Runtime Parameters: Units

Compilation Options

Related options: COMOVING  

Runtime Parameters

Parameters described on this page: OPT__UNIT,   UNIT_L,   UNIT_M,   UNIT_T,   UNIT_V,   UNIT_D  

Other related parameters: NEWTON_G,   ELBDM_MASS,   ELBDM_PLANCK_CONST  

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

• OPT__UNIT   (0=off, 1=on)   [0]

  • Description: Enable code units. One must also set exactly three units among UNIT_L, UNIT_M, UNIT_T, UNIT_V, UNIT_D (unless the compilation option COMOVING is adopted; see Units in Cosmological Simulations for details). See also Unit Consistency.
  • Restriction: It will be turned on automatically when enabling the compilation option COMOVING.

• UNIT_L   (>0.0; ≤0.0 → set by other units)   [none]

  • Description: Length unit in CGS.
  • Restriction: It will be overwritten when enabling the compilation option COMOVING. See Units in Cosmological Simulations for details.

• UNIT_M   (>0.0; ≤0.0 → set by other units)   [none]

  • Description: Mass unit in CGS.
  • Restriction: It will be overwritten when enabling the compilation option COMOVING. See Units in Cosmological Simulations for details.

• UNIT_T   (>0.0; ≤0.0 → set by other units)   [none]

  • Description: Time unit in CGS.
  • Restriction: It will be overwritten when enabling the compilation option COMOVING. See Units in Cosmological Simulations for details.

• UNIT_V   (>0.0; ≤0.0 → set by other units)   [none]

  • Description: Velocity unit in CGS.
  • Restriction: It will be overwritten when enabling the compilation option COMOVING. See Units in Cosmological Simulations for details.

• UNIT_D   (>0.0; ≤0.0 → set by other units)   [none]

  • Description: Mass density unit in CGS.
  • Restriction: It will be overwritten when enabling the compilation option COMOVING. See Units in Cosmological Simulations for details.

Remarks

Unit Consistency

When enabling units (i.e., OPT__UNIT=1), all input values (e.g., runtime parameters, refinement thresholds, and initial conditions) must be set consistently with the adopted unit system unless otherwise specified. All input physical constants (e.g., NEWTON_G, ELBDM_MASS, and ELBDM_PLANCK_CONST) will be reset automatically to conform to the adopted unit system.

The code does not distinguish external and internal units; In other words, all internal variables in the code should also conform to the same unit system (except for the predefined physical constants).

Physical Constants

Some physical constants in CGS units are predefined in the header include/PhysicalConstant.h with the prefix Const_. They can be useful for unit conversion. For example, to convert particle velocity vel_code from code units to km/s, one can use vel_kms = vel_code*UNIT_V*Const_s/Const_km;.

Units in Cosmological Simulations

When enabling COMOVING, the unit system is currently fixed as follows.

• UNIT_L = h^-1 Mpc, where h=HUBBLE0 is the dimensionless Hubble constant.
• UNIT_T = H₀^-1, where H₀ = 100 h km s^-1 Mpc^-1 is the Hubble constant.
• UNIT_D = ρ_m0 = 3Ω_m0H₀² / 8πG, where ρ_m0 is the present matter density, Ω_m0=OMEGA_M0 is the present matter density parameter, and G is the gravitational constant.

Links

• Main page of Runtime Parameters