridfunction to be decomposed, \(u\), is first interpolated from the 3D Cactus grid onto this 2D grid at a given radius, \(r_i\), and the \(C^{lm}\) are computed by evaluating the integral in Eq. ?? for different values of \(l\) and \(m\). The interpolation is performed using the Cactus interpolation interface, so any Cactus interpolator can be used. The numerical method used for interpolation should be specified in the documentation for the thorn which provides it. One such thorn is AEILocalInterp. The integration is performed using either the midpoint rule, yielding a result which is second order accurate in the angular spacings \(\Delta \theta = \pi /n_\theta \) and \(\Delta \varphi = 2\pi /n_\theta \), or Simpson’s rule, which is fourth order accurate.
This thorn is available as part of the Einstein Toolkit via:
svn checkout https://svn.einsteintoolkit.org/cactus/EinsteinAnalysis/Multipole/trunk Multipole
or by using the Einstein Toolkit GetComponents script and thornguide.
Suppose that you have a real grid function, \(u\), for which you want to compute the spherical harmonic coefficients \(C^{lm}\). Start by including Multipole and an interpolator (for example AEILocalInterp) in the ActiveThorns line of your parameter file (for interpolation thorns other than AEILocalInterp, you will need to modify the interpolator and interpolator_options accoording to the documentation of the interpolation thorn). Next decide the number and radii of the coordinate spheres on which you want to decompose. Set the number of spheres with the nradii parameter, and the radii themselves with the radius[i] parameters (the indices \(i\) are zero-based). For example,
ActiveThorns = ".... AEILocalInterp Multipole" Multipole::nradii = 3 Multipole::radius[0] = 10 Multipole::radius[1] = 20 Multipole::radius[2] = 30 Multipole::variables = "MyThorn::u"
The default parameters will compute all \(l = 2\) modes assuming a spin-weight \(s = 0\) on every iteration of your simulation. The coefficients \(C^{lm}\) will be output in the files with names of the form mp_<var>_l<lmode>_m<mmode>_r<rad>.asc, for example
mp_u_l2_m2_r10.00.asc mp_u_l2_m-1_r20.00.asc
For the filename, the radius is converted to a string with two decimal places, which should be sufficient. These are ASCII files where each line has columns
\(t\), \(\mathrm {Re} \, C^{lm}\), \(\mathrm {Im} \, C^{lm}\).
Often it will be necessary to go beyond the basic usage described in the previous section.
By default, Multipole computes only \(l = 2\) modes. You can choose whether to extract a single mode, or all modes from \(l = \) l_min to \(l = \) l_mode, with \(|m| \le \) m_mode. This is controlled by the mode_type parameter, which can be set to "all_modes" or "specific_mode". The parameters l_min and l_mode specify the lowest and highest modes to compute. The parameter m_mode specifies up to which value of \(m\) to compute. When using mode_type = "specific_mode", the mode \(l\) and \(m\) are given by l_mode and m_mode respectively.
Several variable-specific options can be listed as tags in the variables parameter:
Multipole::variables = "<imp>::<var>{<tagname> = <tagvalue> ... } ..."
Valid tags are:
| cmplx | A string giving the fully qualified variable name for the imaginary part of a complex field, assuming that <imp>::<var> is the real part. |
| name | A string giving an alias to use for the decomposed variable in the output filename, for use in the case of complex variable when otherwise the name of the real part would be used, which might be confusing. |
| sw | The integer spin-weight of the spherical harmonic decomposition to use. |
Strings should be enclosed in single quotes within the list of tags.
In order to decompose a complex quantity, Multipole currently requires that the field is stored in two separate CCTK_REAL grid functions, one for the real and one for the imaginary part. Suppose the complex function \(u\) is stored in two gridfunctions u_re and u_im. In order to correctly decompose \(u\), specify the real variable in the variables parameter, and use the tag cmplx to specify the name of its imaginary companion:
Multipole::variables = "MyThorn::u_re{cmplx = ’MyThorn::u_im’}"
In some cases, you might want the name of the variable in the output filename to be different to the name of the grid function. This can be done by setting the name tag of the variable:
Multipole::variables = "MyThorn::u{name = ’myfunction’}"
For example, in the case of a complex variable where the output file contains the name of the real part, you can rename it as follows:
Multipole::variables = "MyThorn::u_re{cmplx = ’MyThorn::u_im’ name = ’u’}"
Depending on the nature of the field to be decomposed, a spin-weight other than 0 might be required in the spherical harmonic basis. Use the tag sw for this
Multipole::variables = "MyThorn::u_re{cmplx = ’MyThorn::u_im’ name = ’u’ sw = -2}"
The interpolator to be used can be specified in the interpolator_name parameter, and a string containing interpolator parameters can be specified in the interpolator_pars parameter. See the interpolator (for example AEIThorns/AEILocalInterp) documentation for details of interpolators available and their options.
