New data format
The data files have changed format. Gas properties are now defined
only for gas particles; the gas particles must precede dark matter
particles in the arrays and the order of the arrays has changed. The
new data format is described in the DOCUMENTATION.
A conversion program to change old-style to new-style data formats is
provided (see UTILITIES).
New units
The i/o units for non-expanding simulations have changed. The
positions are now given in length units, as specified in inunit.F
(they used to be in units of the box100). The boxsize and all
cosmological parameters are now ignored.
The input positions for isolated simulations need no longer be in
the range [0,1). It is possible to (optionally) specify an outer
cut-off radius from the centre of the particle distribution, beyond
which particles are ignored.
If you do non-expanding, periodic boxes, the the positions need to be
in the range [0,1), but box100 is no longer used to fix the units;
hence the boxsize must be specified as lunit.
New program structure Modifications to the SPH New -DBYTE_SWAP option New -DCPUTIME option CMBFAST transfer functions Change to the SPH New -DEXACTNSPH option New -DENTROPY option New -DPHOTO_SS option New -DSAVE_ACCEL option -DRELAX option reimplemeted
© 2002 The Hydra Consortium
There are numerous changes. The most important one is that the
gravity and SPH accelerations are now separated from one another.
This makes the code run MUCH faster in gravity-only mode than before.
For most applications, the SPH also seems to show a speed-up; however
this is yet to tested on a wide range of data-sets. There remains a
lot of optimisation to be done for the SPH.
Minor changes introduced in Hydra4.1
If -DBYTE_SWAP is set in makeflags then Hydra will convert to/from
little-endian on input/output, storing the data files in the opposite
endianness to the internal representation.
This allows easy transfer of data files between different machine
architectures. i/o is a little slower so if only one architecture is
to be used then leave this option unset.
The cputime used is no longer automatically sent to stdout but will be
if the -DCPUTIME option is set.
There is a new option in cosmic to read the transfer function from
a CMBFAST output file (see COSMIC).
Minor changes introduced in Hydra4.2
The SPH PdV work term is modified by a factor, fhvar, to correct for
the spatial variation of the smoothing kernel, as described in
Springel & Hernquist (astro-ph/0111016). This vastly improves entropy
conservation. It can be switched off using the -HNOVAR flag.
New -DEXACTNSPH flag to force the SPH routines to do a partial sort to
find exactly nsph neighbours. Surprisingly, this does not seem to
slow the code down.
Exact entropy conservation from PdV work can be ensured by using the
-DENTROPY flag. This leads to acceptable (but not fantastic) energy
conservation. It is possible that the implementation of this could be
improved and if anyone works out how then please let me know
(p.a.thomas@sussex.ac.uk).
This flag implements a steady-state ionization field modelled on that
in Haardt & Madau 1996. It essentially limits particle temperatures
to be hotter than about 10^4K (with the precise value depending upon
density and redshift).
New SAVE_ACCEL option to save the accelerations in dump files in order
that restarts do not have to re-evaluate them. This both saves time
and ensures that restarts give identical results (without this option
there is an extra evaluation step).
The -DRELAX option to create a glass-like distribution of particles
has been reimplemented.
Last Updated February 5, 2002