5.3.10 Radiation Transport Control

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 13 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 13: List of radiation transport control parameters and the modules they are relevant for.

Parameter	Section	LHDrad	MSrad	SHORTrad
radscheme	5.3.10	*	*	*
n_radminiter	5.3.10	*	*	*
n_raditer	5.3.10	*	*	*
n_radmaxiter	5.3.10	*	*	*
radraybase	5.3.10	*		*
radraystar	5.3.10	*		*
n_radtheta	5.3.10		*
n_radphi	5.3.10		*
n_radsubray	5.3.10		*
n_radthinpoint		*
n_radthickpoint	5.3.10	*	*
n_radtaurefine	5.3.10	*	*
n_radband	5.3.10		*
c_radimplicitmu	5.3.10	*		*
c_raditereps	5.3.10	*		*
c_raditerstep	5.3.10	*		*
c_radtvisdtau	5.3.10	*
c_radtvis	5.3.10	*
c_radhtautop	5.3.4		*
c_radcourant	5.3.12	*	*	*
c_radcourantmax	5.3.12	*	*	*
c_radmaxeichange	5.3.12	*	*	*

• 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. 3.7).
  • MSrad: (``solar module'') 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. 3.7).
  • SHORTrad: (new ``supergiant module'') 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. 3.7).

• 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).

• 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 starts with shorter radiative time scales values around 10 may be considered. All three radiation transport modules understand this parameter.

• 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 values somewhat larger than the recommended number of iterations (n_raditer) or to 0 which disable 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.

• 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='unity/random/randomgroup'
  random
  
  Allowed values 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
  • randomgroup: At each new time step a new base axis system is chosen at random. It is kept for all radiative sub-steps.
  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.

• 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 (Ops! ``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).

• integer n_radtheta:
  Using the MSrad module the ray directions have to specified in a different way: The number of ray sets in theta direction can be chosen with e.g.
  integer n_radtheta f=I4 b=4 &
      n='NTHETA: Number of ray sets in theta direction' c0=2
  2
  

• integer n_radphi:
  Using the MSrad module the number of ray sets in phi direction can be set e.g. with
  integer n_radphi f=I4 b=4 &
      n='NPHI: Number of ray sets in phi direction' c0=2
  2
  

• integer n_radsubray:
  Using the MSrad module the number of rays per cell (with the same direction) can be specified e.g. with
  integer n_radsubray f=I4 b=4 n='KPHI: Number of rays per cell' c0=2
  2
  

• integer n_radthickpoint:
  With the MSrad module the lower part of the model can be computed in diffusion approximation. The number of points in diffusion approximation can be set with e.g.
  integer n_radthickpoint f=I4 b=4 &
    n='Number of grid points with optically thick (diff.) approximation' &
    c0='0: no diffusion approximation'
  0
  
  The value should be choosen so that for all points in that region $\Delta\tau > 1$ is valid. Setting this value to 0 means that the diffusion approximation is not used in any part of the model.

• integer n_radtaurefine:
  With the LHDrad and the MSrad module the number of points on the rays can be finer than the number of points in the basic numerical grid. The refinement can be set e.g. with
  integer n_radtaurefine f=I4 b=4 &
    n='Refinement factor'
  0
  

• 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)'
  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
  Only the MSrad module so far can handle non-grey tables.

• real c_radimplicitmu:
  So far only the LHDrad and the SHORTrad module (tentatively) 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.

• real c_raditereps:
  With activated implicit radiation transport (LHDrad module 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
  

• real c_raditerstep:
  With activated implicit radiation transport (LHDrad module 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.

• real c_radtvisdtau:
  Using the LHDrad module the limit in delta optical depth (rho*kappa*dx) below which the ``radiative temperature viscosity'' (=temperature smoothing) is to be applied can be set with e.g.
  real c_radtvisdtau f=E15.8 b=4 &
    n='Optical depth limit for temperature viscosity' u=1
  0.1
  
  The introduction of this ``temperature diffusion'' is a somewhat desperate and inelegant attempt to improve the behavior of the Greens function (hot cells should be cooled, cool cells should be heated). This diffusion is necessary for not well resolved models. It is switched off with c_radtvisdtau $\leq$ 0.0.

• real c_radtvis:
  Using the LHDrad module the amount of the ``radiative temperature viscosity'' (=temperature smoothing) can be specified e.g. with
  real c_radtvis f=E15.8 b=4 n='Temperature viscosity' u=1
  1.6
  
  For well resolved models it should be switched off (with c_radtvis $\leq$ 0.0). But often its use is necessary.