Commit e8c4a9c7 authored by Matthieu Schaller's avatar Matthieu Schaller
Browse files

Added description of the unit system, physical constants and cosmology...

Added description of the unit system, physical constants and cosmology parameters in the YAML files.
parent 79256906
......@@ -176,7 +176,9 @@ as peculiar velocities divided by ``sqrt(a)``. This can be undone by swicthing
on the parameter ``InitialConditions:cleanup_velocity_factors`` in the
.. _ICs_units_label:
Optional Components
......@@ -64,17 +64,139 @@ ignored (for instance because the code is not configured to use some
The rest of this page describes all the SWIFT parameters, split by
section. A list of all the possible parameters is kept in the file
This section describes the units used internally by the code. This is
the system of units in which all the equations are solved. All
physical constants are converted to this system and if the ICs use a
different system (see :ref:`ICs_units_label`) the particle quantities
will be converted when read in.
The system of units is described using the value of the 5 basic units
of any system with respect to the CGS system. Instead of using a unit
of time we use a unit of velocity as this is more intuitive. Users
hence need to provide:
* a unit of length: ``UnitLength_in_cgs``,
* a unit of mass: ``UnitMass_in_cgs``,
* a unit of velocity ``UnitVelocity_in_cgs``,
* a unit of electric current ``UnitCurrent_in_cgs``,
* a unit of temperature ``UnitTemp_in_cgs``.
All these need to be expressed with respect to their cgs counter-part
(i.e. :math:`cm`, :math:`g`, :math:`cm/s`, :math:`A` and :math:`K`). Recall
that there are no h-factors in any of SWIFT's quantities; we, for instance,
use :math:`cm` and not :math:`cm/h`.
For instance to use the commonly adopted system of 10^10 Msun as a
unit for mass, mega-parsec as a unit of length and km/s as a unit of
speed, we would use:
.. code:: YAML
# Common unit system for cosmo sims
UnitMass_in_cgs: 1.98848e43 # 10^10 M_sun in grams
UnitLength_in_cgs: 3.08567758e24 # 1 Mpc in centimeters
UnitVelocity_in_cgs: 1e5 # 1 km/s in centimeters per second
UnitCurrent_in_cgs: 1 # 1 Ampere
UnitTemp_in_cgs: 1 # 1 Kelvin
Note that there are currently no variables in any of the SWIFT physics
schemes that make use of the unit of electric current. There is also
no incentive to use anything else than Kelvin but that makes the whole
system consistent with any possible unit system.
If one is interested in using the more humourous `FFF unit
system <>`_ one would use
.. code:: YAML
# FFF unit system
UnitMass_in_cgs: 40823.3133 # 1 Firkin (fir) in grams
UnitLength_in_cgs: 20116.8 # 1 Furlong (fur) in cm
UnitVelocity_in_cgs: 0.01663095 # 1 Furlong (fur) per Fortnight (ftn) in cm/s
UnitCurrent_in_cgs: 1 # 1 Ampere
UnitTemp_in_cgs: 1 # 1 Kelvin
The value of the physical constants in this system is left as an
exercise for the reader [#f1]_.
For some idealised test it can be useful to overwrite the value of
some physical constants. In particular the value of the gravitational
constant. SWIFT offers an optional parameter to overwrite the value of
that constant.
.. code:: YAML
G: 1
Note that this set :math:`G` to the specified value in the internal system
of units. Setting a value of `1` when using the system of units (10^10 Msun,
Mpc, km/s) will mean that :math:`G_N=1` in these units [#f2]_ instead of the
normal value :math:`G_N=43.00927`.
This option is only used for specific tests and debugging.
When running a cosmological simulation, this section set the values of the
cosmological model. The epanded :math:`\Lambda\rm{CDM}` parameters governing the
background evolution of the Univese need to be specified here. These are:
* The reduced Hubble constant: :math:`h`: ``h``,
* The matter density parameter :math:`\Omega_m`: ``Omega_m``,
* The cosmological constant density parameter :math:`\Omega_\Lambda`: ``Omega_lambda``,
* The baryon density parameter :math:`\Omega_b`: ``Omega_b``,
* The radiation density parameter :math:`\Omega_r`: ``Omega_r``.
The last parameter can be omitted and will default to :math:`\Omega_r = 0`.
This section als specifies the start and end of the simulation expressed in
terms of scale-factors. The two parameters are:
* Initial scale-factor: ``a_begin``,
* Final scale-factor: ``a_end``.
Two additional optional parameters can be used to change the equation of
state of dark energy :math:`w(a)`. We use the evolution law :math:`w(a) =
w_0 + w_a (1 - a)`. The two parameters in the YAML file are:
* The :math:`z=0` dark energy equation of state parameter :math:`w_0`: ``w_0``
* The dark energy equation of state evolutio parameter :math:`w_a`: ``w_a``
If unspecified these parameters default to the default
:math:`\Lambda\rm{CDM}` values of :math:`w_0 = -1` and :math:`w_a = 0`.
For a Planck+13 cosmological model (ignoring radiation density as is
commonly done and running from :math:`z=127` to :math:`z=0`, one would hence
use the following parameters:
.. code:: YAML
a_begin: 0.0078125 # z = 127
a_end: 1.0 # z = 0
h: 0.6777
Omega_m: 0.307
Omega_lambda: 0.693
Omega_b: 0.0455
Omega_r: 0. # (Optional)
w_0: -1.0 # (Optional)
w_a: 0. # (Optional)
......@@ -100,3 +222,7 @@ DomainDecomposition
.. [#f1] The thorough reader (or overly keen SWIFT tester) would find that the speed of light is :math:`c=1.8026\times10^{12}\,\rm{fur}\,\rm{ftn}^{-1}`, Newton's contant becomes :math:`G_N=4.896735\times10^{-4}~\rm{fur}^3\,\rm{fir}^{-1}\,\rm{ftn}^{-2}` and Planck's constant turns into :math:`h=4.851453\times 10^{-34}~\rm{fur}^2\,\rm{fir}\,\rm{ftn}^{-1}`.
.. [#f2] which would translate into a constant :math:`G_N=1.5517771\times10^{-9}~cm^{3}\,g^{-1}\,s^{-2}` if expressed in the CGS system.
Supports Markdown
0% or .
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment