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Compile-time parameters

As mentioned in the Quickstart guide, compile-time parameters are baked into the executable at build time — changing any of them requires a fresh make. They live in parameters.h inside your build_archive/ directory (copied there from prob/PROB/parameters.h when you first compile), so you can edit them freely without touching the original repository.

The torus problem (prob/torus/parameters.h) is a good representative example, and we will use it throughout this page.

Grid resolution

#define N1TOT 256
#define N2TOT 256

N1TOT and N2TOT set the total number of grid zones in the \(x^1\) and \(x^2\) coordinate directions, respectively. In Kerr-Schild coordinates, as used in the black hole accretion setup, these directions map to functions of the radial coordinate \(r\) and polar angle \(\theta\). Unlike iharm3d, there is no companion NiCPU parameter here — iharm2d_v4 does not use MPI, so the NiTOT values fully describe the grid with no further decomposition.

Metric and coordinate system

#define METRIC MKS
#define DEREFINE_POLES 1

METRIC defines the spacetime metric used in the simulation. The two supported values are:

  • MINKOWSKI — flat spacetime in Cartesian coordinates, appropriate for many test problems.
  • MKS — Modified Kerr-Schild coordinates, employing an exponential radial coordinate and a modified polar coordinate that increases the resolution close to the midplane. The exact transformation is given by Equation (F1) in the PATOKA paper.
  • DEREFINE_POLES takes a value of 0 or 1. Setting it to 1 activates FMKS (Funky MKS) coordinates, which enlarge zones in the \(\theta\) direction near the polar axis and close to the event horizon. This relaxes the severe timestep restriction caused by very small cells, whose light-crossing times become extremely short. For torus problems, leaving this at 1 is strongly recommended.

Floors

#define WIND_TERM    0
#define BSQORHOMAX   100.
#define UORHOMAX     100.

Floors prevent the fluid state from reaching unphysical values in low-density regions. BSQORHOMAX and UORHOMAX cap the maximum allowed ratios of magnetic energy density to rest-mass density (plasma magnetization) and internal energy density to rest-mass density (proportional to temperature), respectively. The defaults of 100. are appropriate for most torus runs; you may want to raise them if your problem has strongly magnetised jets or lower them to keep the floors from activating too aggressively. Additional floors include BSQOUMAX (which caps the inverse plasma beta), geometric floors (RHOMIN and UUMIN) that inject rest-mass density and internal energy as functions of radius, and a velocity ceiling (GAMMAMAX). If these are not set in parameters.h, their default values are taken from decs.h; see that file for the complete list. WIND_TERM enables a small mass-injection source term (1) that can help stabilise the evacuated funnel region in torus problems; it is off (0) by default.

Electron thermodynamics

#define ELECTRONS           0
#define ALLMODELS           0
#define SUPPRESS_HIGHB_HEAT 1

Setting ELECTRONS to 1 activates a separate electron entropy equation, enabling two-temperature plasma modelling. This is based on the entropy-tracking procedure of Ressler et al. (2015). ALLMODELS (0 or 1) tracks separate electron entropies for all built-in heating prescriptions simultaneously, which avoids re-running the GRMHD simulation for each heating model. SUPPRESS_HIGHB_HEAT (0 or 1) disables electron heating in strongly magnetised regions (magnetisation \(\sigma > 1\)) when set to 1. NOTE: The electron heating prescription is applicable to the black hole accretion problem for which it was devised.

Reconstruction algorithm

#define RECONSTRUCTION WENO

Controls the spatial interpolation scheme used to interpolate the primitive variables to zone faces for flux calculation. The two options are:

  • LINEAR — piecewise-linear reconstruction with monotonized central (MC) slope limiter; faster but more diffusive.
  • WENO — fifth-order weighted essentially non-oscillatory reconstruction; more accurate and the recommended default.

Boundary conditions

#define X1L_BOUND OUTFLOW
#define X1R_BOUND OUTFLOW
#define X2L_BOUND POLAR
#define X2R_BOUND POLAR

#define X1L_INFLOW 0
#define X1R_INFLOW 0

The four _BOUND parameters set the boundary condition at each face of the domain. Supported values are:

  • OUTFLOW — outflow boundary condition; copy the outermost active zone into all ghost zones. Material flows out freely of the domain. Standard for the radial (X1) boundaries in a black hole accretion simulation.
  • POLAR — reflective polar boundary; reflect the fluid state across the polar axis. The sign of U2 and B2 is flipped across the polar boundary. Required for the X2 boundaries when using MKS coordinates.
  • PERIODIC — periodic boundary, used in most flat-space test problems.

X1L_INFLOW and X1R_INFLOW (0 or 1) control whether inflow through the corresponding OUTFLOW boundary is permitted. Setting either to 0 prevents matter from being pulled back into the domain if the velocity at that face reverses — a common safeguard at the inner radial boundary near the black hole horizon.