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SWIFT
SWIFTsim
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e7d2480c
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e7d2480c
authored
Nov 03, 2018
by
Matthieu Schaller
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Added description of the gravity related parameters to the RTD.
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ef9a1474
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doc/RTD/source/ParameterFiles/index.rst
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e7d2480c
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@@ -185,8 +185,62 @@ The behaviour of the self-gravity solver can be modifed by the parameters
provided in this section. The theory document puts these parameters into the
context of the equations being solved. We give a brief overview here.
* The co-moving softening used for all particles :math:`\epsilon_{c}`: ``comoving_softening``
* The
* The Plummer-equivalent co-moving softening length used for all particles :math:`\epsilon_{com}`: ``comoving_softening``,
* The Plummer-equivalent maximal physical softening length used for all particles :math:`\epsilon_{max}`: ``comoving_softening``,
At any redshift :math:`z`, the Plummer-equivalent softening length used by the
code will be :math:`\epsilon=\min(\epsilon_{max},
\frac{\epsilon_{com}}{z+1})`. This is expressed in internal units.
* The opening angle (multipole acceptance criterion) used in the FMM :math:`\theta`: ``theta``,
* The time-step size pre-factor :math:`\eta`: ``eta``,
The time-step of a given particle is given by :math:`\Delta t =
\eta\sqrt{\frac{\epsilon}{|\overrightarrow{a}|}}`, where
:math:`\overrightarrow{a}` is the particle's acceleration. Power et al. (2003) recommend using :math:`\eta=0.025`.
The last tree-related parameter is
* The tree rebuild frequency: ``rebuild_frequency``.
Thqe tree rebuild frequency is an optional parameter defaulting to
:math:`0.01`. It is used to trigger the re-construction of the tree every time a
fraction of the particles have been integrated (kicked) forward in time.
Simulations using periodic boundary conditions use additional parameters for the
Particle-Mesh part of the calculation. The last three are optional:
* The number cells along each axis of the mesh :math:`N`: ``mesh_side_length``,
* The mesh smoothing scale in units of the mesh cell-size :math:`a_{\rm
smooth}`: ``a_smooth`` (default: ``1.25``),
* The scale above which the short-range forces are assumed to be 0 (in units of
the mesh cell-size multiplied by :math:`a_{\rm smooth}`) :math:`r_{\rm
cut,max}`: ``r_cut_max`` (default: ``4.5``),
* The scale bewlo which the short-range forces are assumed to be exactly Newtonian (in units of
the mesh cell-size multiplied by :math:`a_{\rm smooth}`) :math:`r_{\rm
cut,min}`: ``r_cut_min`` (default: ``0.1``),
For most runs, the default values can be used. Only the number of cells along
each axis needs to be sepcified. The remaining three values are best described
in the context of the full set of equations in the theory documents.
As a summary, here are the values used for the EAGLE :math:`100^3~{\rm Mpc}^3`
simulation:
.. code:: YAML
# Parameters for the self-gravity scheme for the EAGLE-100 box
Gravity:
eta: 0.025
theta: 0.7
comoving_softening: 0.0026994 # 0.7 proper kpc at z=2.8.
max_physical_softening: 0.0007 # 0.7 proper kpc
rebuild_frequency: 0.01 # Default optional value
mesh_side_length: 512
a_smooth: 1.25 # Default optional value
r_cut_max: 4.5 # Default optional value
r_cut_min: 0.1 # Default optional value
SPH
---
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