5. Description of the parameters in PION parameter files¶

PION is controlled mainly by a parameter file that sets what kind of initial conditions to set up, what equations to use, the properties of the computational grid, source terms such as stellar wind, and numerical parameters specifying the integration scheme. There are described here.

5.1. Initial Conditions Generator¶

There are a number of different problems that can be set up with PION, with source files located in source/ics/:

Problem	Location of source file	Brief description
1Dto2D	`read_1Dto2D.cpp`	Read 1D snapshot and map to 2D or 3D grid
Advection	`basic_tests.cpp`	Sets up advection of an overdense clump
BlastWave	`blast_wave.cpp`	Set up supernova blastwave problems
DivBPeak	`basic_tests.cpp`	Sets up a uniform medium with a peak in \(\nabla\cdot\mathbf{B}\)
DoubleMachRef	`basic_tests.cpp`	The Double Mach-Reflection test
FieldLoop	`basic_tests.cpp`	Advection of a magnetic field loop
Jet	`jet.cpp`	Sets up 2D axisymmetric jet or 3D jet
KelvinHelmholtz	`basic_tests.cpp`	Set up Kelvin-Helmholtz instability test
LaserAblationAxi	`laser_ablation.cpp`	Set up a HEDP laser ablation test calc
LiskaWendroffImplosion	`basic_tests.cpp`	The Liska and Wendroff implosion test
OrszagTang	`basic_tests.cpp`	The 2D Orszag-Tang Vortex test
PE_MC_FN or PE_MC_FM	`photoevaporating_multiclumps.cpp`	Set up photoevaporation of dense clouds
PhotoEvaporatingClump	`photoevaporating_clump.cpp`	Sets up photoevaporation of dense cloud
PhotEvap_RandomClumps	`photoevaporating_random_clumps.cpp`	Set up photoevaporation of randomly placed dense clouds
RadiativeShock	`radiative_shock.cpp`	Sets up 1D or 2D radiative shock test
ShockCloud	`shock_cloud.cpp`	Set up shock-cloud interaction problems
Uniform	`basic_tests.cpp`	Sets up a uniform medium

5.2. Equations¶

There are three different types of equations that can be set up:

Euler equations: the inviscid equations of hydrodynamics.
Ideal MHD equations: magnetohydrodynamics, only usable in 1D because there is no scheme for dealing with DivB errors.
GLM-MHD equations: magnetohydrodynamics plus the Dedner et al. (2002) divergence cleaning mechanism for mitigating DivB errors.

There are a number of different flux solvers that can be used, with different levels of robustness and accuracy:

List of solvers

Parameter	Type	Units	Description
eqn	string	none	System of equations to solve: euler, mhd, glm-mhd
GAMMA	double	none	adiabatic index of the gas (usually 5/3=1.66666666666667)
solver	integer	none	Which flux solver to use (see above for options)
OrderOfAccSpace	integer	none	spatial order of accuracy: 1 = piecewise constant; 2 = piecewise linear
OrderOfAccTime	integer	none	temporal order of accurasy: 1 = 1st order piecewise constant; 2 = 2nd order, piecewise linear
CFL	double	none	Courant-Friedrichs-Lewy number (<1 1D, <0.5 2D, <0.33 3D)
ArtificialViscosity	integer	none	0=no AV, 1=Falle+(1998) AV, 3=Sanders+(1998) H-correction H-correction only works with some solvers.
EtaViscosity	double	none	Viscosity parameter for Falle+(1998) AV, in range [0,1], default value is 0.15.

5.3. Computational Grid¶

The PION computational grid is a uniform rectilinear grid in 1, 2 or 3 spatial dimensions. Static mesh-refinement (nested grids) is implemented for the case where each refinement level has the same shape and number of cells as the coarser level above it, but the spatial resolution is a factor of 2 higher and so the grid covers 1/2 of the domain of the coarser grid in each dimension. The focus of the nested grid can be at the centre of the domain, the negative boundary or the positive boundary for each dimension. The number of refinement levels is in principle unlimited, but snapshots can only be written every coarse timestep and this imposes a practical limit: for 10 grid levels there are 512 finest-level timesteps per coarsest-level step, and one probably wants to save a snapshot at least this often. The coarsest level is denoted level 0, and each refined level has a higher level number.

The parameters are:

Parameter	Type	Units	Description
ndim	integer	none	Number of spatial dimensions to simulate
coordinates	string	none	Geometry. 1D: spherical, 2D: cylindrical/cartesian, 3D: cartesian
NGridX	integer	none	Number of grid points in \(\hat{x}\)-direction
NGridY	integer	none	Number of grid points in \(\hat{y}\)-direction
NGridZ	integer	none	Number of grid points in \(\hat{z}\)-direction
Xmin	double	cm	negative boundary in \(\hat{x}\)-direction (level 0)
Ymin	double	cm	negative boundary in \(\hat{y}\)-direction (level 0)
Zmin	double	cm	negative boundary in \(\hat{z}\)-direction (level 0)
Xmax	double	cm	positive boundary in \(\hat{x}\)-direction (level 0)
Ymax	double	cm	positive boundary in \(\hat{y}\)-direction (level 0)
Zmax	double	cm	positive boundary in \(\hat{z}\)-direction (level 0)

5.4. Source terms¶

5.4.1. Stellar Winds¶

Stellar winds in PION are implemented in 3 modules: a constant wind, an evolving wind, and a latitude-dependent wind. The parameter file for a constant wind has a number of parameters that should be set, quoted below with the expected units. They are stored in these units in the SWP struct of type stellarwind_params.

Parameter	Description	Units/values
WIND_[i]_pos[n]	Star position \(n\in[0,1,2]\)	cm
WIND_[i]_velocity[n]	Star velocity \(n\in[0,1,2]\)	cm/s (if moving) default is 0
WIND_[i]_radius	Radius of wind injection region	cm
WIND_[i]_type	[Constant, evolving, latitude dependent]	[0,1,2]
WIND_[i]_mdot	Mass-loss rate from star (dot{M})	\(\mathrm{M_{\odot}\,yr}^{-1}\)
WIND_[i]_vinf	Terminal velocity of wind	\(\mathrm{km\,s}^{-1}\)
WIND_[i]_vrot	Equatorial rotation velocity	\(\mathrm{km\,s}^{-1}\)
WIND_[i]_temp	Effective Temperature of star, \(T_\mathrm{eff}\)	K
WIND_[i]_Rstr	Radius of star	cm
WIND_[i]_Bstr	Strength of surface magnetic field	Gauss
WIND_[i]_TR[n]	Value of tracer \(n\) in wind	Usually \(\in [0,1]\)
WIND_[i]_evofile	Text file with data for evolving wind	Default is NOFILE
WIND_[i]_t_offset	offset of sim time from evolution time	Default is 0.0
WIND_[i]_t_scalefac	scaling simulation time to evolution time	Default is 1.0
WIND_[i]_updatefreq	How often to update the wind	seconds
WIND_[i]_enhance_mdot	ad-hoc flag to increase \(\dot{M}\)	Default is 0
WIND_[i]_moving	is star moving? yes==1, no==0	Default is 0
WIND_[i]_acceleration	is wind accelerated? yes==1, no==0	Default is 0 (inject at v_inf)

For constant winds, these data are stored in the global struct SWP, defined in pion/source/constants.h, using these units. In simulation snapshots they also have the same units. The evolving wind file should have all units in CGS.