When used with mesh-refinement, it is common to require mode decomposition less frequently than every iteration. The parameter out_every can be used to control this. 1D and 2D output of the coordinate spheres can be enabled using out_1d_every and out_2d_every.
This thorn obtains grid function data via the standard Cactus interpolator interface. To use this, one needs the parallel driver (for example PUGHInterp or CarpetInterp) as well as the low-level interpolator (e.g. AEILocalInterp).
To use this thorn with WeylScal4 to compute modes of the complex \(\Psi _4\) variable, one could use the following example:
ActiveThorns = ".... WeylScal4 CarpetInterp AEILocalInterp Multipole"
Multipole::nradii = 3
Multipole::radius[0] = 10
Multipole::radius[1] = 20
Multipole::radius[2] = 30
Multipole::variables = "WeylScal4::Psi4r{sw=-2 cmplx=’WeylScal4::Psi4i’}"
Multipole::l_mode = 4
Multipole::m_mode = 4
This thorn was developed in the Penn State Numerical Relativity group and contributed to the Einstein Toolkit.
This thorn was written by Ian Hinder and Andrew Knapp, with contributions from Eloisa Bentivegna and Shaun Wood.
[1] J. N. Goldberg, A. J. MacFarlane, E. T. Newman, F. Rohrlich, and E. C. G. Sudarshan. Spin s spherical harmonics and edth. J. Math. Phys., 8:2155, 1967.
coord_system | Scope: restricted | STRING |
Description: What is the coord system?
| ||
| Range | Default: cart3d | |
.* | Any smart string will do
| |
enable_test | Scope: restricted | BOOLEAN |
Description: whether to set a spherical harmonic in the ’harmonic’ grid functions
| ||
| Default: no | ||
hdf5_chunk_size | Scope: restricted | INT |
Description: How many iterations to preallocate in extensible HDF5 datasets
| ||
| Range | Default: 200 | |
1:* | Any integer
| |
integration_method | Scope: restricted | KEYWORD |
Description: How to do surface integrals
| ||
| Range | Default: midpoint | |
midpoint | Midpoint rule (2nd order)
| |
trapezoidal | Trapezoidal rule (2nd order)
| |
Simpson | Simpson’s rule (4th order) [requires even ntheta and nphi]
| |
DriscollHealy | Driscoll & Healy (exponentially convergent) [requires even ntheta]
| |
interpolator_name | Scope: restricted | STRING |
Description: Which interpolator should I use
| ||
| Range | Default: Hermite polynomial interpolation | |
.+ | Any nonempty string
| |
interpolator_pars | Scope: restricted | STRING |
Description: Parameters for the interpolator
| ||
| Range | Default: order=3 boundary_off_centering_tolerance={0.0 0.0 0.0 0.0 0.0 0.0} boundary_extrapolation_tolerance={0.0 0.0 0.0 0.0 0.0 0.0} | |
.* | ”Any string that Util_TableSetFromStr ing() will take”
| |
l_max | Scope: restricted | INT |
Description: The maximum l mode to extract
| ||
| Range | Default: 2 | |
0:* | l >= 0
| |
l_min | Scope: restricted | INT |
Description: all modes: above which l mode to calculate/ specific mode: which l mode to extract
| ||
| Range | Default: -1 | |
-1:* | Deprecated
| |
l_mode | Scope: restricted | INT |
Description: The maximum l mode to extract
| ||
| Range | Default: -1 | |
-1:* | Deprecated
| |
m_mode | Scope: restricted | INT |
Description: all modes: Up to which m mode to calculate/ specific mode: which m mode to extract
| ||
| Range | Default: -100 | |
-100:* | Deprecated
| |
mode_type | Scope: restricted | KEYWORD |
Description: Which type of mode extraction do we have
| ||
| Range | Default: deprecated | |
all modes | Extract all modes up to (l_mode, m_mode).
| |
specific mode | Select one specific (l_mode, m_mode) mode
| |
deprecated | Deprecated
| |
nphi | Scope: restricted | INT |
Description: The number of points in the phi direction, minus one. (E.g., if this is set to 100,
then 101 points will be chosen.)
| ||
| Range | Default: 100 | |
1:* | ||
nradii | Scope: restricted | INT |
Description: How many extraction radii?
| ||
| Range | Default: 1 | |
0:100 | ||
ntheta | Scope: restricted | INT |
Description: The number of points in the theta direction, minus one. (E.g., if this is set to 50,
then 51 points will be chosen.)
