[yt-dev] ramses length units

[Desika Narayanan]

Hey YT-dev,

I've discovered something that I think could maybe be a bug (though maybe
not) in the Ramses front end but am unsure about the motivation behind the
code so thought I'd check in and try to learn a bit.

In frontends/ramses/data_structures.py, I think the length_unit is set to
be the length_unit that comes from the info_xxx.txt file multiplied by the
box length.   For the example yt data sets, this doesn't impact anything
because the box lengths are unity.

In examining some Agora snapshots, however the box lengths are not unity -
one that I'm looking at for example has a boxlen of 600.  This results in
length units that aren't similar to what the info_xxx.txt file says, and
more dramatically, mass units that are very different.  For example, in one
such snapshot, I read from the info file:

boxlen      =  0.600000000000000E+03
time        =  0.500035810875167E+00
aexp        =  0.100000000000000E+01
H0          =  0.100000000000000E+01
omega_m     =  0.100000000000000E+01
omega_l     =  0.000000000000000E+00
omega_k     =  0.000000000000000E+00
omega_b     =  0.000000000000000E+00
unit_l      =  0.308656802500000E+22
unit_d      =  0.157339152800000E-25
unit_t      =  0.308687241173998E+17

but if I load this snapshot in yt, I get:

>In [2]: print ds.mass_unit
9.9935115109e+46 g

>In [3]: print ds.mass_unit.in_units("Msun")
5.02586592269e+13 Msun

>In [4]: print ds.length_unit
1.851940815e+24 cm

which consequently results in derived quantities from the simulation (like
stellar mass) that are totally off (by a factor boxlen**3).

So, I'm writing (as a total Ramses novice) to try to understand the
motivation behind multiplying the length unit by the boxlen in the
frontend, and if I should be accounting for this factor of boxlen**3 in my
analysis, or if this is a bona fide bug.

Thanks,
-desika

ps I updated this morning:

thundersnow:yt desika$ hg id
8d3663ac217d+ (yt) tip
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