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__FLUID,   DT__FLUID_INIT,   DT__GRAVITY,   DT__PHASE,   DT__PARVEL,   DT__PARVEL_MAX,   DT__PARACC,   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  

Other related parameters: [[]]  

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

• 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 for the cases where the first step requires a much smaller timestep.
  • Restriction: Useless for restart.

• 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 can be useful when, for example, all particles are initially at rest, for which the timestep can be 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__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 adjacent levels must differ by 2
    • 3: The only constraint is 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 when the program fails until the timestep reducing factor becomes smaller than AUTO_REDUCE_DT_FACTOR_MIN.
  • Restriction: For OPT__DT_LEVEL=3 only.

• AUTO_REDUCE_DT_FACTOR   (>0.0)   [0.8]

  • Description: See AUTO_REDUCE_DT.
  • Restriction:

• AUTO_REDUCE_DT_FACTOR_MIN   (≥0.0)   [0.1]

  • Description: See AUTO_REDUCE_DT.
  • Restriction:

Links

• Main page of Runtime Parameters