| ||
| Range | Default: 50 | |
0:* | Positive please
| |
out_1d_every | Scope: restricted | INT |
Description: How often to output 1d data
| ||
| Range | Default: (none) | |
no output
| ||
1:* | output every to many iterations
| |
out_dir | Scope: restricted | STRING |
Description: Output directory for Extract’s files, overrides IO::out_dir
| ||
| Range | Default: (none) | |
.+ | A valid directory name
| |
 | An empty string to choose the default from IO::out_dir
| |
out_every | Scope: restricted | INT |
Description: How often to output
| ||
| Range | Default: 1 | |
no output
| ||
1:* | output every to many iterations
| |
output_ascii | Scope: restricted | BOOLEAN |
Description: Output a simple ASCII file for each mode at each radius
| ||
| Default: yes | ||
output_hdf5 | Scope: restricted | BOOLEAN |
Description: Output an HDF5 file for each variable containing one dataset per mode at each
radius
| ||
| Default: no | ||
radius | Scope: restricted | REAL |
Description: The radii for extraction
| ||
| Range | Default: 0.0 | |
0.0:* | Please keep it in the grid
| |
test_l | Scope: restricted | INT |
Description: which mode to put into the test variables
| ||
| Range | Default: 2 | |
* | Any integer
| |
test_m | Scope: restricted | INT |
Description: which mode to put into the test variables
| ||
| Range | Default: 2 | |
* | Any integer
| |
test_mode_proportional_to_r | Scope: restricted | BOOLEAN |
Description: whether the test spherical harmonic coefficient is proportional to the radial
coordinate
| ||
| Default: no | ||
test_sw | Scope: restricted | INT |
Description: which spin weight to put into the test variables
| ||
| Range | Default: -2 | |
* | Any integer
| |
variables | Scope: restricted | STRING |
Description: What variables to decompose
| ||
| Range | Default: (none) | |
.* | A list of variables
| |
verbose | Scope: restricted | BOOLEAN |
Description: Output detailed information about what is happening
| ||
| Default: no | ||
io_out_dir | Scope: shared from IO | STRING |
Implements:
multipole
Inherits:
grid
| Group Names | Variable Names | Details | |
| harmonics | compact | 0 | |
| harmonic_re | description | Spherical harmonics | |
| harmonic_im | dimensions | 3 | |
| distribution | DEFAULT | ||
| group type | GF | ||
| timelevels | 1 | ||
| variable type | REAL | ||
| test_integration_convergence_orders | compact | 0 | |
| test_midpoint_convergence_order | description | Test integration convergence orders | |
| test_trapezoidal_convergence_order | dimensions | 0 | |
| test_simpson_convergence_order | distribution | CONSTANT | |
| group type | SCALAR | ||
| timelevels | 1 | ||
| variable type | REAL | ||
| test_integration_results | compact | 0 | |
| test_midpoint_result_low | description | Test integration results | |
| test_midpoint_result_high | dimensions | 0 | |
| test_trapezoidal_result_low | distribution | CONSTANT | |
| test_trapezoidal_result_high | group type | SCALAR | |
| test_simpson_result_low | timelevels | 1 | |
| test_simpson_result_high | variable type | REAL | |
| test_integration_symmetries | compact | 0 | |
| test_midpoint_pi_symmetry | description | Test integration symmetries | |
| test_trapezoidal_pi_symmetry | dimensions | 0 | |
| test_simpson_pi_symmetry | distribution | CONSTANT | |
| test_driscollhealy_pi_symmetry | group type | SCALAR | |
| timelevels | 1 | ||
| variable type | REAL | ||
| test_orthonormality | test_orthonormality | compact | 0 |
| description | Test orthonormality of spherical harmonics | ||
| dimensions | 2 | ||
| distribution | CONSTANT | ||
| group type | ARRAY | ||
| size | 1 | ||
| size | (10*10)*(10*10+1)/2 | ||
| timelevels | 1 | ||
| variable type | REAL | ||
This section lists all the variables which are assigned storage by thorn EinsteinAnalysis/Multipole. Storage can either last for the duration of the run (Always means that if this thorn is activated storage will be assigned, Conditional means that if this thorn is activated storage will be assigned for the duration of the run if some condition is met), or can be turned on for the duration of a schedule function.
| Conditional: | |
| harmonics[1] | |
| test_integration_convergence_orders | |
| test_integration_results | |
| test_integration_symmetries | |
| test_orthonormality | |
CCTK_ANALYSIS (conditional)
multipole_calc
calculate multipoles
| After: | calc_np | |
| psikadelia | ||
| accelerator_copyback | ||
| Language: | c | |
| Options: | global | |
| Type: | function | |
CCTK_INITIAL (conditional)
multipole_setharmonic
populate grid functions with spherical harmonics
| Language: | c | |
| Reads: | grid::coordinates(everywhere) | |
| Type: | function | |
| Writes: | multipole::harmonics(interior) | |
CCTK_INITIAL (conditional)
multipole_testorthonormality
loop over modes and integrate them to check orthonormality
| Language: | c | |
| Type: | function | |
CCTK_PARAMCHECK (conditional)
multipole_paramcheck
check multipole parameters
| Language: | c | |
| Options: | global | |
| Type: | function | |
CCTK_PARAMCHECK (conditional)
multipole_testintegrationconvergence
test convergence of integration
| Language: | c | |
| Type: | function | |
CCTK_PARAMCHECK (conditional)
multipole_testintegrationsymmetry
test symmetry of integration
| Language: | c | |
| Type: | function | |