[yt-users] chemical species abundances discrepancy

[Britton Smith]

I think there is a bigger problem here.
When you do a profile of a field (with some weight field), the values you
get out are the weight-average of the field such that:
profile (field, weight_field) = Sum (field * weight_field) /
Sum(weight_field)

You are comparing
profile(H2_Density) / profile(Density)
with
profile(H2_Density / Density)

Mathematically, these are not the same.

Or, in other words:
mean(x * y) != mean(x) * mean(y)


On Mon, Jan 27, 2014 at 5:24 PM, Stefano Bovino <poetaste at gmail.com> wrote:

> ah... I'm binning, it also depends on the number of bins I'm using, but
> the discrepancy (slight) is still there):
>
> prof3 = BinnedProfile1D(sph3, 64, "Radiuspc", 1.0e-6, 1.0e6)
>
> In my initial setup I'm using a box of 300 pc.
>
> Stefano
>
>
> On Mon, Jan 27, 2014 at 6:20 PM, Stefano Bovino <poetaste at gmail.com>wrote:
>
>> Hi Matt,
>> thanks for the quick reply.
>> I'm using simple data from the enzo Collapse Test (27).
>> It seems is mainly related to the size of the sphere I'm taking... but I
>> don't know.
>>
>> Here following a piece of my script:
>>
>>  pf3=load("DD0003/DD0003")
>> c3= pf3.h.find_max("Density")[1]
>> sph3 = pf3.h.sphere(c3, (100, 'pc'))
>> prof3.add_fields("H2I_Density")
>> prof3.add_fields("H2I_Fraction")
>> prof3.add_fields("Radius")
>>
>> ...
>> ...
>>
>> d_ax2.loglog(prof3['Radius'], prof3['H2I_Density']/prof3['Density'],
>>             lw=1.5, linestyle='--', color='r')
>> d_ax2.loglog(prof3['Radius'], prof3['H2I_Fraction'],
>>             lw=1.5, linestyle=':', color='y')
>>
>> Thanks in advance
>> Stefano
>>
>>
>>  On Mon, Jan 27, 2014 at 6:11 PM, Matthew Turk <matthewturk at gmail.com>wrote:
>>
>>> On Mon, Jan 27, 2014 at 12:14 PM, poetaste at gmail.com <poetaste at gmail.com>
>>> wrote:
>>> > Hi Guys,
>>> > I did a simple test with enzo and I tried to plot a radial profile of
>>> the
>>> > chemical species fractions with YT.
>>> >
>>> > The problem is that if I use the YT function which intrinsically plot
>>> the
>>> > fraction for a species, let's say H2I_Fraction (as usual), and compare
>>> this
>>> > with a direct evaluation of the mass fraction:
>>> >
>>> > data['H2I_Density']/data['Density']
>>> >
>>> > I obtain some slightly different results, mostly at large radii.
>>> >
>>> > Anyone might explain this discrepancy!? This happens with all the
>>> species.
>>>
>>> Without knowing what "data" here is, or how you generated it, it's tough
>>> to say.
>>>
>>> -Matt
>>>
>>> >
>>> > Thank you in advance
>>> > Stefano_______________________________________________
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>>
>>
>
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