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From: Volker Springel <volker_at_MPA-Garching.MPG.DE>

Date: Fri, 12 May 2006 20:20:20 +0200

Evgeny Kurbatov wrote:

*>
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*> Hi!
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*>
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*> Please help me to shed light on four questions:
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*> 1. The drift factor looks as if the internal velocity variables would be
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*> a^2 \dot{x} ,
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*> where `x' is the comoving coordinates and `a' is the scale factor. But
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*> procedures `init' and `fill_write_buffer' tell us that internal velocity is
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*> a^{3/2} \dot{x}
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*> (I mean that the `\dot{x}' stores in a snapshots).
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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
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*> \int da / (a^2 H) ?
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you can find the answer to this in the code paper.

*> 3. Why the hydrokick factor differs from the gravkick factor?
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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
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*> realizes? Looking into the code I didn't find corresponding term in the
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*> DtEntropy calculation.
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The expansion of the universe doesn't change the entropy, only the density.

Volker

*>
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*> Thanks!
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*>
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*>
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*>
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*>
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Received on 2006-05-12 20:19:49

Date: Fri, 12 May 2006 20:20:20 +0200

Evgeny Kurbatov wrote:

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}.

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

because the pressure force term has a different dependence on a than the

gravity term (for fixed comoving coordinates)

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

Volker

Received on 2006-05-12 20:19:49

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