Runtime Parameters: Timestep

This page describes various timestep constraints. See also Sections 2.1 and 2.6 in the GAMER-2 code paper.

Compilation Options

Related options: none

Runtime Parameters

Parameters described on this page: DT__MAX,   DT__FLUID,   DT__FLUID_INIT,   DT__SPEED_OF_LIGHT,   DT__GRAVITY,   DT__PARVEL,   DT__PARVEL_MAX,   DT__PARACC,   DT__CR_DIFFUSION,   DT__MAX_DELTA_A,   DT__SYNC_PARENT_LV,   DT__SYNC_CHILDREN_LV,   OPT__DT_USER,   OPT__DT_LEVEL,   OPT__RECORD_DT,   AUTO_REDUCE_DT,   AUTO_REDUCE_DT_FACTOR,   AUTO_REDUCE_DT_FACTOR_MIN,   AUTO_REDUCE_MINMOD_FACTOR,   AUTO_REDUCE_MINMOD_MIN,   AUTO_REDUCE_INT_MONO_FACTOR,   AUTO_REDUCE_INT_MONO_MIN  

Other related parameters: [[]]  

Parameters below are shown in the format:   Name   (Valid Values)   [Default Value]

• DT__MAX   (≥0.0; <0.0 → off)   [-1.0]

  • Description: Maximum allowed time-step on all levels. This can be used to, for example, avoid too large time-steps on lower levels.
  • Restriction:

• DT__FLUID   (≥0.0; <0.0 → set to default)   [-1.0]

  • Description: Courant–Friedrichs–Lewy (CFL) safety factor for the hydrodynamic solver. The default value and stable regime depend on the adopted fluid scheme. See Section 2.6, Eqs. [1-2] in the GAMER-2 code paper for the exact formulae.
  • Restriction:

• DT__FLUID_INIT   (≥0.0; <0.0 → set to default)   [DT__FLUID]

  • Description: CFL safety factor for the hydrodynamic solver at the first step. This could be useful when the first step requires a much smaller timestep.
  • Restriction: Useless for restart.

• DT__SPEED_OF_LIGHT   (0=off, 1=on)   [0]

  • Description: Using the speed of light as the maximum information propagation speed for determining timesteps in special relativistic hydrodynamic simulations.
  • Restriction: Only applicable when adopting the compilation option SRHD.

• DT__GRAVITY   (≥0.0; <0.0 → set to default)   [-1.0]

  • Description: Safety factor when determining timestep from the gravitational acceleration of fluid. See Section 2.6, Eq. [3] in the GAMER-2 code paper for the exact formula.
  • Restriction:

• DT__PARVEL   (≥0.0)   [0.5]

  • Description: Safety factor when determining timestep from the particle velocity. See Section 2.6, Eq. [4] in the GAMER-2 code paper for the exact formula.
  • Restriction:

• DT__PARVEL_MAX   (≥0.0; <0.0 → off)   [-1.0]

  • Description: Maximum allowed value of the timestep determined from the particle velocity. This could be useful when, for example, all particles are initially at rest, for which the timestep determined from the particle velocity is infinity.
  • Restriction:

• DT__PARACC   (>0.0; ≤0.0 → off)   [0.5]

  • Description: Safety factor when determining timestep from the particle acceleration. See Section 2.6, Eq. [3] in the GAMER-2 code paper for the exact formula.
  • Restriction: Only applicable when adopting the compilation option STORE_PAR_ACC.

• DT__CR_DIFFUSION   (≥0.0)   [0.3]

  • Description: CFL safety factor for cosmic-ray diffusion.
  • Restriction: Only applicable when adopting the compilation option CR_DIFFUSION.

• DT__MAX_DELTA_A   (≥0.0)   [0.01]

  • Description: Maximum allowed fraction of increase in the cosmic scale factor a. Specifically, it enforces Δa ≤ DT__MAX_DELTA_A ⋅ a.
  • Restriction: Only applicable when adopting the compilation option COMOVING.

• DT__SYNC_PARENT_LV   (≥0.0)   [0.1]

  • Description: Allow timestep to increase by 1.0+DT__SYNC_PARENT to help synchronize with the parent level. See also Section 2.1 in the GAMER-2 code paper for more details
  • Restriction: For OPT__DT_LEVEL=3 only.

• DT__SYNC_CHILDREN_LV   (≥0.0; <0.0 → off)   [0.1]

  • Description: Allow timestep to decrease by 1.0+DT__SYNC_CHILDREN_LV to help synchronize with the children level. See also Section 2.1 in the GAMER-2 code paper for more details
  • Restriction: For OPT__DT_LEVEL=3 only.

• OPT__DT_USER   (0=off, 1=on)   [0]

  • Description: Enable a user-defined timestep criterion. Edit src/Miscellaneous/Mis_GetTimeStep_User.cpp or a problem-specific function (for the latter, see Add Problem Specific Functionalities).
  • Restriction:

• OPT__DT_LEVEL   (1=shared, 2=differ by two, 3=flexible)   [3]

  • Description: Constraints on the timesteps of adjacent levels.
    • 1: All levels must share the same timestep
    • 2: Timesteps of a parent level is fixed to be twice smaller than its parent level
    • 3: No constraint (except that the timestep of a children level cannot be larger than that of a parent level)
  • Restriction:

• OPT__RECORD_DT   (0=off, 1=on)   [1]

  • Description: Record the timesteps set by various constraints in the file Record__TimeStep.
  • Restriction:

• AUTO_REDUCE_DT   (0=off, 1=on)   [1]

  • Description: Automatically reduce timestep by a factor of AUTO_REDUCE_DT_FACTOR, MINMOD_COEFF by a factor of AUTO_REDUCE_MINMOD_FACTOR, and INT_MONO_COEFF by a factor of AUTO_REDUCE_INT_MONO_FACTOR when the program fails until the timestep reducing factor becomes smaller than AUTO_REDUCE_DT_FACTOR_MIN or the interpolation coefficients become smaller than AUTO_REDUCE_MINMOD_MIN or AUTO_REDUCE_INT_MONO_MIN.
  • Restriction: For OPT__DT_LEVEL=3 only.

• AUTO_REDUCE_DT_FACTOR   (>0.0)   [1.0]

  • Description: See AUTO_REDUCE_DT.
  • Restriction:

• AUTO_REDUCE_DT_FACTOR_MIN   (≥0.0)   [0.1]

  • Description: See AUTO_REDUCE_DT.
  • Restriction:

• AUTO_REDUCE_MINMOD_FACTOR   (>0.0)   [0.8]

  • Description: See AUTO_REDUCE_DT.
  • Restriction: Must enable AUTO_REDUCE_DT.

• AUTO_REDUCE_MINMOD_MIN   (≥0.0)   [0.01]

  • Description: See AUTO_REDUCE_DT.
  • Restriction: Must enable AUTO_REDUCE_DT.

• AUTO_REDUCE_INT_MONO_FACTOR   (>0.0)   [0.8]

  • Description: See AUTO_REDUCE_DT.
  • Restriction: Must enable AUTO_REDUCE_DT.

• AUTO_REDUCE_INT_MONO_MIN   (≥0.0)   [0.01]

  • Description: See AUTO_REDUCE_DT.
  • Restriction: Must enable AUTO_REDUCE_DT.

Links

• Main page of Runtime Parameters