7.1.16 Radiation-transport control (general)

In this part of the parameter file the radiation transport module has to be selected. Depending on this selection a couple of additional parameters have to be specified. Table 7.2 gives a list of the parameters and the modules they apply to. The standard routines are now in the MSrad module for local models and the SHORTrad module for global ``star-in-a-box'' models. The LHDrad module is not maintained very much anymore.

Table 7.2: List of radiation transport control parameters and the modules they are relevant for.

Parameter	Section	LHDrad	MSrad	SHORTrad
radscheme	7.1.16.1	*	*	*
opafile	7.1.7.1	*	*	*
opapath	7.1.7.2	*	*	*
n_radband	7.1.16.2		*	*
n_radminiter	7.1.16.3	*	*	*
n_raditer	7.1.16.4	*	*	*
n_radmaxiter	7.1.16.5	*	*	*
radraybase	7.1.16.6	*	*	*
radraystar	7.1.16.7	*		*
c_tminlimit	7.1.16.8		*	*
c_radimplicitmu	7.1.16.9	*		*
c_raditereps	7.1.16.10	*		*
c_raditerstep	7.1.16.11	*		*
n_radtheta	7.1.17.1		*
n_radphi	7.1.17.2		*
n_radsubray	7.1.17.3		*
n_radthickpoint	7.1.17.4	*	*
n_radtaurefine	7.1.17.5	*	*
n_radrsyslevel	7.1.17.6		*
n_radoutput	7.1.17.7		*
c_radtcool	7.1.17.8		*
c_raddcool	7.1.17.9		*
c_radscool	7.1.17.10		*
c_radtinci	7.1.17.11		*
c_raddinci	7.1.17.12		*
c_radtvisdtau	7.1.18.1	*
c_radtvis	7.1.18.2	*
c_radhtautop	7.1.4.19		*	*
c_radcourant	7.1.20.13	*	*	*
c_radcourantmax	7.1.20.14	*	*	*
c_radmaxeichange	7.1.20.15	*	*	*

7.1.16.1 character radscheme

So far, there exist three different radiation transport modules. The active on can be selected e.g. with

character radscheme f=A80 b=80 n='Radiation transport scheme' &
  c0='LHDrad/MSrad/SHORTrad' &
  c1='None (skip radiation transport step entirely)'
SHORTrad

Possible values are

• None: Skip radiation transport entirely.
• LHDrad: (old ``supergiant module'') It uses long characteristics and is restricted to an equidistant grid and open boundaries at all surfaces. Note that the switch -Drhd_r01=1 has to be set during compilation (see Sect.4.4.7.1), usually by setting export F90_LHDRAD=1 before calling the configure script (see Sect.4.3.1.8).
• MSrad: (``solar module'', local models, ``box-in-a-star'' models) It uses long characteristics. The lateral boundaries have to be periodic. Top and bottom can be closed or open. Note that the switch -Drhd_r02=1 has to be set during compilation (see Sect.4.4.7.2), usually by setting export F90_MSRAD=1 before calling the configure script (see Sect.4.3.1.9).
• SHORTrad: (new ``supergiant module'', global models, ``star-in-a-box'' models) It uses short characteristics and is restricted to an equidistant grid and open boundaries at all surfaces. Note that the switch -Drhd_r03=1 has to be set during compilation (see Sect.4.4.7.3), usually by setting export F90_SHORTRAD=1 before calling the configure script (see Sect.4.3.1.10).

7.1.16.2 integer n_radband

It can be specified whether the grey opacity table or the binned frequency-dependent part of the opacity table is used during the computation. The grey part contains only one bin. The other (possibly non-grey) contains one or more bins depending on the table chosen. The parameter is specified with e.g.

integer n_radband f=I4 b=4 n='Number of frequency bins' &
  c0='1: grey opacities' &
  c1='2: non-grey opacities (if available from table)' &
  c2='3: two bands: 1.grey, 2.CO (density via XCO)' &
  c3='4: two bands: 1.grey, 2.CO (density from chemistry)'
1

Allowed values are

• 1: Use the grey part of the table
• 2: Use the other (possibly non-grey, frequency-dependent) part of the table
• 3: Use a continuum band plus an infrared band with CO opacity, calculated with CO equilibrium density
• 4: Use a continuum band plus an infrared band with CO opacity, calculated with CO density resulting from time-dependent chemistry

7.1.16.3 integer n_radminiter

Usually the stability considerations dictate a radiative time step smaller than the hydrodynamics or tensor-viscosity time step. To remedy this situation it is possible to allow several radiation transport steps per global time step. Hitherto, all three radiation transport modules support this iteration. The minimum number of iterations (radiative sub-steps) can be specified e.g. with

integer n_radminiter f=I4 b=4 &
  n='Minimum number of radiation transport iterations' c0=8
1

If less iterations are needed the time step limit for the next step is increased. This value will in almost any case (for explicit radiation transport) be set to 1. In the implicit case it is set to a higher value (typically 5).

7.1.16.4 integer n_raditer

After each complete radiative time step the recommendation for the next time step will be chosen so that n_raditer iterations will (probably) needed. The parameter can be set e.g. with

integer n_raditer f=I4 b=4 &
    n='Number of radiation transport iterations' c0=10
8

For a simulation of a solar-type star (with comparatively long radiative time scales) it will typically be set to 1. For stars with shorter radiative time scales values around 10 may be considered. All three radiation transport modules understand this parameter.

