# Re: Comoving integration

From: Volker Springel <volker_at_MPA-Garching.MPG.DE>
Date: Fri, 12 May 2006 20:20:20 +0200

Evgeny Kurbatov wrote:
>
> Hi!
>
> 1. The drift factor looks as if the internal velocity variables would be
> a^2 \dot{x} ,
> where x' is the comoving coordinates and a' is the scale factor. But
> procedures init' and fill_write_buffer' tell us that internal velocity is
> a^{3/2} \dot{x}
> (I mean that the `\dot{x}' stores in a snapshots).

No, what's stored in the snapshots is u = a^{1/2} \dot{x}, as explained in
the user guide. This is done for compatibility with gadget1. The internal
velocity variable in gadget2 is indeed a^2\dot{x}.

> 2. Why the gravkick factor is
> \int da / (a^2 H) ?

you can find the answer to this in the code paper.

> 3. Why the hydrokick factor differs from the gravkick factor?

because the pressure force term has a different dependence on a than the
gravity term (for fixed comoving coordinates)

> 4. How in the GADGET-2 the cooling due to cosmological expansion
> realizes? Looking into the code I didn't find corresponding term in the
> DtEntropy calculation.

The expansion of the universe doesn't change the entropy, only the density.

Volker

>
> Thanks!
>
>
>
>
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