7.1.16.5 integer n_radmaxiter

The absolute maximum number of iterations can be specified e.g. with

integer n_radmaxiter f=I4 b=4 &
    n='Maximum number of rad. transport iterations' c0=30
0

If more iterations are needed the computation for the current time step is stopped and resumed with a smaller one (which means that the hydrodynamics and the tensor-viscosity step have to be done again). Usually, n_radmaxiter will either be set to a value somewhat larger than the recommended number of iterations (n_raditer) or to 0 which disables the check for too many iterations completely. This can be safely allowed in many cases and has the advantage that there is no need to save the initial model before calling the radiation transport module, which saves some memory. To disable the iteration of the radiation transport sub-step set n_radminiter=n_raditer=n_radmaxiter=1. All three radiation transport modules understand this parameter.

7.1.16.6 character radraybase

Using the modules LHDrad or SHORTrad, the orientation of the base axis system can be selected e.g. with

character radraybase f=A80 b=80 n='Base axis system' &
  c0='SHORTrad: unity/random/randomcube/randomgroup/alternate' &
  c1='MSrad: lobatto/dblgaus'
random

Allowed values for radscheme=SHORTrad are:

• unity: (default) During all time steps and radiative sub-steps the direction of the rays stays the same.
• random: At each time step (and radiative sub-step) a new base axis system is chosen at random
• randomcube: The distribution of the random angles is tuned to optimize the isotropy of the time-averaged radiation field. Use together with radraystar=cube.
• randomgroup: At each new time step a new base axis system is chosen at random. It is kept for all radiative sub-steps.
• alternate: Alternate between unity and random orientation matrix.
• drift01: Slow drift, algorithm 1.
• drift02: Slow drift, algorithm 2.

Because typically only a relatively small number of rays is chosen per time step (with radraystar) it is advisable to vary the directions of the rays (by choosing radraybase=random or randomgroup) to cover the entire sphere at least over a longer time.
Allowed values for radscheme=MSrad are:

• lobatto:
• dblgaus:

7.1.16.7 character radraystar

Using the modules LHDrad or SHORTrad, the list of ray directions (i.e. the number of rays and their coordinates) relative to the base axis system can be specified with e.g.

character radraystar f=A80 b=80 n='List of relative ray directions' &
  c0='x1(1)/x2(1)/x3(1)/oktaeder(3)/tetraeder(4)/cube(4)' &
  c1='ikosaeder(6)/dodekaeder(10)'
oktaeder

Examples for allowed values are

• x1: (N=1) one single ray along x1 axis (not enough to specify fluxes in all directions)
• x2: (N=1) one single ray along x2 axis (not enough to specify fluxes in all directions)
• x3: (N=1) one single ray along x3 axis (not enough to specify fluxes in all directions)
• oktaeder: (N=3, default) octahedron
• tetraeder: (N=4) tetrahedron
• cube: (N=4)
• ikosaeder: (N=6) icosahedron
• dodekaeder: (N=10) dodecahedron
• list-01, list-01(3): Choose ray systems from a list (oktahedrons, tetrahedrons). If character radraybase is set to unity the rays will only be aligned to the axes or diagonals and thus avoid the time-consuming interpolation step of the short-characteristics method.

Several other choices are possible, which are meant for test purposes only. Choosing one of the five Platonic solids (Oops! ``German-Greek'' names only, so far) means that the 3 to 10 rays are equally distributed over the solid angle (from the center to each corner of the respective solid). Check the complete list of options in subroutine rhd_rad3d_RaySystem.

7.1.16.8 real c_tminlimit

A minimum temperature can be set, that is enforced by the radiation transport modules MSrad3D and SHORTrad by adding energy to the too cool cells, e.g. with

real c_tminlimit f=E15.8 b=4 &
  n='Enforced minimum temperature for radiation transport step' u=K &
  c0='<=0.0: off (default); 500.0: reasonable value'
400.0

7.1.16.9 real c_radimplicitmu

So far, only the LHDrad and the SHORTrad module support implicit radiation transport. It can be activated with the parameter

real c_radimplicitmu f=E15.8 b=4 &
    n='Implicitness parameter for radiation transport'            u=1 &
    c0='0.0: explicit / 0.5: time centered / 1.0: fully implicit'
0.0

Allowed values are

• 0.0: Fully explicit radiation transport (possible with all modules)
• 0.0 $<$ C $<$ 1.0: Partly implicit radiation transport
• 0.5: Radiation transport time-centered
• 1.0: Fully implicit radiation transport

Values outside this range do not have much meaning. The implicit transport does not work efficiently yet: It does not yield significantly larger time steps than possible with a sequence of purely explicit sub time steps. Additionally, it turns out that the hydrodynamics runs into trouble if a too large time step (still well within the Courant condition) is requested.

7.1.16.10 real c_raditereps

With activated implicit radiation transport (LHDrad and SHORTrad modules only) the requested convergence accuracy of the iteration can be set e.g. with

real c_raditereps f=E15.8 b=4 &
    n='Relative accuracy for radiation iteration'                 u=1 &
    c0='Typical value: 1.0E-03'
2.0E-03

7.1.16.11 real c_raditerstep

With activated implicit radiation transport (LHDrad and SHORTrad modules only) the step size of the iteration can be restricted with e.g.

real c_raditerstep f=E15.8 b=4 &
    n='Step size of radiation iteration'                          u=1 &
    c0='Typical values: 0.7,0.81'
1.0

Allowed values are

• 0.0 $<$ 1.0: Restricted step size
• 1.0: No restriction, standard step size
• $>$ 1.0: Extra large steps

This value has to be chosen carefully to get optimal performance. Is the step size too small the convergence is safe but too slow. A too large step size inhibits convergence and leads to a decrease in the time step, which results in a bad performance, too.